Question

In a di-hybrid cross, when one pair of alleles show incomplete dominance, genotypic ratio comes to
A) 3: 6: 3: 1: 2: 1
B) 1: 2: 2: 4: 1: 2: 1: 2: 1
C) 9: 3: 3: 1
D) 1: 2: 1

Answer 1

Hint: The cross between two different genes or lines that differ in two observed traits is known as Di-hybrid cross. A Di-hybrid cross does not always yield the 9:3:3:1 phenotypic ratio. Incomplete dominance will alter the phenotypic ratios.

Complete answer:
A Di-hybrid cross does not always yield the 9:3:3:1 phenotypic ratio. If the mode of inheritance is different, this ratio will be different. Not all the genes show complete dominance. Incomplete dominance will alter the phenotypic ratios. Incomplete dominance is when a form of a gene or a dominant allele, does not completely mask the effects of the recessive allele, and the organism’s physical appearance shows a blend of both alleles. It is also known as partial dominance or semi-dominance. Incomplete dominance occurs because neither of the two alleles is fully dominant over another allele, or because the dominant allele does not completely dominate the recessive allele. This causes a phenotype that is different from both the recessive and dominant alleles, and appears to be a mixture of both, but the genotype remains unaffected by this. Therefore, the genotypic ratio of a di-hybrid cross will always remain the same. Therefore, the ratio becomes, 1: 2: 1: 2: 4: 2: 1: 2: 1.
When a gene has no dominant factors, the phenotype of a heterozygous dominant individual will be a blend of recessive and dominant traits. This is called as Incomplete dominance. Here, the ratio 1: 2: 1 phenotypic ratio in \[{{F}_{2}}\] generation is obtained in the monohybrid cross between two pure varieties. The phenotypic ratio of \[{{F}_{2}}\] ​generation of a di-hybrid cross is 9: 3: 3: 1 provided, the alleles follow recessive- dominant-relationship. When alleles of one gene exhibits the recessive - dominant relationship but the second gene alleles are codominant, the \[{{F}_{2}}\] ​phenotypic ratio of a di-hybrid cross becomes 3: 6 : 3: 1 : 2: 1. Therefore, the option A, C and D are wrong.

Hence, the correct answer is option (B).

Note: The process, where the fertilization occurs between the two true-breeding parents which differ in only one characteristic, is known as a monohybrid cross. The resulting offspring are called monohybrids.