Principles of Inheritance and Variation – Class 12 Biology Chapter 4 Notes & NCERT PDF

Introduction

Genetics deals with the inheritance and variation of characters from parents to offspring.

As early as 8000–1000 B.C., people used natural variation in artificial selection and domestication, which produced breeds such as Sahiwal cows in Punjab, but they did not know its scientific basis. This chapter moves from Mendel's pea experiments to chromosomes, linkage, sex determination, mutation and human genetic disorders. It carries heavy weight in board exams through numerical crosses, pedigrees and disorder-based questions.

Key Concepts

4.1 Mendel's Laws of Inheritance

Gregor Mendel ran hybridisation experiments on garden pea for seven years (1856–1863). It was the first time statistical analysis and mathematical logic were applied to problems in biology, and his large sample sizes gave his data credibility.

1. stem height: tall/dwarf;

2. flower colour: violet/white;

3. flower position: axial/terminal;

4. pod shape: inflated/constricted;

5. pod colour: green/yellow;

6. seed shape: round/wrinkled;

7. seed colour: yellow/green.

4.2 Inheritance of One Gene

- Mendel's "factors" are now called genes, the units of inheritance.

- Alleles are genes coding for a pair of contrasting traits, "slightly different forms of the same gene".

- Homozygous means TT or tt; heterozygous means Tt.

- The genotype is the gene make-up (TT, Tt, tt). The phenotype is the observable trait (tall, dwarf).

- Monohybrid: Tt is heterozygous for one character, so TT × tt is a monohybrid cross.

- The F₂ genotypes come out as 1/4 TT : 1/2 Tt : 1/4 tt, a genotypic ratio of 1:2:1, while the phenotypic ratio is 3:1.

- In binomial form: (½T + ½t)² = ¼TT + ½Tt + ¼tt.

- Vv × vv gives violet : white = 1:1.

- VV × vv gives all violet.

4.2.1 Law of Dominance

This law explains why only one parental character appears in the F₁, why both appear in the F₂, and the 3:1 ratio.

4.2.2 Law of Segregation

Alleles do not blend. During gamete formation the two factors of a pair segregate, so each gamete receives only one. A homozygote produces one kind of gamete, and a heterozygote produces two kinds in equal proportion.

4.2.2.1 Incomplete dominance is seen in the snapdragon (dog flower, Antirrhinum).

- a normal or less efficient enzyme, which gives an equivalent allele;

- a non-functional enzyme; or

- no enzyme at all.

In the last two cases, the working, unmodified allele is dominant and the modified allele is usually recessive.

4.2.2.2 Co-dominance is where the F₁ resembles both parents. The example is the ABO blood groups, controlled by gene I.

- IᴬIᴬ and Iᴬi give A;

- IᴮIᴮ and Iᴮi give B;

- IᴬIᴮ gives AB;

- ii gives O.

- BB seeds make large starch grains and are round. bb seeds make small grains and are wrinkled.

- Bb seeds are round, so for seed shape B is dominant.

- Bb starch grains are intermediate in size, so for grain size the alleles show incomplete dominance.

- The lesson: dominance depends on the phenotype chosen for study.

4.3 Inheritance of Two Genes

- Mendel published in 1865, but his work was unrecognised until 1900. There were four reasons:

1. poor communication;

2. his discrete, non-blending factors did not fit the continuous variation people saw;

3. using mathematics in biology was new;

4. he had no physical proof of the factors.

- In 1900, de Vries, Correns and von Tschermak independently rediscovered his results.

- By 1902, chromosome movement during meiosis had been worked out. Walter Sutton and Theodore Boveri noticed that chromosomes behave like genes: both occur in pairs, both segregate at gamete formation, and pairs segregate independently. Sutton named this synthesis the chromosomal theory of inheritance.

- Thomas Hunt Morgan verified it experimentally using Drosophila melanogaster. The fly suits genetics because:

- it grows on a simple synthetic medium;

- its life cycle takes about two weeks;

- one mating gives many progeny;

- males and females are clearly distinct;

- it has many hereditary variations visible under low-power microscopes.

- Morgan crossed yellow-bodied, white-eyed females with brown-bodied, red-eyed males. The F₂ deviated significantly from 9:3:3:1.

- Linkage is the physical association of genes on a chromosome. Recombination is the generation of non-parental gene combinations.

- The genes white and yellow are tightly linked, with 1.3% recombination. White and miniature wing show 37.2% recombination.

- Alfred Sturtevant used recombination frequency to measure the distance between genes, producing genetic maps.

4.4 Polygenic Inheritance

A polygenic trait is spread across a gradient, is generally controlled by three or more genes, and is also influenced by the environment. Examples are human height and skin colour.

4.5 Pleiotropy

A pleiotropic gene is a single gene with multiple phenotypic expressions. It usually works through its effect on metabolic pathways.

4.6 Sex Determination

- There are 23 pairs of chromosomes, of which 22 pairs are autosomes.

- Males produce X- and Y-bearing sperms, 50% each. Females produce only X-bearing ova.

- X sperm with an ovum gives XX (female), and Y sperm gives XY (male). So "it is the genetic makeup of the sperm that determines the sex of the child", and every pregnancy has a 50% chance of either sex.

- A fertilised egg becomes a female (queen or worker) with 32 chromosomes.

- An unfertilised egg becomes a male (drone) by parthenogenesis, with 16 chromosomes.

- Males make sperm by mitosis. They have no father and cannot have sons, but they have a grandfather and can have grandsons.

4.7 Mutation

A mutation alters DNA sequences and so changes the genotype and phenotype. Mutation and recombination are the two sources of variation discussed.

4.8 Genetic Disorders

- Colour blindness: sex-linked recessive. It is a defect in the red or green cones, found in about 8% of males and 0.4% of females. A carrier mother's son has a 50% chance of being colour blind. A daughter is normally unaffected unless her mother is a carrier and her father is colour blind.

- Haemophilia: sex-linked recessive. A protein in the blood-clotting cascade is affected, so a simple cut causes non-stop bleeding. The disorder passes from carrier females to some sons, and affected females are extremely rare. Queen Victoria was a carrier.

- Sickle-cell anaemia:

- Inheritance: autosomal recessive, with alleles Hbᴬ and Hbˢ. Only HbˢHbˢ individuals are diseased, while HbᴬHbˢ individuals are carriers with the sickle-cell trait.

- Cause: Glu is replaced by Val at the sixth position of the β-globin chain, from the codon change GAG → GUG.

- Effect: under low oxygen tension the haemoglobin polymerises, and the RBCs change from a biconcave disc to a sickle shape.

- Phenylketonuria: autosomal recessive. The enzyme that converts phenylalanine to tyrosine is missing, so phenylpyruvic acid accumulates in the brain and causes mental retardation. It is also excreted in the urine.

- Thalassemia: autosomal recessive. There is a reduced rate of synthesis of one globin chain.

- α-thalassemia involves the genes HBA1 and HBA2 on chromosome 16.

- β-thalassemia involves the gene HBB on chromosome 11.

- Thalassemia is a quantitative problem (too few globin molecules). Sickle-cell anaemia is a qualitative problem (an incorrectly functioning globin).

- Aneuploidy is the gain or loss of chromosome(s) through failure of chromatid segregation. Polyploidy is an increase by a whole set, from failure of cytokinesis, and is often seen in plants.

- Down's syndrome is trisomy 21, described by Langdon Down in 1866. Features: short stature, small round head, furrowed tongue, partially open mouth, a broad palm with a characteristic crease, and retarded physical, psychomotor and mental development.

- Klinefelter's syndrome is 47, XXY. It gives overall masculine development with gynaecomastia, and the individual is sterile.

- Turner's syndrome is 45, XO. The individual is a sterile female with rudimentary ovaries who lacks secondary sexual characters.

Diagrams to Know (in words)

Key-Facts Sheet

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Key Terms

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