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Learn beyond limits
This is NEET Biology's genetics chapter, and it's one of the highest-yield topics in the exam because the underlying logic (Mendelian ratios, Punnett squares, pedigree reading) repeats across many question styles. Getting comfortable with the reasoning matters more than memorising ratios by rote.
NEET weightage: High · NCERT Biology Class 12, Part 1, Chapter 4 (Principles of Inheritance and Variation)
Law of Dominance: in a heterozygote, one allele (dominant) masks the expression of the other (recessive). Law of Segregation: the two alleles of a gene separate during gamete formation, so each gamete carries only one allele. Law of Independent Assortment: alleles of different genes (on different chromosome pairs) segregate independently of one another during gamete formation.
A cross tracking a single gene, e.g. Tt × Tt (tall × tall, both heterozygous) gives a 3:1 phenotypic ratio (tall:dwarf) and a 1:2:1 genotypic ratio (TT:Tt:tt) in the F2 generation, following segregation.
A cross tracking two genes simultaneously, e.g. RrYy × RrYy (round-yellow × round-yellow, both heterozygous for seed shape and colour) gives a 9:3:3:1 phenotypic ratio in F2, following independent assortment — provided the two genes are on different chromosomes (unlinked).
Incomplete dominance produces an intermediate phenotype in the heterozygote (e.g. red × white Mirabilis jalapa → pink F1). Co-dominance expresses both alleles fully and simultaneously (e.g. AB blood group, where both A and B antigens are expressed).
Genes located close together on the same chromosome tend to be inherited together (linkage) rather than assorting independently, which is why Mendel's independent assortment law strictly applies only to genes on different chromosome pairs. Sutton and Boveri's chromosomal theory connected Mendel's 'factors' to the physical behaviour of chromosomes during meiosis.
A pedigree traces a trait through a family tree to determine whether it is dominant/recessive and autosomal/sex-linked. NEET commonly tests disorders such as haemophilia and colour blindness (X-linked recessive), sickle-cell anaemia (autosomal recessive, single point mutation) and Down syndrome (trisomy of chromosome 21).
Monohybrid F2 phenotypic ratio
3 : 1
Monohybrid F2 genotypic ratio
1 : 2 : 1
Dihybrid F2 phenotypic ratio (unlinked genes)
9 : 3 : 3 : 1
Down syndrome
Trisomy of chromosome 21 (47 chromosomes)
Turner syndrome
45, X0 (monosomy of X)
Klinefelter syndrome
47, XXY
In pea plants, tall (T) is dominant over dwarf (t). A heterozygous tall plant (Tt) is crossed with a dwarf plant (tt). What phenotypic ratio do you expect in the offspring?
This is a test cross: Tt × tt. Gametes from Tt are T and t (each 1/2); gametes from tt are all t. Offspring: 1/2 Tt (tall) : 1/2 tt (dwarf) — a 1:1 phenotypic ratio, not the 3:1 you'd get from Tt × Tt. Recognising a test cross (heterozygote × homozygous recessive) is a common NEET trap.
A colour-blind man (X-linked recessive) marries a woman who is homozygous normal. What are the chances their daughters are carriers, and their sons are colour-blind?
Father's genotype: XᶜY. Mother's genotype: XᴺXᴺ. Daughters receive Xᶜ from father and Xᴺ from mother → all daughters are XᴺXᶜ, i.e. all carriers (none colour-blind, since it's recessive). Sons receive Y from father and Xᴺ from mother → all sons are XᴺY, i.e. all normal. So: 100% of daughters are carriers, 0% of sons are colour-blind.
A cross between a plant with round, yellow seeds (heterozygous for both traits, RrYy) and a plant with wrinkled, green seeds (rryy) is a test cross. What phenotypic ratio is expected in the offspring, assuming the genes are unlinked?
1:1:1:1 — round-yellow : round-green : wrinkled-yellow : wrinkled-green. A dihybrid test cross (heterozygote × double homozygous recessive) directly reveals the gamete ratio produced by the heterozygous parent.
Why do haemophilia and colour blindness appear much more frequently in males than in females?
Both are X-linked recessive conditions. Males are hemizygous (XY), so a single recessive allele on their one X chromosome is enough to cause the condition. Females (XX) need the recessive allele on both X chromosomes to be affected, and are otherwise carriers.
What distinguishes co-dominance from incomplete dominance, using the ABO blood group system as an example of one of them?
In co-dominance, both alleles are fully and separately expressed in the heterozygote — the ABO blood group system shows this in the AB genotype, where both A and B antigens are produced. In incomplete dominance, the heterozygote instead shows a blended, intermediate phenotype rather than both parental phenotypes together.
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