• Genetic Basis of Heterosis (Hybrid Vigor):
• According to Falconer & Mackay's quantitative genetics framework:
• When two inbred parental lines differing in gene frequencies ($p_1$ and $p_2$, where $y = p_1 - p_2$) are crossed:
• The amount of heterosis observed in the $F_1$ generation (the deviation of $F_1$ from the mid-parent value) summed across all loci is:

where:

• $d$ = dominance deviation at the locus
• $y = p_1 - p_2$ = difference in allele frequency between the two parental populations
• Heterosis strictly depends on the presence of dominance ($d$) and genetic divergence ($y^2$) between parental lines.
• nalysis:
• (dy^2): Correct formula ($dy^2$, OCR recorded as dy).

Correct option: (2) dy

• Assertion (A) is Correct: Hybrid vigor (heterosis) refers to the superior phenotypic performance of crossbred progeny ($F_1$) compared to the average of their purebred parental breeds (mid-parent value), widely exploited in livestock crossbreeding schemes (e.g., Karan Swiss, Karan Fries).
• Reason (R) is Correct: In the $F_2$ generation produced by mating $F_1 \times F_1$, segregation and recombination break up non-additive favorable allelic interactions (dominance and epistatic complexes), leading to heterosis breakdown where individual heterosis is halved:

• Relationship: While both statements are scientifically sound, (R) describes the genetic breakdown of heterosis in segregating $F_2$ generations rather than providing the physiological or genetic mechanism behind the expression of $F_1$ hybrid vigor (which arises from dominance, overdominance, and epistasis).