Albino rabbits (lacking pigment) are homozygous for the...

Albino rabbits (lacking pigment) are homozygous for the recessive $c$ allele ( $C$ allows pigment formation). Rabbits homozygous for the recessive $b$ allele make brown pigment, while those with at least one copy of $B$ make black pigment. True-breeding brown rabbits were crossed to albinos, which were $B B . \mathrm{F}_1$ rabbits, which were all black, were crossed to the double recessive ( $b b c c$ ). The progeny obtained were 34 black, 66 brown, and 100 albino. a. What phenotypic proportions would have been expected if the $b$ and $c$ loci were unlinked? b. How far apart are the two loci?

Albino rabbits (lacking pigment) are homozygous for the recessive $c$ allele ( $C$ allows pigment formation). Rabbits homozygous for the recessive $b$ allele make brown pigment, while those with at least one copy of $B$ make black pigment. True-breeding brown rabbits were crossed to albinos, which were $B B . \mathrm{F}_1$ rabbits, which were all black, were crossed to the double recessive ( $b b c c$ ). The progeny obtained were 34 black, 66 brown, and 100 albino.
a. What phenotypic proportions would have been expected if the $b$ and $c$ loci were unlinked?
b. How far apart are the two loci?

Key Concepts

Recombination Frequency

Recombination frequency is a measure of the proportion of recombinant offspring produced from a cross and is used as an indicator of the physical distance between two genes on a chromosome. The frequency, expressed in map units (centimorgans), quantifies the likelihood that a crossover event will occur between the loci during meiosis.

Epistasis

Epistasis describes an interaction between different gene loci where the alleles of one gene mask or modify the phenotypic effect of alleles at another gene. In such cases, the expected phenotypic ratios can deviate from those predicted by independent assortment alone, as one gene's expression can override or modify the effects of another.

Independent Assortment

In classical Mendelian genetics, genes that are located on different chromosomes or far apart on the same chromosome segregate independently during gamete formation. This independent segregation leads to the expected phenotypic ratios in dihybrid crosses when the loci are unlinked.

Gene Linkage

Gene linkage refers to the phenomenon where genes that are physically near each other on the same chromosome tend to be inherited together more often than would be expected by independent assortment. Linkage can cause deviations in the expected phenotypic ratios of crosses, as recombinant events between linked genes are less frequent.

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