If the organism with the genotype Ab/aB produces 10% each of the crossover gametes, AB and ab in a test cross, what is the distance between A and B gene loci?

Question: If the organism with the genotype Ab/aB produces 10% each of the crossover gametes, AB and ab in a test cross, what is the distance between A and B gene loci?

Given:

Genotype of organism: Ab/aB
This is a repulsion (trans) heterozygote, meaning A is with b on one chromosome and a is with B on the other chromosome.

The organism is test crossed (i.e., crossed with ab/ab).

The crossover gametes are:
  • AB = 10%
  • ab = 10%

Total crossover frequency:

Crossover gametes are produced only due to recombination. In this case:
  • AB and ab are the recombinant gametes
  • Ab and aB are the parental (non-recombinant) gametes
So,
Crossover frequency = AB + ab = 10% + 10% = 20%

Distance between A and B gene loci:

In genetics, 1% recombination = 1 map unit (centiMorgan or cM)

So,
Distance between A and B = 20 cM

Answer: 20 cM




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