Q.49 In a hypothetical trihybrid cross of three loci (viz. A, B, C), all were inherited in a complete dominant

manner over their recessive alleles 𝐚, 𝐛, 𝐜, respectively. When a test cross between F 1 and parent ‘aabbcc’

was performed, following genotypes of eight phenotypically distinct classes were observed with

respective numbers

The genetic distance (up to one decimal) between A and C loci will be ___ cM.

In a trihybrid test cross involving loci A, B, and C with complete dominance, observed progeny numbers reveal linkage and allow precise genetic distance calculation between specific loci. The query focuses on the distance between A and C loci, derived from recombination frequencies in the test cross data.​

Progeny Data Overview

The test cross progeny classes and counts are: ABC (416), ABc (80), AbC (8), Abc (3), aBC (5), aBc (7), abC (1), abc (1). Total progeny sums to 521, confirming the dataset for analysis. Parental classes dominate (ABC and abc most frequent at 416 and 1? Wait, no: actually ABC 416 likely parental dominant, but with low abc, suggests coupling phase with rare doubles.​​

Identifying Classes

Parental gametes produce ABC (416) and abc (1), indicating tight linkage overall but imbalance due to B involvement. Single crossovers between A and C appear rare: AbC (8) and aBc (7), while doubles (requiring crossovers in both intervals) are abc? Standard identification: recombinants for A-C are those differing in A and C from parentals. For A-C pair, recombinant progeny total 8 + 3 + 7 + 1 = 19.​​

Recombination Calculation

Recombination frequency (RF) for A and C equals recombinants divided by total progeny: 19 / 521 ≈ 0.0365 or 3.65%. Genetic distance equals RF in centimorgans (cM), rounded to one decimal: 3.6 cM. This low value indicates A and C loci lie very close on the chromosome.​​

Detailed Class Explanations

• ABC (416): Parental, no crossover.​

• ABc (80): Single crossover between B and C (assuming order A-C-B).​

• AbC (8): Single crossover between A and C.​

• Abc (3): Double crossover.​

• aBC (5): Single between A-C and B? Wait, fits doubles or other.​

• aBc (7): Single between A and C.​

• abC (1): Double.​

• abc (1): Parental (rare due to linkage).​
  RF A-B ≈18.4% (96/521), B-C high ~96.7% suggests B distant; A-C closest at 3.6 cM confirms query focus. Double crossovers (3+1=4) are lowest, verifying order with C between A and B? But distance A-C directly computed.​​