Consider two hypothetical recessive autosomal genes a and b, where a heterozygot
ID: 60820 • Letter: C
Question
Consider two hypothetical recessive autosomal genes a and b, where a heterozygote is testcrossed to a double-homozygous mutant. Predict the phenotypic ratios under the following conditions.
Part A: a and b are located on separate autosomes.
Part B: a and b are linked on the same autosome, but are so far apart that a crossover always occurs between them.
Part C: a and b are linked on the same autosome, but are so close together that a crossover almost never occurs.
Part D: a and b are linked on the same autosome about 10 mu apart
Explanation / Answer
QA). Under independent assortment we would expect 25% of each corresponding phenotype.
QB). Under independent assortment (no linkage) we would expect 25% of each corresponding phenotype, even though the genes are present on the same autosome.
QC). Under complete linkage, only two phenotypes would be produced, which phenotypes would depend on the phase of the alleles in the dihybrid. The dihybrid could either be AB/ab or Ab/aB
QD).
We would get a ratio of AB 45:Ab 5:aB 5:ab 45 if the alleles in the dihybrid were in cis-phase.
We would get a ratio of AB 5:Ab 45: aB 45:ab 5 if the dihybrid were in trans-phase .