Comprehension:
Principles of Inheritance
Read the following passage carefully and answer the given questions.
Gregor Mendel conducted hybridisation experiments on garden peas for seven contrasting characters. He crossed tall and dwarf pea plants to study the inheritance of characters. Mendel observed that all the F1 progeny plants were tall. Mendel then self-pollinated tall F1 plants and surprisingly observed the proportion of tall and dwarf plants in 3:1 ratio. Hence, the tall and dwarf traits were identical to their parental type and did not show any blending that is all the offspring were either tall or dwarf and none were of in between height. This study explains the Law of Segregation that the alleles do not show any blending and that both the characters are recovered as such in the F2 generation though one of these is not seen at the F1 generation. Further, Mendel observed the four different phenotypes from the cross of two contrasting characters (seed shape and seed color) which led to the conclusion that when two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters as stated by Law of Independent Assortment. Walter Sutton and Theodore Boveri noted that the behaviour of chromosomes was parallel to the behaviour of genes and used chromosome movement to explain Mendel's law. The important things to remember that chromosomes as well as genes occur in pairs. The two alleles of a gene pairs are located on homologous sites on homologous chromosomes.
In Mendel's dihybrid cross, the phenotypic ratio in F2 generation is:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 9 : 3 : 3 : 1
(1) 3 : 1 — Incorrect. This is the phenotypic ratio in the F2 of a monohybrid cross, in which one pair of characters is followed.
(2) 9 : 3 : 3 : 1 — Correct. A dihybrid cross follows two pairs of characters, such as seed shape and seed colour. In the F2 the four phenotypes round yellow, round green, wrinkled yellow and wrinkled green appear in the ratio 9 : 3 : 3 : 1. It is the product of two monohybrid ratios: (3 round : 1 wrinkled) × (3 yellow : 1 green).
(3) 1 : 2 : 1 — Incorrect. This is the genotypic ratio in the F2 of a monohybrid cross (1 TT : 2 Tt : 1 tt).
(4) 1 : 1 — Incorrect. This is the ratio from the test cross of a monohybrid hybrid, such as Tt × tt.
Note: The passage is taken from NCERT. NTA has put lines of the NCERT text together to make it. Most CUET (UG) questions come straight from NCERT, so study the book line by line, including figures and their titles.
NCERT Extract:
Inheritance of Two Genes – …
