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NEET Biology Gene Expression And Gene Regulation

NEET Biology Gene Expression

It is the mechanism at a molecular level by which a gene can express itself in the phenotype of an organism. The mechanism of gene expression involves biochemical genetics.

• It consists of the synthesis of specific RNAs, polypeptides, structural proteins, proteins biochemicals such as enzymes.
• DNA is the master molecule that directs gene expression.

NEET Biology Gene Expression And Gene Regulation Operon Concept

Francois Jacob and Jacques Monod, in 1%1, proposed the operon concept to explain how genes express themselves. They worked on the bacterium Escherichia coli.

• When lactose is added to the culture medium of E. coli, three enzymes are needed for the breakdown of lactose.
• Structural genes code for the synthesis of these enzymes. The functioning of these genes is regulated as a single unit by the operator gene. This entire unit is called an operon. A regulatory gene switches on or off the operator gene, which in turn regulates the activity of structural genes.

Jacob and Monod (1961) investigated the nature of induction of enzyme synthesis by E. coli. Of the 800 enzymes thought to be synthesized by E. coli, some are believed to be synthesized continuously and are called constitutive enzymes; others are synthesized only in the presence of an inducer compound, which may be the substrate, and are called inducible enzymes. β-galactosidase is an inducible enzyme.

NEET Biology Gene Expression And Regulation Notes

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Gene Regulation In Eukaryotes

• The regulation of gene expression in eukaryotes is more complex than in prokaryotes.
• In eukaryotes, the gene is split into exons and introns.
• Intron is also termed spacer DNA or IVS (Intervening sequence) The molecular basis of processes such as differentiation and development, cancer, and aging, all involve differential gene expression.
• Spliccosome Mediated Splicing

Spliceosonie. Spliceosomes are ellipsoid particles of RNA and protein; they arc -25 nm x 50 nm in size. They are assembled on the RNA precursors by an association of five different small nuclear ribonucleoproteins (snRNPs) and many proteins.

• Eukaryotic nuclei and cytoplasm contain many discrete small RNA species, called small nuclear RNAs, (snRNA) and small cytoplasmic RNAs (sc RNAs), respectively.
• The processing of the bulk of mRNA precursors is mediated by particles called spliceosomes. These mRNA precursors have many introns of variable lengths.
• Interestingly, the excision of introns does not seem to proceed sequentially, i.e., beginning at one end and progressing to the other, as is the case with transcription.
• Any intron of the mRNA precursor could be removed at a given point in time, but there seem to be some preferred patterns of their excision.

Genes

1. Housekeeping genes (Constitutive genes): The genes are constantly transcribed and expressed in the cells because the products they code for are constantly needed for cellular activities eg. genes for glycolysis, and ATP-ase to keep one alive.
2. Non Constitutive (Luxury Genes): These genes remain inactive and are expressed in certain cells only when the product they code for is needed. They are also called specialist genes e.g. gene for nitrate reductase in plants and lactose system in E. coli.
3. Viral oncogenes: A gene that can cause cell transformation in animal cells growing in culture and tumor formation in animals. They are twenty in number.
4. Proto-oncogene. A normal cellular gene that can be changed to an oncogene by mutation.
5. Nif (Nitrogen fixing) of plasmids: Plasmids are extrachromosomal DNA molecules present in addition to chromosomes in most bacteria. They can replicate like and independently of the chromosome. The plasmids carry genes for Resistance—R-genes and Nif (Nitrogen fixing).
6. Gram +ve gene: The gram +ve gene is present in gram +ve bacteria which take up the gram stain. They retain the blue color and are called Gram +ve bacteria.
7. Polygenes (Multiple genes/cumulative genes). There are many characteristics in both plants and animals that are controlled by cumulative action of dominant alleles of generally two or more independent genes e.g. height, weight, skin color in man, number of grains in an ear, and size of fruits.
8. The grani-ve gene is Present in Gram-ve bacteria which do not take up the blue colour of the Gram stain. The gram-ve bacteria can be observed with another stain such as safranin.
9. Inducible genes. Non-constitutive = Luxury gene, = Specialist gene.
10. Structural genes These genes code for chemical substances that contribute to morphological or functional traits of the cells.
    • These are nowadays called cistrons. These are of two types-Monocistronic and Polycistronic (code for many RNAs)
11. Polypeptide-coding genes These code for mRNA which in turn code for polypeptides.
12. Polyprotein-coding genes These code for more than one polypeptide per gene.
13. RNA coding genes Code for sRNA and tRNA. They are randomly repeated in the chromosomes.
14. Regulator genes Code for repressor proteins to regulate the transcription of cistrons.
15. The operator gene acts as a switch to turn on or off the transcription of a structural gene.
16. Promoter genes These genes are DNA sites where RNA polymerase binds for the transcription of RNA by the structural genes.
17. Terminator genes These genes are DNA regions where RNA polymerase activity stops to suspend the transcription of structural genes for example., UAA(Ochre), UAG (Amber), UGA (Opal)
18. Split genes Genes carrying information in pieces rather than in a continuous stretch are known as split genes such as introns /exons. The eukaryotic gene is split into exons and introns.
19. Introns. Eukaryote genes have inserts called introns which are transcribed but not translated between segments.
20. Exons are inserts of eukaryotic genes which are translated as well as transcribed.
21. F-gene (Fertility factor) contains genes for producing sex pilli and other characteristics needed for gene transfer.
22. Muton A segment of DNA that takes part in mutation
23. Subvital Genes Many mutant genes reduce the viability of the individuals carrying them in the appropriate genotype as compared to that of the normal individuals.
    • Such genes kill less than 90% of the individuals in which they are present in the appropriate genotype.
    • Most of the mutant genes are subviral in their effect, for example., miniature wings in Drosophila, some viridis mutants of barley, etc.
24. Vital Genes Those genes that do not affect the survival of the individuals in which they are present are known as vital genes.
    • Such genes neither enhance nor reduce the viability of the individuals carrying them. It does not imply that these genes are not necessary for the survival of the concerned organism.
    • It simply means that the survival of the organism is not affected by the fact that the concerned vital genes are present in the homozygous or heterozygous state. Wild-type alleles of all the genes of an organism are regarded as vital genes; they serve as the reference point in determining the effect of mutant alleles on survival.
25. Supervital Genes Some mutant alleles enhance the survival of those individuals that carry them in the appropriate genotype as compared to that of the wild-type allele, such genes are known as super ital genes.
    • Genes for disease resistance in a crop species may be regarded as superficial genes as they protect the individuals carrying them against the concerned disease and thus increase the chances of their survival in the presence of the concerned disease.
    • Similarly, genes conferring resistance/tolerance to the various abiotic stresses, for example., salinity, alkalinity, high temperature, low temperature, drought, etc., may also be regarded as superficial genes in that they enhance the fitness of the plants in the presence of the concerned stress.
26. Single Copy Gene They are present in a single copy (occasionally 2-3). They form 60-70% of the functional genes. Duplication mutation and exon reshuffling between two genes result formation of new genes
27. Repeated Genes These genes occur in multiple copies for example.,
    1. Histone genes
    2. Actin genes
    3. t-RNA and
    4. rRNA genes.
28. Multigenes (Multiple Gene Family). It is a group of similar or nearly similar genes for meeting the requirement of time and tissue-specific products e.g. globin gene family (ε,β,δ,γ) on chromosome 11, α, and β on chromosome 16.
29. Pseudogenes These are genes that have homology to functional genes but are unable to produce functional products due to intervening non-sense codons, insertions, deletions, and inactivation of promoter regions for example., several snRNA genes.
30. Processed genes Eukaryotic genes which lack introns, are formed probably due to reverse transcription or retroviruses. They are generally non-functional due to a lack of promoter genes.

NEET Biology Gene Expression And Regulation Notes

NEET Biology Gene Expression In Viruses

The term virus’ was coined by Dmitry Iwanovsky in 1892. Viruses are nucleoprotein structures devoid of cytoplasm, motility, irritability’ and energy generation they are intracellular obligate parasites using the metabolic machinery of their host for the multiplication and assembly of their parts.

• In a cell-free environment, a virus is inert and called a virion. It can be crystallized. Smallpox is the first viral disease studied by man.
• The first virus studied was the Tobacco Mosaic Virus (TMV). Mayer (1880) found that the sap of plants infected by Tobacco Mosaic could cause disease in healthy plants.
• Swarovski (1892) found that the infective property of sap remained intact even after passing through bacteria-proof filters.
• Viruses were first seen under a microscope by Takahashi and Rawlins (1933). TMV was crystallized by Stanley (1936).

Viral Reproduction Occurs By Following Ways

1. Lytic cycle in case of T₄ and other T even bacteriophages
2. Lysogenic cycle in case of λ (Lambda) phage.
3. Reproduction in single-stranded viruses such as Φ×l74 and coliphage fd. This single-stranded DNA is a plus strand and it forms a complementary negative strand, thus temporarily viruses become double-stranded.
   1. The double-stranded DNA or replicative DNA takes control over the machinery of the host and synthesizes strand DNA and protein for the assemblage of new viral particles.
4. Pinocytic Reproduction. The whole virus enters the host cells except the envelope. It is quite common in RNA viruses which are of two types :

RNA-RNA Viruses. DNA has no role in their multiplication.

After entering the host cell the viruses produce enzyme replicase which helps in producing more genetic RNA over the template of the present RNA genome. The latter also produces mRNAs for the synthesis of viral proteins.

RNA-DNA Viruses. They are also called retroviruses (Temin, 1970, e.g., Tumour/ Cancer Viruses, HIV) The viruses possess enzyme reverse transcriptase (Temin and Baltimore, 1972; in Rous Sarcoma Virus or RSV of Mouse).

• The enzyme builds DNA over the RNA genome. The phenomenon is called reverse transcription or feminism.
• The copied DNA (cDNA) builds its complementary strand. The double-strand copy DNA attaches to the host DNA/chromosome and is now called a provirus.
• It produces a chemical that disturbs cellular machinery. The cell with provirus may dedifferentiate and start dividing leading to cancer (uncontrolled growth forming tumor which sends branches/roots to invade different parts.)
• Some 20 viral oncogenes (genes that trigger cancer) have been recorded. However, no virus has been proven to cause cancer in humans.
• Humans have some protooncogenes (similar to viral oncogenes)which take part in growth but under certain circumstances, they get changed to cellular oncogenes.
• Jumping genes/transposons can also cause cancer. There are several chemicals, physical factors, and some viruses (e.g., Hepatitis B, Herpes, Papilloma, and Epstein-Barr Virus) that promote cancer.
• HIV (Human Immunodeficiency Virus) destroys the immune system of the body and causes AIDS. The provirus ultimately dissociates, transcribes viral RNA genomes, and produces reverse transcriptase and viral proteins.
• Different parts assemble to form retroviruses. The latter comes out through exocytosis or passes out to other cells through protoplasmic connections.

NEET Gene Expression And Regulation Chapter Notes

NEET Biology Gene Expression Gene Expression In Prokaryotes

In 1946, two American scientists, J. Lederbcrgand E.L.Tatum demonstrated sexuality in bacteria for the first time, and this opened a new era of research.

• The prokaryotic chromosome is circular. It is represented by a single double-stranded DNA molecule.
• It is usually attached to a cell membrane at one point and does not integrate with histones and is not bound by a membrane for example., bacteria and blue-green algae (cyanobacteria). Bacterial DNA is associated with few proteins. The genome of E.cali contains 2000 to 3000 genes.
• The bacterium can survive on a glucose diet giving the idea that its genes have information for the synthesis of practically all the organic compounds it needs.
• Mycoplasma (PPLO) has a genome of a few hundred genes that code for a few proteins. Bacteria divide very rapidly.
• The doubling time is also called generation time and it may be as low as 20 minutes. Bacteria mainly reproduce by asexual reproduction but do not exhibit true sexual reproduction as they do not produce a diploid phase. Thus, meiosis is lacking. However, bacteria exchange genetic material between two cells.

Modes Of Genetic Transfer In Bacteria

Three modes of genetic transfer between bacterial cells are:

1. Transformation
2. Transduction
3. Conjugation.

Transformation. It is also called the Griffith effect or the Recombination effect.

• The phenomenon by which DNA isolated from one type of cell, when introduced into another type can bestow some of its properties into the latter, is referred to as transformation.
• It was confirmed by Griffith with his experiments on the bacteria Streptococcus pneumoniae. Griffith used S-3 and R-2 strain out of S-1, S-2, S-3, and R-1, R-2 and R-3 strains

Transduction. The transfer of genetic material from one bacterium to another through bacteriophage is called transduction.

Transduction It Is Of Two Types:

1. Restricted and
2. Generalized transduction.

Conjugation:

Conjugation refers to the unidirectional transfer of DNA between cells via a cytoplasmic bridge. The process is analogous to sexual reproduction in eukaryotes.

• Occasionally, the F factor integrates into bacterial chromosomes. Such bacteria can transmit their genetic material into female cells with high frequency (Hfr) in a specific sequence.
• These are referred to as Hfr strains. Conjugation was initially established by Lcdcrberg and Tatum in Escherichia coli.
• The recombination frequency was exceedingly low in Lederberg’s studies. Hayes (1952) identified a strain of E. coli exhibiting a recombination frequency 100 to 1000 times greater than that reported by Lcdcrberg.
• The strain was designated as the high-frequency recombinant (Hfr) strain.

Conjugation It Is Of Two Types :

1. Sterile male method
2. A fertile male method that results in the formation of Hfr maniac or Supermale.

NEET Biology Gene Expression And Regulation Revision Notes

NEET Biology Gene Expression And Gene Regulation Important Contributors

Robert Briggs and Thomas Kings. Conducted experiments on nuclear transplantation and concluded that nuclei undergo some change during differentiation.

• Johanssen. Introduced and defined the term gene.
• Beadle And Tatum. Conducted experiments on Nenrospora and proposed one gene-one enzyme hypothesis. Yanofsky. Proposed one gene-one polypeptide hypothesis.
• Jacob And Monod. Found that genetic material possesses regulator gene units called operons. Shared Nobel Prize with Lwoff of France (1965).
• Pirk And Bowden. They found the nucleoprotein nature.
• Stanley. He was the first to isolate Tobacco Mosaic Virus (TMV) and crystallize the same. Lcdcrberg and Hayes discovered Plasmids.
• N.C. Jacole and Wolfman found the conversion of the plasmid into episome.
• Paul Berg— father of genetic engineering transferred gene SV 40 Virus into E.coli
• Safferman and Morris discovered cyanophages Barbera McClintock (Lady Mendel) discovered jumping genes. (The term transposon was given by Hedges and Jacob)

NEET Biology Gene Expression And Regulation Revision Notes

NEET Biology Gene Expression And Gene Regulation Gene Expression Synopsis

Prions (Prp) self self-duplicating protein molecules discovered first by Alper et al (1966) and cause diseases like scrapie and Kuru

• The term gene was given by Johanssen and genes are arranged in a linear order and the term Genetic code was given by Gamow.
• Cosmids are plasmids in which particular (passenger) DNA sequences have been inserted. They are novel vectors that combine the features of a plasmid and 1-phage. It carries genes for resistance to antibiotics. A cosmid has two sites of the RE enzyme
• Episomes are genetic elements of dual capacity to existing either as a chromosomal or as an extrachromosomal genetic element and are non-essential to the cell.
• Pseudogenes are DNA sequences derived from m-RNA through reverse transcription.
• Cancer-causing genes are called oncogenes (Discovered by Michael Bishop and Harold Varmin).

Nearly 20 viral genes have been identified which are involved in triggering cancerous cells.

• In 1988 Watson accepted the job as co-ordinator of “The Human Genome Project. ” Recent work in genetics is being done on this project.
• Carlson et. al. 1972 for the first time used the leaf protoplasts of Nicotiana glauca and N.langsdorjfii and successfully regenerated an interspecific hybrid plant.
• 2, 4-D promotes callus formation in cereals.
• Mitra and Kaul (1964) were among the first to produce risperidone from Rauwoljfia serpentina tissue culture. H.Kohn et. al. (1985) produced somatic hybrids in tobacco.
• Repetitive DNA. It consists of DNA nucleotides or base sequences which are few to several human base pairs (bp) long and are present in several to million copies per genome. Short sequences of bases are repeated hundreds of times in DNA.

Unique DNA. It consists of those DNA segments whose base sequences are present only in a single copy per genome i.e. their base sequences are not repeated in the genome.

• Promiscuous DNA. The DNA segments that have migrated from one organelle to another organelle i.e. chlo- roplast to mitochondria or vice versa.
• The discovery of promiscuous DNA has lent support to the endosymbiotic hypothesis of the origin of chloroplast and mitochondria.

Replication is of three types: viz

1. Conservative. A new strand is formed over the existing strand
2. Disruptive. Old strand undergoes fragmentation, the fragments synthesize complementary structures both of which assemble to form two replicas
3. Semiconservative. Because of the sequential opening of the DNA double chain and its replication to form two new chains DNA replication is called Zipper duplication.

Effect Of Medium Supplements On The Growth Of Mutants Of Neurospora Crassa:

NEET Biology Gene Expression And Regulation Study Material

Some Inhibitors Of Bacterial Protein Synthesis:

• RNA Polymerase 1. This enzyme is located in the nucleolus and is responsible for the transcription of ribosomal RNA. Polymerase I is not sensitive to a-ainanitin which is a bicyclic octapeptide.
• RNA Polymerase 2. This polymerase is located in the nucleoplasm which is part of the nucleus other than nucleolus. This enzyme is the major component of RNA polymerase activity, and it transcribes all the genes that produce mRNA.
• In eukaryotes, the transcript is initially in the form of mRNA precursors, called heterogeneous nuclear RNA (mRNA) which are later processed into mRNA (see, later). Polymerase I activity is rapidly inhibited by low concentrations of α-ainanitin.
• RNA Polymerase 3. It occurs in nucleoplasm and transcribe RNA and 5 RNA genes. It is inhibited by high levels of amanitin in animal cells but not in yeast and insects.

Inducer. A substance of low molecular weight that combines with a repressor and thus decreases the repression of protein synthesis. Repressor. A protein that switches on or off the operator gene is termed a repressor.

Gene Expression And Regulation NEET Chapter Summary

• Promoter. A part of an operon; is a nucleotide sequence to which RNA polymerase binds and initiates transcription. Also, a chemical substance that enhances the transformation of benign cells to cancerous cells.
• Regulator gene.  A gene that controls the rate of expression of another gene or genes. Example: The operon involved in lactose production of Escherichia coli has a regulator, an operator, and structural genes.
• Operator gene. A part of an operon that controls the activity of one or more structural genes.
• Operon. A group of genes makes up a regulatory or control unit. The unit includes an operator, a promoter, and structural genes.

Structural genes. A gene that controls actual protein production by determining the amino acid sequence (cf. Operator gene and Regulator gene.)

• Cistron. A gene segment that transcribes a single stretch of RNA is called a cistron or it is a unit of function in DNA
• Polycistronic. The long messenger RNA covering all the cistrons is called polycistronic.
• Introns. The regions of a gene which does not form part of mRNA and are removed during RNA processing before mRNA formation are referred to as introns.
• Exons. The region of a gene, which becomes part of mRNA and codes for different regions of the protein is referred to as exons.

Repressive enzyme.

Enzymes are synthesized in a cell in the absence of its final product. It is suppressed when the final product accumulates.

• The enzyme that can be activated. An enzyme is generated only in the presence of a substrate that functions as an inducer. Constitutive enzyme. An enzyme is synthesized in constant amounts irrespective of need (compared with inducible enzyme and repressive enzyme).
• Sexduction.The integration of bacterial genes via sex factors and their subsequent transmission through conjugation to a recipient cell.
• Sexual element. An episome (F+ in E. coli) allows a bacterial cell to serve as a donor of genetic material. The episome can be moved inside the cytoplasm during conjugation or integrated into the bacterial chromosome.
• Genetic equilibrium. State in a population of interbreeding organisms where specific gene frequencies remain unchanged across several generations.

Hfr. High-frequency recombination strain of Escherichia coli: the F episome is integrated into the bacterial chromosome.

• Lysis and Lytic bacteria. Bursting of a bacterial cell by the destruction of the cell membrane following infection by bacteriophage and such bacteria are called lytic bacteria.
• Lysogenic bacteria. Those harboring temperate bacterial phages.

NEET Biology Principles Of Inheritance And Variation

Genetics:(G.genesis_ descent) The study of the mechanism of inheritance and the control of the characteristics of an organism by its genes is called genetics. The name ‘genetics’ was first proposed by Bateson in 1906.

Heredity: (L he red inis = heirship). The transmission of characters, resemblances, and variations from one generation to another is called heredity.

Variations: The differences shown by the individuals of a species and also by the offspring of the same parents are referred to as variations.

Epigenetic Theory: A German zoologist, C.F. Wolff, proposed that neither the egg nor the sperm had a structure like a homunculus, but the gametes contained living substances capable of forming the embryo after fertilisation.

Magnetic power theory: Proposed by W. Harvey, it suggested that the utensils, through the friction of coitus, acquire some magnetic power to conceive an embryo. f-3 Kolreuter, the German botanist (1868), worked on crosses between different species of tobacco and gave some experimental evidence to prove that hereditary units tended to remain discrete in different generations.

Mendel: Mendel was the first to explain that heredity involved the transmission of units from the reproductive cells of the parents to the offspring.

NEET Biology Principles Of Inheritance And Variation Notes

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Hugo de Vries, Correns, and Tschermak rediscovered the laws of Mendel and extended the work by experiments on various plants and animals.

• The physical basis of heredity is genes, present on chromosomes, and the chemical basis of heredity is DNA.
• Various pre-Mendelian concepts such as Moist vapour theory, Fluid Theory (Empedocles), Reproductive Blood Theory by Aristotle, and Preformation Theory (Malpighi). Theory of Pangenesis- Maupertius, had been proposed

NEET Biology Principles Of Inheritance And Variation Mendelism

Gregor Johann Mendel is called the ‘Father of Genetics’ and the study of principles of heredity’ laid down by Mendel is called Mendelism.

• Mendel was born in 1822 and became a schoolteacher in 1854.
• Mendel studied physics, mathematics, and the natural sciences at Vienna University.
• Mendel procured 34 varieties of pea that showed clear differences in characteristics and used them in breeding experiments.

The seven traits (characters) which interested Mendel were:

1. Length of stem (plant height),
2. Shape of seed,
3. Seed colour ;
4. Pod colour,
5. Pod shape,
6. Flower colour
7. Flower position.

Mendel studied the law of heredity. Mendel simply described the results of his crosses based on meiosis and fertilization and drew certain conclusions. Correns gave these conclusions the shape of laws.

• But his laws were rediscovered and finally got recognition in 1900 through the discoveries of Hugo de Vries, Carl Correns, and E.V. Tschermak.
• Mendel presented his “Experiments in Plant Hybridization” before the Brunn Natural Society on February 8, 1865. The society published Mendel’s research in 1866.

Mendel’s Law Of Dominance. This law states that the character that expresses itself in the F₁ generation is dominant, and the alternative character not expressed is recessive. The law of purity of gametes, also called the law of splitting of hybrids, states that each gamete receives only one gene of a trait.

Menders laws Of segregation: According to this law, the factors regulating character are separated during the formation, and hence only one of the pairs is transmitted by a particular gamete.

Mendel Law Of Independent Assortment: This law states that when there are two or more pairs of factors. The members of one of the factors segregate and assort independently of other pairs.

NEET Biology Principles Of Inheritance And Variation Notes

NEET Biology Principles Of Inheritance And Variation Terminology Used In Genetics

Gene: According to Mendel, a gene (factor or determinant) is a unit all is ivs|vnsiMe tor die transmission and expression of a hereditary trait. A gene has two different forms or alleles.”
Or
The concept of the gene resulted from a convergence of cytology and genetics is gene is a segment of chromosomes which acts as a carrier of characters.
Or
Gene is a segment of nucleic acid that is responsible for the transmission and expression of a hereditary trail. A gone may have a large number of hereditary alleles that arise by mutation or by recombination.
Or
Gene is a segment of DNA which expresses itself by directing the synthesis of specific polypeptides.

• The term gene was given by Johannsen (in 1909). A gene is a unit of inheritance which is carried from a parent by a gamete in a chromosome and controls the expression of a specific character in the young one.
• Genotype: It refers to the genetic makeup of an organism (Johannsen 1909). Genotype is the gene complement or genetic constituent of an individual about one or more characters irrespective of whether genes are expressed or not.

Phenotype: (Johannsen, 1911) It is a measurable or observable distinctive structural and functional characteristic of an individual about one or more characters which is a result of gene products brought to expression in a given environment.

• The characteristic may be visible to the eyes Example (the height of a plant) or may require speculation for its identification (for example., a serological test for a blood group.)
• The tall pea plants have two types of genetic combinations (TT and Tt). Irrespective of the genetic set up they develop the same character. Thus phenotype is dependent upon genotype and expressibility of its genes.

Alleles or Allelomorphs: (Allele-belonging to one another). It is used to refer to one member of a gene pair.

• According to Mendel, two genes representing two alternatives of a character are present on two separate chromosomes of a homologous pair but at the corresponding loci Example in the gene pair, A (tall) is present on one chromosome and (dwarf) is on the other chromosome.
• Each of them is called an allele, i.e. T. an allele to T and vice versa.
• Pseudoalleles: Genes having separate but nearby loci and controlling the same trait In Drosophila, the genes for apricot and white eye colour are pseudoalleles. Duplicate Genes.
• Two pairs of genes lie in different chromosomes and produce a single trait with or without a cumulative effect. The coat colour in the Duroc-jersey breed of pigs depends on 2 pairs of genes.

Lethal Genes: Genes that make the organism having them inviable to live. In the yellow race of mouse, Mus musculus, the genes YY kill the embryos.

Atavism (Reversion). Occasional reappearance of a remote ancestral trait in some individuals. Example A short tail in some babies.

Dominant Factor: A heterozygote possesses two contrasting genes but only one of the two can express itself while the other remains hidden.

• The gene which gains expression in the F₁ hybrid is known as the dominant Example (T is dominant and masks the t).
• Recessive factor: The factor of an allelomorphic pair which is unable to express itself in the presence of its contracting factor is called the recessive factor in the case of Tt.
• Wild type: The commonly occurring phenotype in the natural breeding population is called wild type.

Hoinozvgotc: It is the individual whose homologous chromosomes bear identical genes or alleles of a given allelic pair and produces gametes all example RR.rr, TT. u.TTRR etc.

Heterozygote: It is the individual whose homologous chromosomes bear unidentical or dissimilar genes or alleles of a given allelic pair and produces dissimilar or different types of gametes, for example, Tt, Rr, Tt Yy etc. (The number of the types of gametes produced by heterozygote can be found out by the formula 2n where ‘n’ stands for the number of pairs of contrasting characters present in the heterozygote). It is also called hybrid.

NEET Biology Principles Of Inheritance And Variation Notes

• Hybrid vigour or Heterosis: It is the increased vigour or strength of the hybrids than their parents. It is of superior quality to either of the parents.
• Pure Line (Johannsen, 1900). The series of generations of organisms or descendants which breed true because they are derived from a single self-fertilized homozygous ancestor or identical homozygous ancestors (members of a pure line are therefore homozygous for one or more characters).
• Linkage: Linkage was first seen by Sutton. Term linkage was given by Morgan. It is the phenomenon of staying together of certain genes during inheritance through several generations without any change or separation.
• Back Cross: It is a cross which is performed between a hybrid and one of its parents. In plant breeding, a back cross is performed a few times to increase the traits of that parent.

Test Cross: It is a cross to know whether an individual is homozygous or heterozygous for a dominant character. The individual is crossed with a recessive parent.

• The offspring will be 100% dominant if an individual is homozygous dominant. The ratio will be 50% dominant and 50% recessive in the case of hybrid or heterozygous individuals.
• Thus monohybrid test cross ratio is 1:1 In case of double heterozygote (Rr Yy) crossed with double recessive (rryy) the ratio will be 1:1:1:1.

Out Cross: Cross between F₁ individuals and homozygous dominant parent to improve or produce hybrid plants which are superior to parents, we make out-cross.

• Eugenics: The application of genetics in an attempt to improve the hereditary qualities of humans.
• Gene locus: Location of a particular gene (or one of its alleles) on a chromosome.
• Polyploidy: Having three or more complete sets of chromosomes.
• Qualitative inheritance: It is a type of inheritance in which a single gene influences a complement.

Principles Of Inheritance And Variation NEET Notes

Penetrance: A genotype can produce its phenotype in an individual. It is of 2 types :

1. Complete Penetrance Full expression of a genotype in an individual. In humans, the allele B for brown eyes and the allele b for blue eyes have complete penetrance in the homozygous condition, BB, bb. The allele B has complete penetrance even in the heterozygous condition, Bb also.
2. Incomplete (Reduced) Penetrance. Failure of a genotype to express in all cases. In humans, diabetes Mellitus is genetically controlled. However, not all persons carrying the genes for this disorder develop it.

Relation among pairs of independent alleles Gametes F₂ genotypes and F₂ phenotypes when domainer is prevented:

Self Stecrility: Hound in plants having multiple alleles for compatibility incompatibility reaction – S₁, S₂, S₃, S₄, S₅, S₆, etc. A plant carries two such allelesExample S₁, S₂, S₂, S₃, S₁, S₃,S₂,S_4,S_3,S₅.

• A pollen grain carries only one allele. If it happens to be one of the two alleles of the pistil, the pollen grain fails to form a pollen tube.
• Heterodominance or Overdominance or Superdominance: Scrra( 1959) described when F₁ generation heterozygotes have a more complex phenotype than that of a corresponding parent, it is termed hetero-dominance.

F₁ Generation: The F₁ or First filial (Filus — son and filial—daughter) generation is the generation of hybrids produced from a cross between two genetically different individuals called parents as Tt individuals are produced from a cross between TT and tt parent.

F₂Generation. F₂ or Second filial generation is the generation of individuals which arises as a result of inter-breeding amongst individuals of the F₁ generation.

• Principle of Paired Factors. Each character is represented in an individual by two determinants, factors or genes. They are similar in homozygous individuals which breed true. The factor or a character represents two alternatives of a trait in hybrids.
• Monohybrid Cross: It involves the study of the inheritance of one pair of contrasting characters.
• Monohybrid Ratio: It is the ratio obtained in the F₂ generation when a monohybrid cross is made and the offspring of the F₁ generation are self-bred. Monohybrid ratio is usually 3 :1 (phenotypically) and 1: 2 :1 (genotypically).
• Dihvbrid Cross: It is a cross between two organisms of a species which is made to study the inheritance of two pairs of characters.

Trihybrid Cross: A cross between homozygous parents that differ in three gene pairs (i.e. producing trihybrids) is a combination of three single-pair crosses operating together. Thus (AA x aa), (BB x bb) and (CC x cc) could be combined in the same cross.

Principles Of Inheritance And Variation NEET Notes

• Dihybrid Ratio: It is the ratio obtained in the F9 generation when a dihybrid cross is made and the offspring of the F₁ generation are self-bred. Phenotypically it is 9:3:3:1.
• Trihybrid ratio: Ratio in F₂ obtained in a trihybrid cross represented by a Punnet square. 64 (8n) squares would be required with a phenotypic ratio of 27:9:9:9: 3:3:3:1.
• Polyhybrid Cross: It includes those characters in which inheritance of more than two pairs of genes are considered.
• Reciprocal Crosses: These crosses involve two concerning the same characters but with reversed sexes. It means if in a first cross, A is used as the female parent and B as the male parent, then in 2nd or reciprocal crosses A will be used as the male parent and B as the female parent.

Punnet Square: It is a chequerboard used to show the results of a cross between two organisms. The chequerboard was devised by R.C. Punnet. It depicts both the genotype and phenotype of the progeny.

• Dihybrid: An individual that is heterozygous for two pairs of alleles. The progeny of a cross between homozygous parents differs in two respects.
• Homozygous: The condition in which only one allele of a pair is present, as in sex linkage or as a result of deletion.

The formula for number of genotypes in case of multiple alleles is \(\frac{n}{2}(n+1)\) = n Alleles. i.e. in the case of the blood group system, 3 alleles regulate the ABO blood group thus number of the genotype formed will be six.

• If a cross is made between two yeasts having genotype Aa Bb Cc. then the probability of getting genotype aa bb cc will be 1/64 because if we consider individuals the probabilities of occurring of aa, bb and cc is — \(\frac{1}{4}\) therefore = \(\frac{1}{(4)^3}=\frac{1}{64}\)
• Three genotypes are produced by two alleles.
• Even Aa Bb Cc DD Ee Ff produces 32 different gametes.
• A recessive allele in the zygote appears only if both parents possess this allele.
• Progeny = offspring ir
• Siblings or sibs, Individuals having the same parents, brothers and sisters.

Principles Of Inheritance And Variation NEET Notes

NEET Biology Principles Of Inheritance And Variation Modifications Of Mendelian Ratio And Gene Interaction

Gene interaction. It is the influence of one gene (allele) over another which causes a change in expression, and phenotype of normal Mendelian ratios like 3: 1 (monohybrid cross) and9:3:3:l(dihybrid cross.).It may be intragenic (intr alleles) or intergallic (non-allelic). They are of the following types :

• ID Incompletedominance (Intermediateinheritance). It is the phenomenon where none of the two Mendelian factors, alleles or genes is dominant over the other.
• Genotypic and phenotypic ratios are the same in the F2 generation i.e. 1:2:1. Example. Illustrated in Mirabilis jalapa (Four o’clock), Antirrhinum (Snapdragon or Dog flower) and Andalusian fowls.
• In a dihybrid cross if one trait follows the law of dominance the other shows incomplete dominance, the ratio comes to be 3: 6: 3:1 : 2: 1 and if both traits show incomplete dominance, it will be1: 2:1: 2: 4: 2: 1.
• Codominance. It is the equal and independent expression of two alleles of a trait when they are present together in an individual. The phenotype of a heterozygous individual is different from either of the homozygous.
• Example. A, B, AB and O blood group.

NEET Principles Of Inheritance And Variation Chapter Notes

Duplicate Genes (15: 1) Observed by G.H. Shell Pseudoalleles or Duplicate genes or factors are two or more independent genes present on different chromosomes which determine the same or nearly the same phenotype in a dominant state so that either of them can produce the same character. The independent genes do not have a polymetric effect, e.g. Endosperm colour in maize, fruit shape in Shepherd’s’ purse, and inheritance of grain colour in Avena sativa (Oat).

• Polymetric or Additive genes (9:6: 1). They are duplicate genes with cumulative effects. Two independent dominant genes, whether present in homozygous or heterozygous conditions have similar phenotypic effects when present individually but produce a cumulative or double effect when found together for example Summer squash plant (Cucurbita pepo) has three types of fruit forms.

Polygenes (Quantitative inheritance) or Multiple genes or Cumulative genes. There are many characteristics in both plants and animals which are controlled by the cumulative action of dominant alleles of generally two or more independent genes for example., height, weight, size of fruits, number of grains in an ear, skin colour in mah.

NEET Principles Of Inheritance And Variation Chapter Notes

Illustrations of Polygenes in Human Skin Colour

• Human skin colour is controlled by the polygenic effect of at least three separate genes.
• Each gene contributes to a unit of darkness’ due to incomplete dominance. ‘
• Skin colour is determined by cumulative genes and this hypothesis was designed by Davenport and Davenport in 1910.
• Davenport and Davenport designated five phenotypic classes controlled by genes A . and B (like kernel colour of wheat)

Hughes (1944) recognized seven phenotypic classes designating genes A, B and C.

• The skin shades vary from very dark in AABBCC individuals to very light in an aabbcc individual.
• A person with AaBbCc (i.e. heterozygous for all three genes) will have an intermediate colour termed mulatto.
• The number of possible allele combinations in the gametes is eight (ABC, ABc, AbC, aBC, abC, aBC, abe. abc) for such a person.
• So a total of 64 phenotypic combinations is possible when two persons of ABC x abc gene combinations marry.

NEET Principles Of Inheritance And Variation Chapter Notes

NEET Biology Principles Of Inheritance And Variation Characteristics Of Polygene

1. Negligible effect of environment.
2. No involvement of linkage and epistasis.
3. The quality of the gene does not matter.
4. Each contributing gene (allele) in a series produces an equal effect.
5. The effect of each contributing gene is additive (cumulative).

Short Cut Method To Predict The Phenotype Ratio Of Polygenic Inheritance :

• We should know the numbers of total alleles involved in a quantitative inheritance to predict the phenotype ratio of F-, generation.
• This prediction can be shov/n with the help of Pascal’s triangle: e.g. Kernel colour of wheat there are 4 alleles and a ratio of 1: 4 : 6: 4: 1 is obtained.
• Nilsson-Eble (1908) obtained the first experimental proof of polygenic inheritance.

Principles Of Inheritance And Variation Class 12 Notes For NEET

Some Ratios Of Polygenic Inheritance:

Quantitative or polygenic inheritance was first studied by J. Kolreuter in the case of tobacco and F. Galton in the case of human beings, Nilsson-Ehle in kernel colour in wheat and Emerson and East studied it in cob length in maize. Davenport studied skin colour in man

The relative contribution of each polygene

= \(\frac{\text { Maximum height }- \text { minimum average height }}{\text { Total number of polygenes }}\)

• Complementary genes (9:7) (Bateson and Punnet 1906). These non-allelic genes independently show a similar effect but produce a new (rail when present together in dominant form for example., flower colour in sweet pea, grain colour In lice, grain colour in Sorghum.
• Supplementary genes (9:3: 4): They are a pair of non-allelic genes; one of which produces its effect independently when in the dominant state while the dominant allele of the other is without any independent effect but can produce a new trait along with the dominant allele of the form example Glume colour in Sorghum emulation, Seed colour in Lab lab, Coat colour in mice and guinea pig.
• Collaboration: Epistasis of complementary and supplementary genes produces a ratio of 27: 37. c.g. Maize colour pigment anthocyanin is due to two complementary genes and one supplementary gene.

Principles Of Inheritance And Variation Class 12 Notes For NEET

Epistasis: It is the phenomenon of masking or suppressing the expression of a nonallelic gene. The gene which suppresses other non-allelic genes is called the epistatic gene while the gene or locus which is suppressed by the non-allelic gene is called the hypostatic gene.

• Dominant epistasis (12: 3:1): The dominant allele at one locus suppresses the expression of another gene regardless of its allelic condition ( dominant or recessive)Example Fruit colour in cucurbita (yellow, green and white), coat colour in the dog (White, black and brown).
• Recessive epistasis (9:3:4): In this case, recessive homozygous genotype at one locus (aa) suppresses the expression of non-alleles at another locus. The latter can produce their effect only when a dominant allele occurs at the first example Pigmentation in onion bulb, and coat colour in mice.
• Dominant-recessive epistasis (13 : 3): Here the dominant allele at one locus (A-) and recessive allele at another locus (bb) give rise to the same effect. (A-B, A-bb, aabb) other gene combinations produce different phenotypesExample Plumage colour in poultry birds.

Summary Of Epistatic Ratios:

Lethal genes. These genes kill the organism when they can express their effect. Dominant or homozygous recessive, for Example, Sickle cell anaemia, Fur colour in mice (first studied by Cuenot in 1905)

Principles Of Inheritance And Variation Class 12 Notes For NEET

• E. Baur. Observed lethal genes in. Snapdragon (Antirrhinum) ill Yellow Lethals in Mice
• Lucien Cuenot (1905) reported an incompletely dominant allele Y for yellow coats in mice.
• ‘A’ represents a gene for yellow and ‘n’ one for black.
• W. Castle and C. Little (1910) proposed that the yellow homozygotes were aborted in the uterus.
• In other words, the yellow allele has a dominant effect on coat colour but acts as a recessive allele concerning the lethality phenotype.

Albinism In Corn (Zea mays)

• Albinism in corn is due to a lethal gene.
• The gene ‘G’ is for normal chlorophyll production. It is completely dominant to its allele.
• G plants contain chlorophyll and are photosynthetic.
• On the other hand, gg plants produce no chlorophyll and are yellowish white and die.

Pleiotropic Genes: These genes have multiple effects because they influence several traits simultaneously.

Examples

1. Marphan’s syndrome is caused in human beings by a pleiotropic gene which is characterised by a slender body, limb elongation, hypermobility in joints, lens dislocation and a tendency to develop heart diseases.
2. Sickle cell anaemia. Sickle cell anaemia disease is caused by gene (Hbs) which is lethal in the homozygous condition but has a slightly detectable effect in the heterozygous condition. In sickle cell anaemia, a change in the shape of red blood cells occurs in the venous blood.

Being deficient in oxygen tension, these erythrocytes show a marked change in their structure attaining a sickle-shaped structure. As a result, rupturing of cells may take place and chronic haemolytic anaemia is caused.

• This disease is caused when the gene responsible for haemoglobin produced by recessive alleles differs in one amino acid i.e. it incorporates valine in place of glutamic acid, at the 6th position of the β-chain.
• Sickle cell anaemia is common in persons of African descent and is also found in some other parts of the world where malaria is, or has been major cause of death.
• In heterozygotes Hb^AHb^S, some red cells contain haemoglobin A, and other haemoglobin S. Because both types of haemoglobin, rather than a single intermediate form are produced, it is also a case of codominance.
• Under normal conditions, heterozygotes manifest none of the severe symptoms of HbsHbs persons, though they may suffer some periodic discomfort and even develop anaemia after a time at high altitude.

Principles Of Inheritance And Variation Class 12 Notes For NEET

Explanations for various genotypes are :

• Hb^AHb^S — Normal (Haemoglobin A only: no sickling)
• Hb^AHb^S — Sickle cell trait (Haemoglobin A and S: sickling under reduced oxygen tension)
• Hb^SHb^S — Sickle cell anaemia (Haemoglobin S only: sickling under normal oxygen tension)

Sickle cell anaemia can also be considered a case of lethal genes.

• A lethal gene can be defined as a gene whose phenotypic effect is sufficiently drastic to kill the bearer.
• The expected ratio in lethal gene cases comes equal to 2: 1.
• In sickle cell anaemia, death is caused by Hb^SHb^S condition.

Multiple Alleles:

Genes possessing multiple alternative forms, such as 15 alleles for eye color in Drosophila.

• Four alleles determine coat colour in rabbits. ABO blood groupings.
• Multiple alleles exhibit the following characteristics
• Several alleles are situated at the identical locus on the chromosome.
• Several alleles govern a specific trait.
• Crossover events do not occur across multiple alleles since they occupy the same locus.
• Multiple alleles can exhibit dominant or intermediate traits, but the wild type in the series is often dominant.
• Wild type or dominant alleles are denoted by uppercase letters, whereas recessive alleles are indicated by lowercase letters

Blond groups:

Viv determined by two types of antigens (A and B) present on the surtax coating to R.B.C.These antigens occur in the oligosaccharides-rich head regions of a glycophorin.

NEET Biology Principles Of Inheritance And Variation Revision Notes

Showing phenotype, genotype and antigen.

• Additive Factors: Polygenes affecting the same trait, with each enhancing the phenotype.
• Concordance: Identity of matched pairs or groups for a given trait; for example, identical twins both expressing the same genetic syndrome.

NEET Biology Principles Of Inheritance And Variation Revision Notes

NEET Biology Principles Of Inheritance And Variation Important Contributors

E. Haeckel:

Inheritance transmission via the nucleus.

• Pythagoras: He posited the hypothesis of moist vapour.
  Empedocles proposed that each bodily portion generated a fluid, so introducing the fluid theory of inheritance.
• Aristotle proposed the hypothesis of reproductive blood.
• Darwin posited that each somatic cell and tissue in the body generates minute particles known as gemmules or pangene, which encapsulate both hereditary and acquired traits.
• Leeuwenhoek: First observed spermatozoa.
• Malpighi posited that a homunculus, or miniature individual, exists within sperm or egg cells.
• Maupertuis proposed the notion of pangenesis, positing that heredity is governed by minute particles.
• Kolreuter, a German botanist. He produced viable interspecific hybrids in tobacco. He is the ‘‘father of polygenic inheritance.”

Gregor Johann Mendel. Father of genetics as he was the first to demonstrate the mechanism of transmission of characteristics and biochemical disorders of man.

• Johannsen. Coined the terms gene and genotype, phenotype and pure line.
• Gallon. Started pedigree analysis.
• R.C. Funnel. Used a chequerboard called Punnet square to show the results of a cross between two organisms.
• Sir Archibald Garrod. Studied ABO blood types.
• F. Gallon. He studied polygenic inheritance in man and coined the term eugenics.
• Nilsson-Ehlc. Obtained the first experimental proof of polygenic inheritance in the case of kernel colour of wheat.
• Karl Landsteiner. (1907) discovered A. B, O blood groups.
• Gastello and Steini (1902) discovered the A B blood group.
• Kolreuter: (German botanist). He obtained fertile interspecific hybrids in tobacco. He is the “father of polygenic inheritance.
• Karl Landsteiner and Weiner discovered the Rh Factor.
• Corrrens, de Vries and Tsechermak rediscovered Mendel’s theory ol inheritance. Also discovered incomplete dominance and cytoplasmic inheritance.

NEET Biology Principles Of Inheritance And Variation Revision Notes

NEET Biology Principles Of Inheritance And Variation Some Important Ratios

NEET Biology Principles Of Inheritance And Variation Revision Notes

NEET Biology Principles Of Inheritance And Variation Variations

All living beings exhibit variations in practically every character and almost all directions.

• The differences shown by the individuals of a species also by the offspring of the same parents are referred to as variations.
• All organisms change themselves to adapt themselves to the changing environment, otherwise they fail to survive. Variations are classified in two ways i.e.

As per the nature of the cells, it affects.

Somatic variations. Such variations are not inherited from parents and affect the omalic cells only.

• They are acquired by an organism during its lifetime and are lost with death Hence, such variations are also called acquired variations. There are possible causes for such variations :

Three possible causes of acquired variations are :

1. Environmental Factors i.e. medium, light, temperature, nutrition and water.
2. Use of disuse of organs
3. Conscious efforts

Germinal variations (Blastogenic Variations). These variations cause the line to grow cells and are thus inheritable for haemophilia, blood groups, colour-blindness, hairiness, eve vvIihii etc SikIi satiations may be due to the following reasons. As per the degree of difference produced. 2 types

1. Merstic = Influence the mnnlicr of parts
2. Substantive = Inllncnec the shape, size, weight and colour.

Continuous sanctions: Variation not represented by distinct classes. Individuals grade into each other and measurement data are required for analysis (cf. Discontinuous variation), Multiple genes or polygenes are usually responsible for Ibis type of variation, (or Fluctuating variations).

• Discontinuous variations. Distinct classes such as red versus white, tall versus dwarf (rf. Continuous variation).
• Transgressive variation. The appearance in (lie F₂ (or later) generation of individuals, showing n more extreme development of a trait than shown in either original parent. It is due to polygenic inheritance discovered by Punnet and Hailey.
• Determinate Variations (Orthogenic variation). Adaptive and selective variations of definite evolutionary lines, developing progressively generation after generation.
• Indeterminate variations. Variations occurring in any conceivable direction and are not governed by any law may give a distinct advantage to the possessor in its light for existence.

Techniques Used To Study Lutinuii Genetics

1. Pedigree analysis
2. Population genetics
3. Human karyotype.

Pedigree analysis: Pedigree is a record of the inheritance of certain traits for two or more generations presented in the form of a diagram family tree or symbols. Different symbols used are.

• The idea dial disorders are inherited has been prevailing in human society for a long. Dus was based on the heritability of certain characteristic features in families.
• After the rediscovery of Mendel’s work, the practice of analysing the inheritance patterns of traits in human living began.
• Since it is evident that control crosses that can be performed in plants or some other organisms, are not possible in the case of human beings, the study of the family history about inheritance of a particular trail provides an alternative.
• Such an analysis of traits in several of generations of a family is called the pedigree analysis. In the pedigree analysis, the inheritance of a particular trait is represented in the family tree over generations.
• In human genetics, pedigree study provides a strong tool, which is utilised to trace die inheritance of a specific trait, abnormality or disease.

Population genetics:

• It is the study of the distribution of units and the frequency of genes within the total population.
• The formula delineating the genotypic expectations of progeny in relation to the gametic (allelic) frequencies of the gene pool was proposed by Hardy and Weinberg and is referred to as the Hardy-Weinberg principle.

Principles Of Inheritance And Variation NEET Chapter Summary

This law is applicable only if:

1. Populationis is large and pamniclie (Random matiing).
2. No selecting, no imitation.
3. Population is closed.

under these conditions, according to Hardy and Weinberg’s Law. there.should be no change in the gametic or zygotic frequencies from generation to generation.

Expected genotype pic (zygotic) frequencies in generations may be summarised as

⇒ (p+q)²=P²+2pq+q²=1.0

⇒ P² = frequency of dominant gene/alleles.

⇒ q² = frequency of recessive gene/alleles P=k=1

⇒ p² + 2pq = frequency of dominant trait.

⇒ q² = frequency of a recessive trait.

Human Karyotype: Tjio and Levan first prepared a human karyotype.

• It is an arrangement of chromosomes according to the length of chromosomes and the position of the centromere.
• A pencil sketch or photograph of a karyotype is called an idiogram.
• It is an asymmetrical karyotype which is considered an advanced character.

Principles Of Inheritance And Variation NEET Chapter Summary

Classification of chromosomes: The human metaphase chromosomes were First of all classified in a conference of cytogeneticists at Denver, Colorado in I960.

• In humans 23 pairs (46) chromosomes have been numbered from l to 23 according to their decreasing size, Patau (I960) divided the human chromosome into the following seven groups designated A to G.

• Banding Technique: It is the development of risk ami light band of stains on the chlorosome when they are treated with special fluorescent lives.
• Hits are because different parts of the chromosome show differential affinity to these stains.
• Chromosome handling was discovered by Caspersson et al (1970) and Paridue and Galt(1970) almost simultaneously late as different types of stains and staining techniques.
• Each develops a unique banding pattern which remains constant.

Principles Of Inheritance And Variation NEET Chapter Summary

• Continuity of life is made possible by asexual and sexual reproduction Sexual reproduction, besides creating new individuals introduces variability in the offspring by combining trials of parents.
• The genotypic ratio of the dihybrid cross is RRYY: RrYY : RRYy: rrYY: RrYy : rrYy: ItRyy : Rryy: rryy 1:2: 2:1:4:2:1:2:1
• In the case of MN blood groups in humans, alleles show codominance. The red blood cells can carry two types of native antigens, M and N, and an individual can be MM, MN or NN exhibiting either one or both of them
• ABO blood group system shows codominance, multiple alleles and dominant recessive gene interaction.
• Treatment with gibberellic acid only brings phenotypic changes and genes are not affected.
• Pleiotropy results in different expressions at the phenotypic level. For example, sickle cell anaemia and cystic fibrosis in men.
• Morgan is called the father of experimental genetics. Bateson is called the father of modern genetics. Nilsson-Ehle (1908) was the first scientist to prove quantitative inheritance.
• At the time of fertilization, the chance factor is responsible for the fusion of gametes. Infinite new combinations are produced Example 70 × 10¹² for 23 pairs of chromosomes.

Cause Of Variations Is Hidden In Sexual Reproduction

Mendel’s Laws of Inheritance:

1. Law of paired factors
2. Law or Principle of Dominance Law of Segregation.
3. Phenotypic ratio = 3:1 Monohybrid genotypic ratio =1:2:1
4. Law of Independent assortment Dihybrid phenotypic ratio = 9 : 3 : 3: 1 Dihybrid  genotypic ratio = 1: 2 : 1: 2 : 4: 2 : 1: 2: 1

Exceptions To Mendelism

• Incomplete Dominance = Genotypic and phenotypic ratio =1:2:1
• Codominance. I^AI^A (In the case of blood groups)
• Hb^AHb^A (In case of sickle cell anaemia)
• Multiple alleles. 15 alleles for eye colour in Drosophila, 3 alleles for blood groups in humans (I^A, I^A and 1° for four types of blood groups (A, B, AB and O.)
• Chromosomal theory of inheritance proposed by Sutton and Boveri (1902)
• Experimental verification by Morgan with his experiments on Drosophila melanogaster.
• Linkage. Physical association of two genes on a chromosome.
• Recombination. Generation of non-parental gene combination.
• Sex Determination.

Principles Of Inheritance And Variation NEET Chapter Summary

Human : Autosomes + XY → Male, Autosomes + XX → Female

Most of the insects (Male heterogamy).

• Grasshopper AA + XO → Male
• AA + XX → Female
• Birds AA + ZW → Female
• AA + ZZ → Male

Mutation

1. Chromosomal mutation
2. Point mutation — Sickle cell anaemia
3. Frameshift mutation.

Mutagen—Physical agents which cause mutation: UV radiation and Chemical Mutagen.

Pedigree Analysis Male and Female Analysis of Traits.Marriage between close relatives 0=0

NEET Biology Principles Of Inheritance And Variation Study Material

NEET Biology Principles Of Inheritance And Variation Elements Of Heredity And Variations Conclusion

In sickle cell anemia, glutamic acid is substituted by valine at the sixth position in the β-chain of haemoglobin.

• In thalassemia, a genetic blood condition characterized by anemia, the β-chain of haemoglobin is altered due to a frameshift mutation. Bone marrow is absent.
• Either the mother must be Rh positive, or both the mother and the husband must be Rh negative; otherwise, the second kid may suffer erythroblastosis fetalis.
• Rh positive is genetically prevalent in humans.
• The back cross is a genetic cross formulated by Mendel in which the F1 generation is mated with an individual possessing the same genotype as one of the parental organisms.
• The colouration of sweet pea flowers is attributed to a substance known as anthocyanin. It is released by a sequence of metabolic processes involving distinct enzymes.

The formation of each enzyme is determined by separate genes the mutant white tigers now seen in Indian zoos, a defective pleiotropic gene influences both fur colouration and the connection between the eye and brain during development.

• The trihybrid ratio shows the same pattern of inheritance as the dihybrid. The phenotype ratio will be 27: 9 : 9: 9 : 3 : 3 : 3: 1.
• When the heterozygotes have a more extreme phenotype than either of the corresponding homozygotes, then it is usually called over-dominance, super-dominance and hetero-dominance.
• Parasexual hybridisation. It is defined as the technique of hybridisation through protoplast fission. The protoplast fusion opens up the possibility of overcoming the sexual barriers and of mixing and reassorting the genetic element of sexually isolated organisms

• Mendel used the word unit factor or pair factors for controlling units by various traits because he was unaware of the word gene.
• Epistasis is a Greek word which means the act of stopping or inhibiting.
• Non-allelic genes are those present in different loci.
• Genealogy: The history of descent of a person or its family is termed genealogy.
• Modifier Genes: Genes that alter or modify the expression of other non-allelic genes. Modifier genes produce a variety of crests on pigeons’ heads.