In Mendel's dihybrid cross, the F1 plant has the genotype RrYy. Since the segregation of R and r is independent of the segregation of Y and y, which of the following describes the gametes of this plant?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Four types, RY, Ry, rY and ry, each forming 25 per cent of the gametes
Mendel’s law of independent assortment says that when two pairs of traits are combined in a hybrid, the segregation of one pair is independent of the other. Each gamete gets one allele of each gene, and the chances multiply:
- R or r: 1/2 each; Y or y: 1/2 each
- RY = 1/2 × 1/2 = 1/4; Ry = 1/4; rY = 1/4; ry = 1/4
- Four types of gametes, each 25 per cent
Option (1) Four types, RY, Ry, rY and ry, each forming 25 per cent of the gametes — Correct. R goes with Y as often as with y, and r likewise. So the RrYy plant makes four kinds of gametes in equal numbers, which is the basis of the 9 : 3 : 3 : 1 ratio in the F2.
Option (2) Two types, RY and ry, each forming 50 per cent of the gametes — Incorrect. Only RY and ry would form if R always stayed with Y and r with y. That would mean the two pairs do not segregate independently, which goes against the condition given.
Option (3) Four types, with the parental types RY and ry formed more often than Ry and rY — Incorrect. More parental types than non-parental types is what Morgan found when the two genes lie on the same chromosome (linkage). Under independent assortment all four types are equal.
Option (4) Two types, Rr and Yy, each forming 50 per cent of the gametes — Incorrect. A gamete carries only one allele of each gene. Rr or Yy holds both alleles of one gene and none of the other, so neither can be a gamete.
NCERT Extract:
Law of Independent Assortment – “… Consider the segregation of one pair of genes R and r. Fifty per cent of the gametes have the gene R and the other 50 per cent have r. Now besides each gamete having either R or r, it should also have the allele Y or y. The important thing to remember here is that segregation of 50 per cent R and 50 per cent r is independent from the segregation of 50 per cent Y and 50 per cent y. Therefore, 50 per cent of the r bearing gametes has Y and the other 50 per cent has y. Similarly, 50 per cent of the R bearing gametes has Y and the other 50 per cent has y. Thus there are four genotypes of gametes (four types of pollen and four types of eggs). The four types are RY, Ry, rY and ry each with a frequency of 25 per cent or 1/4th of the total gametes produced.”