62. Alleles A, a, B and b can be distinguished on the basis of their mobility on an agarose gel. These genes are present on the same chromosome. In the gel imagebelow, the band pattern reflects the alleles in parents and their progeny (number reflects the progeny counted).
Which one of the following statements correctly explains the band pattern?
(1) In the heterozygous parent, the alleles are in coupling configuration.
(2) In the heterozygous parent, the alleles are in repulsion configuration.
(3) The alleles A and B are in different linkage groups.
(4) The information is insufficient for any conclusion.
Answer: The correct statement is (1): In the heterozygous parent, the alleles are in coupling configuration.
Interpreting the gel counts
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The parents are homozygous for alternative alleles at both loci A/a and B/b, and the genes lie on the same chromosome. The progeny classes show two large groups (75 and 80) and two small groups (22 and 16), a hallmark of linkage where parental combinations are more frequent than recombinants. In such backcross/testcross-type data, the two most frequent classes represent the parental (nonrecombinant) haplotypes, and the two least frequent classes are the recombinant haplotypes. [file:62.jpg]
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Because the two most common classes share the same allele combinations as the parental chromosomes, the heterozygous parent carried A with B on one chromosome and a with b on the other (AB/ab), i.e., coupling configuration. [file:62.jpg]
Why option (1) is correct
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Coupling means both dominant (or both specific) alleles occur together on one homolog and both alternate alleles on the other (AB/ab). The excess of parental classes over recombinant classes in the counts (75, 80 vs 22, 16) fits AB and ab as parental and Ab and aB as recombinants, which is the definition of coupling. [file:62.jpg]
Why the other options are incorrect
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Option (2) Repulsion (Ab/aB) would make Ab and aB the most frequent classes, which contradicts the data where those are the least frequent. [file:62.jpg]
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Option (3) “Different linkage groups” implies independent assortment, predicting approximately equal frequencies for all four classes; the strong deviation from 1:1:1:1 disproves this. [file:62.jpg]
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Option (4) The information is sufficient: four progeny classes with two high and two low counts is the classic pattern to infer phase (coupling vs repulsion). [file:62.jpg]
