A trihybrid test cross (order of loci unknown) produced progeny classes as
follows: 35 AbC/abc, 37 aBc/abc, 8 ABc/abc, 10 abC/abc, 3 ABC/abc, 5 abc/
abc, 1 Abc/abc, 1 aBC/abc. The gene order is
CBA
BAC
BCA
CAB
In a trihybrid test cross, the trihybrid (unknown linkage phase) is crossed with a triple recessive (abc/abc), producing eight progeny classes that reveal gene order through recombination frequencies. The parental classes appear most frequent, single crossovers next, and double crossovers rarest. Here, with progeny 35 AbC/abc, 37 aBc/abc, 8 ABc/abc, 10 abC/abc, 3 ABC/abc, 5 abc/abc, 1 Abc/abc, 1 aBC/abc, the correct gene order is CBA.
Identifying Parental and Crossover Classes
Parental (non-recombinant) classes are most abundant: AbC/abc (35) and aBc/abc (37), indicating the trihybrid configuration as AbC / aBc (cis for A-B, trans for B-C if order is C-B-A). Single crossover classes follow: ABc/abc (8) and abC/abc (10) for one interval, ABC/abc (3) and abc/abc (5) for the other. Double crossovers are rarest: Abc/abc (1) and aBC/abc (1).
Gene order is determined by comparing double crossovers to parentals—the middle gene “flips” relative to parentals in doubles. For parentals AbC and aBc, doubles Abc and aBC show B unchanged (always b in first, B in second? Wait, notation: AbC is A b C, aBc is a B c). Assuming uppercase dominant: parental gametes AbC (A b C), aBc (a B c). Doubles Abc (A b c), aBC (a B C)—C flips in both relative to parentals, confirming C is middle gene in order C-B-A (CBA).
Why Other Options Are Incorrect
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BAC: Assumes B-A-C. Parentals would be bAC/aBc? But doubles wouldn’t match flip of A (middle); observed flips C instead.
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BCA: Assumes B-C-A. Middle C wouldn’t flip consistently; data shows C as middle only in CBA.
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CAB: Assumes C-A-B. Middle A should flip in doubles (e.g., to a b C / A B c), but observed Abc/aBC flip C, not A.
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This analysis confirms CBA via standard three-point mapping, where double crossover rarity pinpoints the middle locus. Total progeny ~101 verifies ratios.
