Which one of the following genetic law is responsible for 3 : 1 expression of the phenotypic ratio in F2 generation?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Law of dominance
In a monohybrid cross, a tall pea plant (TT) is crossed with a dwarf one (tt). All F1 plants are Tt and tall. When F1 plants are selfed, the F2 plants are TT, Tt and tt in the ratio 1 : 2 : 1. Tt looks the same as TT because T is dominant, so the phenotypes come out as 3 tall : 1 dwarf.
Option (1) Law of dominance — Correct. In a dissimilar pair of factors, one member (dominant) hides the other (recessive). Because the Tt plants show only the dominant trait, three of every four F2 plants look alike. This law explains the 3 : 1 proportion at the F2.
Option (2) Law of segregation — Incorrect. This law says that the two alleles of a pair separate during gamete formation, so each gamete gets only one of them. It explains why the recessive trait comes back unchanged in the F2. The 3 : 1 phenotypic proportion itself is credited to the law of dominance.
Option (3) Incomplete dominance — Incorrect. Here neither allele is fully dominant, so the heterozygote shows an in-between phenotype, as in the pink flowers of the snapdragon. The F2 phenotypic ratio then becomes 1 : 2 : 1, not 3 : 1.
Option (4) Co-dominance — Incorrect. Here both alleles express themselves fully in the heterozygote, as in the AB blood group of humans. The heterozygote looks different from both parents, so the F2 ratio is again 1 : 2 : 1.
NCERT Extract:
Law of Dominance – “… (iii) In a dissimilar pair of factors one member of the pair dominates (dominant) the other (recessive).
The law of dominance is used to explain the expression of only one of the parental characters in a monohybrid cross in the F1 and the expression of both in the F2. It also explains the proportion of 3:1 obtained at the F2.”