Principles of Inheritance and Variation

2.1 Core concepts

• Genetics is the branch of biology dealing with inheritance (transmission of characters from parent to progeny) and variation (degree by which progeny differ from parents) (NCERT §4 intro, p. 54).
• Gregor Mendel conducted hybridisation experiments on garden peas for seven years (1856–1863); he used large samples and applied statistical/mathematical logic to biology for the first time and selected 14 true-breeding lines as 7 pairs with contrasting traits (NCERT §4.1, p. 54).
• The seven pairs of contrasting traits Mendel studied: stem height (tall/dwarf), flower colour (violet/white), flower position (axial/terminal), pod shape (inflated/constricted), pod colour (green/yellow), seed shape (round/wrinkled), seed colour (yellow/green) (NCERT §4.1 Table 4.1, p. 55).
• A monohybrid cross (Tt × Tt) gives an F2 phenotypic ratio of 3:1 (tall:dwarf) and a genotypic ratio of 1:2:1 (TT:Tt:tt); the F1 is all Tt and phenotypically tall (NCERT §4.2, pp. 56–58).
• The Punnett square, developed by Reginald C. Punnett, graphically represents probabilities of all possible offspring genotypes by writing gametes on the top row and left column (NCERT §4.2, p. 57).
• A test cross is a cross between an organism showing dominant phenotype (unknown genotype) and the homozygous recessive parent; it is used to determine whether the dominant individual is homozygous (TT) or heterozygous (Tt) (NCERT §4.2, p. 58).
• Law of Dominance (Mendel's First Law): characters are controlled by discrete units called factors that occur in pairs; in a dissimilar pair one factor dominates the other; explains the 3:1 F2 phenotypic ratio (NCERT §4.2.1, p. 59).
• Law of Segregation (Second Law): alleles do not blend; during gamete formation the two alleles segregate so that each gamete carries only one of them; a heterozygote produces two kinds of gametes in equal proportion (NCERT §4.2.2, p. 59).
• Incomplete dominance: in snapdragon (Antirrhinum) / Mirabilis-type cross RR (red) × rr (white) → F1 Rr is pink, F2 is 1 red : 2 pink : 1 white; phenotypic ratio (1:2:1) equals genotypic ratio because R is not fully dominant over r (NCERT §4.2.2.1, p. 60).
• Co-dominance: in human ABO blood groups, gene I has three alleles I^A, I^B and i; I^A and I^B are both dominant over i but co-dominant with each other, so I^A I^B genotype expresses both A and B sugars on RBCs (AB blood type); 6 genotypes give 4 phenotypes (A, B, AB, O) (NCERT §4.2.2.2 Table 4.2, p. 61).
• Multiple alleles: ABO blood grouping illustrates more than two alleles for one gene in a population, though any individual carries only two (NCERT §4.2.2.2, p. 62).
• Pleiotropy (incomplete-dominance angle): starch synthesis in pea seeds is controlled by one gene (B, b) — BB produces large round seeds, bb produces small wrinkled seeds, Bb produces round seeds with intermediate-sized starch grains; so the same gene shows complete dominance for shape but incomplete dominance for grain size (NCERT §4.2.2.2, p. 62).
• A dihybrid cross RrYy × RrYy gives an F2 phenotypic ratio of 9 round-yellow : 3 wrinkled-yellow : 3 round-green : 1 wrinkled-green (NCERT §4.3 and §4.3.1, pp. 63–64).
• Law of Independent Assortment: when two pairs of traits are combined in a hybrid, segregation of one pair is independent of the other; gametes formed by RrYy are RY, Ry, rY, ry each at 25% (NCERT §4.3.1, p. 64).
• Chromosomal Theory of Inheritance: Walter Sutton and Theodore Boveri (1902) noted that the behaviour of chromosomes during meiosis parallels the behaviour of genes — both occur in pairs, both segregate at gamete formation, and independent pairs assort independently (NCERT §4.3.2 Table 4.3, p. 66).
• Morgan & Drosophila: T.H. Morgan used the fruit fly Drosophila melanogaster (simple medium, ~2-week life cycle, large progeny, distinguishable sexes) to experimentally verify the chromosomal theory (NCERT §4.3.2, p. 67).
• Linkage and recombination: when two genes lie on the same chromosome, parental combinations exceed non-parental (recombinant) combinations; Morgan coined "linkage" for physical association and "recombination" for generation of non-parental combinations. y–w showed 1.3% recombination (tightly linked) while w–m showed 37.2% (loosely linked); Sturtevant used recombination frequency to map gene positions (NCERT §4.3.3, pp. 67–68).
• Polygenic inheritance: traits like human skin colour and height are controlled by three or more genes with additive allelic effects and environmental influence; e.g., AABBCC = darkest, aabbcc = lightest skin (NCERT §4.4, p. 69).
• Pleiotropy: a single gene producing multiple phenotypic effects — phenylketonuria is caused by mutation in the gene coding for phenylalanine hydroxylase and manifests as mental retardation plus reduced hair and skin pigmentation (NCERT §4.5, p. 69).
• Sex determination — XO type (grasshopper, insects): males have only one X-chromosome (besides autosomes); females have two X-chromosomes; male-heterogamety (NCERT §4.6, p. 70).
• XY type (humans, Drosophila): males XY, females XX; male-heterogamety; both sexes have same total chromosome number; humans have 22 pairs of autosomes plus one pair of sex chromosomes (NCERT §4.6 / §4.6.1, p. 70–71).
• ZW type (birds): females ZW, males ZZ; female-heterogamety (NCERT §4.6, p. 71).
• Haplodiploidy (honey bees): females (queen/worker) develop from fertilised eggs and are diploid (32 chromosomes); males (drones) develop from unfertilised eggs by parthenogenesis and are haploid (16 chromosomes); drones have no father, cannot have sons, but have a grandfather and grandsons (NCERT §4.6.2, p. 71).
• Mutation: alteration of DNA sequence resulting in change in genotype and phenotype; arises from deletions/insertions/duplications of DNA segments (chromosomal aberrations, common in cancer cells) or single-base-pair changes (point mutations, e.g., sickle-cell anaemia); UV radiation is a mutagen (NCERT §4.7, p. 72).
• Pedigree analysis: study of inheritance of a trait across generations in a family tree, since controlled crosses are impossible in humans; uses standard symbols (NCERT §4.8.1, p. 72).
• Colour blindness: X-linked recessive defect in red or green cone; ~8% of males and ~0.4% of females are affected because the gene is on X chromosome; mother is unaffected carrier if heterozygous (NCERT §4.8.2, p. 73).
• Haemophilia: X-linked recessive; one of the clotting-cascade proteins is defective causing non-stop bleeding from a simple cut; heterozygous female (carrier) transmits to sons; female homozygotes are extremely rare because the father has to be haemophilic; Queen Victoria's family pedigree shows multiple haemophilic descendants (NCERT §4.8.2, p. 74).
• Sickle-cell anaemia: autosomal recessive; controlled by alleles Hb^A and Hb^S; only Hb^SHb^S shows the disease, Hb^AHb^S are carriers; caused by substitution of Glutamic acid by Valine at the sixth position of the β-globin chain (single base substitution GAG → GUG at the sixth codon); mutant haemoglobin polymerises under low oxygen tension changing RBC shape from biconcave to sickle-like (NCERT §4.8.2, pp. 74–75).
• Phenylketonuria: autosomal recessive inborn error of metabolism; affected individual lacks the enzyme that converts phenylalanine to tyrosine; accumulation of phenylalanine and phenylpyruvic acid in brain causes mental retardation; excreted via urine (NCERT §4.8.2, p. 75).
• Thalassaemia: autosomal recessive; reduced synthesis of one of the globin chains; α-thalassaemia controlled by two closely linked genes HBA1 and HBA2 on chromosome 16; β-thalassaemia controlled by single gene HBB on chromosome 11; differs from sickle-cell anaemia in being a quantitative (too few globin molecules) rather than qualitative defect (NCERT §4.8.2, p. 75).
• Chromosomal disorders: caused by absence/excess/abnormal arrangement of chromosomes; aneuploidy from failure of segregation of chromatids during cell division (gain/loss of chromosome); polyploidy from failure of cytokinesis (whole set added) (NCERT §4.8.3, p. 75).
• Down's syndrome: trisomy of chromosome 21 (extra copy of chromosome 21, total 47); first described by Langdon Down (1866); short stature, small round head, furrowed tongue, partially open mouth, broad palm with characteristic crease, retarded physical/psychomotor/mental development (NCERT §4.8.3, p. 76).
• Klinefelter's syndrome: karyotype 47, XXY (extra X); overall masculine development but gynaecomastia (breast development); sterile (NCERT §4.8.3, p. 76).
• Turner's syndrome: karyotype 45, X0 (one X chromosome missing); sterile females with rudimentary ovaries; lack secondary sexual characters (NCERT §4.8.3, p. 76).

2.2 Definitions to memorise

2.3 Diagrams / processes to remember

• Figure 4.1: Seven pairs of contrasting traits Mendel studied in pea (stem height, flower colour, flower position, pod shape, pod colour, seed shape, seed colour), p. 54.
• Figure 4.2: Steps in making a cross in pea — emasculation, dusting pollen, bagging, p. 55.
• Figure 4.3: Monohybrid cross diagrammatic representation (TT × tt → F1 all Tt tall → F2 3:1 phenotype, 1:2:1 genotype), p. 56.
• Figure 4.4: Punnett square for monohybrid cross between true-breeding tall and dwarf plants, p. 57.
• Figure 4.5: Test cross of violet (V) × white (v) showing 1:1 ratio, p. 59.
• Figure 4.6: Monohybrid cross in Snapdragon with incomplete dominance — F2 ratio 1 red : 2 pink : 1 white, p. 60.
• Figure 4.7: Dihybrid cross with seed colour and seed shape — F2 9:3:3:1 grid (16 squares), p. 63.
• Figure 4.8: Meiosis and germ-cell formation in a four-chromosome cell, p. 65.
• Figure 4.9: Independent assortment of chromosomes — two possibilities of metaphase alignment, p. 66.
• Figure 4.10: Drosophila melanogaster male and female, p. 67.
• Figure 4.11: Morgan's two dihybrid crosses showing tight linkage (y–w: 1.3% recombination) vs loose linkage (w–m: 37.2%), p. 68.
• Figure 4.12: Sex determination — (a) human XX/XY (b) Drosophila XX/XY (c) bird ZZ/ZW, p. 70.
• Figure 4.13: Sex determination in honey bee (haplodiploid; diploid females 32, haploid males 16), p. 71.
• Figure 4.13 (also): Symbols used in human pedigree analysis (male = square, female = circle, affected = shaded, mating = horizontal line, sibship = vertical descent), p. 72.
• Figure 4.14: Pedigrees of (a) autosomal dominant trait — myotonic dystrophy and (b) autosomal recessive trait — sickle-cell anaemia, p. 73.
• Figure 4.15: Micrograph of normal vs sickle-cell RBCs and the amino-acid composition of β-chain showing Glu→Val substitution at position 6, p. 74.
• Figure 4.16: Down's syndrome — flat back of head, broad flat face, big wrinkled tongue, palm crease, congenital heart disease; with trisomy-21 karyotype, p. 76.
• Figure 4.17: (a) Klinefelter's syndrome (tall, feminised character, gynaecomastia, 47 XXY); (b) Turner's syndrome (short stature, underdeveloped feminine character, 45 X0), p. 76.

2.4 Common confusions / NTA trap points

• Phenotypic vs genotypic ratios. In a normal monohybrid Tt × Tt cross, phenotypic is 3:1 but genotypic is 1:2:1; in incomplete dominance (Snapdragon RR × rr), both phenotypic and genotypic ratios are 1:2:1. NTA loves swapping these.
• Co-dominance vs incomplete dominance. Co-dominance — both alleles express their distinct phenotypes simultaneously (AB blood). Incomplete dominance — the heterozygote shows an intermediate/blended phenotype (pink Snapdragon).
• Multiple alleles vs polygenic inheritance. Multiple alleles = more than two alleles of the same gene (I^A, I^B, i for ABO). Polygenic = many different genes acting additively on the same trait (skin colour, height).
• X-linked recessive disorders sex bias. Colour blindness and haemophilia are far more common in males because they have only one X — a single recessive allele expresses. NTA distractors will swap the % frequencies (8% male / 0.4% female for colour blindness).
• Sickle-cell anaemia substitution direction. The mutation is GAG → GUG (codon change) and Glu → Val (amino-acid change) at the sixth position of the β-globin chain. Wrong-position (4th, 7th) or wrong-chain (α-chain) distractors are classic NTA traps.
• Down's syndrome karyotype. Trisomy of chromosome 21 → 47 chromosomes total (NOT 47, XX or 47, XY notation alone — the autosomal 21 is the extra one). Don't confuse with Klinefelter (47, XXY — sex-chromosome trisomy).
• Turner vs Klinefelter karyotypes. Turner = 45, X0 (female, missing X). Klinefelter = 47, XXY (male, extra X). Both are sterile, but one has a chromosome missing and the other has an extra one.
• Honey bee haplodiploidy. Males are haploid (16, from unfertilised egg), females are diploid (32). "Drones produce sperm by mitosis" and "drones have no father" are favourite assertion-reason hooks.
• Linkage strength vs recombination frequency. Tightly linked genes show low recombination frequency (y-w: 1.3%), loosely linked show high (w-m: 37.2%). Distractors will invert this relationship.