Gene linkage – crossing over and genetic mapping

What it shows

Genes on the same chromosome tend to be inherited together, but a crossover between them during meiosis produces recombinant gametes. The recombination frequency f is the proportion of recombinant gametes. The farther apart two genes are, the more likely a crossover falls between them, so f can be used as a map distance, with 1% recombination equal to 1 centiMorgan (cM). The two parental gamete types each make up (1 − f)/2 and the two recombinant types f/2. Because a crossover involves only two of the four chromatids, f never exceeds 50%, the value expected for genes on different chromosomes.

How to use

Set the Distance between genes and read the theoretical ratio of the four gamete types in the chart and table. Switch the Heterozygote arrangement from Cis (AB/ab) to Trans (Ab/aB) and note which gametes become parental. Change the Number of meiotic cells simulated and press Run meiosis again to compare counted recombinants with theory, then use the observed f to estimate the distance in cM.

Parameters you can change

• Distance between genes 0–50 cM
• Heterozygote arrangement Cis (AB/ab), Trans (Ab/aB)
• Number of meiotic cells simulated 100–5000

Questions to explore

1. Why does the recombination frequency never exceed 50%, even for genes very far apart?
2. For a trans heterozygote Ab/aB with f = 20%, what proportion of its gametes are AB?
3. How does simulating more meiotic cells change the gap between the observed and theoretical recombination frequency?