Inheritance and variation
Read the following passage carefully and answer the given questions.
Mendel’s approach of using mathematics to explain biological phenomena was totally new and unacceptable to many of the biologists of his time. Finally, though Mendel’s work suggested that factors (genes) were discrete units, he could not provide any physical proof for the existence of factors or say what they were made of. In the dihybrid cross, the phenotypes round, yellow; wrinkled, yellow; round, green and wrinkled, green appeared in the ratio 9:3:3:1. Such a ratio was observed for several pairs of characters that Mendel studied. The important things to remember are that chromosomes as well as genes occur in pairs. Morgan and his group also found that even when genes were grouped on the same chromosome, some genes were very tightly linked while others were loosely linked. In a number of other insects and mammals including man, XY type of sex determination is seen where both male and female have same number of chromosomes. Grasshopper is an example of XO type of sex determination in which the males have only one X-chromosome besides the autosomes, whereas females have a pair of X-chromosomes.
In a dihybrid cross, the plants having round and yellow seeds were crossed with plants having wrinkled and green seeds. What is the ratio of plants having round and green seeds in F2 generation?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 3/16
In a dihybrid cross, two characters are followed together. Round (R) is dominant over wrinkled (r), and yellow (Y) is dominant over green (y).
Parents: RRYY (round, yellow) × rryy (wrinkled, green)
F1: all RrYy (round, yellow)
F1 × F1: each F1 plant makes four kinds of gametes in equal numbers: RY, Ry, rY, ry.
Round and green seeds need at least one R and two y (RRyy or Rryy). The chance of round is 3/4 and the chance of green is 1/4. Since the two characters sort independently:
\( \displaystyle \frac{3}{4} \times \frac{1}{4} = \frac{3}{16} \)
So the F2 shows round yellow : round green : wrinkled yellow : wrinkled green = 9 : 3 : 3 : 1, and round green seeds are 3/16 of the plants.
(1) 9/16 — Incorrect. This is the share of round, yellow seeds, the plants with both dominant traits.
(2) 3/16 — Correct. Round, green seeds are 3 parts out of 16.
(3) 4/16 — Incorrect. No class of the 9 : 3 : 3 : 1 ratio is 4/16.
(4) 1/16 — Incorrect. This is the share of wrinkled, green seeds (rryy), the plants with both recessive traits.
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.