Question

From which of the following crosses, $F_1$ progeny showing maximum hybrid vigour can be obtained?
(I) AAbb $\times$ aaBB (II) AAbb $\times$ aabb
(III) aaBB $\times$ aabb (IV) AABB $\times$ aabb

• A. (II), (III)
• B. (III), (IV)
• C. (I), (IV)
• D. (I), (II)

Answer 1

The correct option is C (I), (IV)

Hybrid vigour has increased the performance of hybrids over their inbred parents. Interbreeding of inbred produces hybrids in which harmful recessive alleles of one parent are masked by dominant allele from other parents thereby increasing the performance of hybrid over parents. Thus hybrid vigour is caused by heterozygosity of hybrids. A cross between AAbb and aaBB (cross I) genotypes would obtain all offspring with AaBb (double hybrid offspring). A cross between AAbb and aabb (cross II) would obtain all offspring with Aabb (offspring heterozygous for one trait but homozygous for the other). Similarly, aaBB × aabb (cross III) would obtain all offspring with aaBb genotype and the cross between AABB and aabb (cross IV) obtains all AaBb offspring. Therefore, we can see that cross I and IV obtain all offspring with double heterozygous genotype while those from cross II and III are heterozygous for one trait and homozygous for other. Since hybrid vigour increases with increasing heterozygosity; cross I and IV would obtain it at maximum in their $F_1$ generation. The correct answer is C.