Q105. A cross is made between two animals of genotypes AaBb, where both A and B are autosomal. The progeny of this dihybrid cross was allowed to self cross. The proportion of the dihybrid cross that showed independent segregation (in loci A, i.e., produce A and a progeny) in % will be
In dihybrid crosses involving autosomal genes like AaBb, understanding dihybrid cross independent segregation proportion is key for CSIR NET aspirants tackling Mendelian genetics. This guide solves Q105: the % of F2 progeny from AaBb self-cross that segregate alleles at locus A (heterozygous Aa, producing A and a gametes).
Answer: 50%
In a dihybrid cross between AaBb × AaBb individuals assuming independent assortment, the F2 progeny follow a 9:3:3:1 phenotypic ratio and specific genotypic ratios across 16 combinations. “Independent segregation” for locus A means the individual is heterozygous (Aa), capable of producing both A and a gametes, which occurs in 50% of F2 progeny.
Cross Setup
Parents AaBb × AaBb produce F1 all AaBb. F1 self-cross (AaBb × AaBb) yields F2 with independent segregation if genes unlinked.
F2 Genotypic Ratios
For locus A alone (1:2:1), AA is 25%, Aa 50%, aa 25%—matching monohybrid despite dihybrid context. Full dihybrid: 1 AABB, 2 AABb, 1 AAbb, 2 AaBB, 4 AaBb, 2 Aabb, 1 aaBB, 2 aaBb, 1 aabb (total 16). Aa genotypes total 8/16 = 50%.
Punnett Square Breakdown
Gametes from each AaBb parent: AB, Ab, aB, ab (1:1:1:1). F2 yields 9:3:3:1 phenotypes, but genotypically, locus A segregates 1 AA : 2 Aa : 1 aa (50% Aa).
| Locus A Genotype | Proportion | Segregates A/a? |
|---|---|---|
| AA | 25% | No |
| Aa | 50% | Yes |
| aa | 25% | No |
Why 50%?
Independent assortment ensures each locus behaves monohybrid-like: P(Aa) = 2pq = 2(0.5)(0.5) = 0.5. Only Aa progeny show segregation upon selfing (1:2:1 offspring).
Exam Options Explained
- 25%: Matches homozygous AA/aa (no segregation).
- 50%: Correct—heterozygotes.
- 75%: Dominant phenotype carriers (AA + Aa).
- 0% or 100%: Irrelevant; ignores segregation test.
Master this for CSIR NET: deviations indicate linkage (not independent).
