Question

In a test cross between a dihybrid exhibiting two dominant phenotypes (AaBb) and a double homozygous recessive (aabb), we obtain progeny that shows a higher ratio of phenotypes that combine a dominant phenotype for one character and the recessive for the other (e.g. A_bb), and viceversa. What is the likely explanation ?

Group of answer choices

the alleles in the heterozygous parent are in coupling configuration.

double crossovers likely occurred between the A and B loci.

the alleles in the heterozygous parent are in repulsive configuration

The recombination frequency is about 50 percent

Answer 1

the alleles in the heterozygous parent are in repulsive configuration

The two parents in the cross are of the genotype AaBb and aabb. The dihybrid parent can, however, have two distinct chromosomal arrangements, the parent can either have AB and ab chromosomes or the parent can have aB and Ab chromosomes. The first arrangement is called the coupling phase or cis arrangement, while the second is called the repulsive or trans configuration.

If the dhybrid parent is in repulsive configuration, then the aB and Ab gametes will be the non-recombinant gametes, and will be present in a larger fration of progeny.