Linkage & Recombination
Read testcross data, spot recombinants, and explain when genes do not assort independently.
What to do when the question looks unfamiliar
- Step 1Use the testcross offspring to infer gametes.
- Step 2Find the parental pair and the recombinant pair.
- Step 3Divide recombinant offspring by total, then explain what the pattern can show.
Read the two chromosomes before the Punnett square
AB/ab and Ab/aB both describe AaBb, but the starting allele combinations differ. In a cross to aabb, the tester provides only ab, so the offspring classes can reveal the gametes made by the heterozygote.
During prophase I, crossing over between nonsister chromatids can swap DNA segments. The common reciprocal classes usually represent parental combinations when the genes are linked and classes survive equally well.
Calculate recombination without overclaiming
With 420 AB, 410 ab, 90 Ab, and 80 aB offspring, the recombinant fraction is (90 + 80) ÷ 1,000 = 0.17. This gives roughly 17 map units in a two-point estimate.
A result near 50% means the genes behave as though unlinked. They may be on different chromosomes or far apart on the same chromosome. Multiple crossovers can hide exchanges by restoring parental marker combinations.