What happens when crossing over can occur in both sexes?
Here is the question:
Genes B D and G are linked as follows:
B—12 mu—D—-24 mu—G
A plant with the genotype Bdg//bDG is crossed to another plant with the same genotype. Assume crossing over can occur in both sexes and 1,000 offspring are produced. How many bdg//bdg individuals will you get?
Okay, so, in order to get a bdg gamete, there has to be only one cross over and it occurs between the B and D loci. This is because the parents are:
B——d————g
b——D————G
Since B and D are 12 mu apart, that means that 12% of the time crossing over occurs. Which would mean that 6% of the gametes are BDG and 6% are bdg.
But, there will also be double crossovers where crossing over occurs between both the B and D loci as well as between the D and G loci. These double crossovers don’t produce the gametes you’re looking for, so you have to subtract them from the total 12%. There’s 24% crossing over between D and G, so the double crossovers would be 12% x 24%, or 2.88%. So you’re not working with 12%, but 12% - 2.88%, or 9.12%. That means that for each parent, 4.56% of the gametes produced will be bdg.
Now, you have to cross the two parents, so the % offspring that will get both bdg gametes from each parent is 4.56% x 4.56%, or 0.2079%. For 1000 offspring, this means only 2 should be bbddgg.
Okay, so, in order to get a bdg gamete, there has to be only one cross over and it occurs between the B and D loci. This is because the parents are:
B——d————g
b——D————G
Since B and D are 12 mu apart, that means that 12% of the time crossing over occurs. Which would mean that 6% of the gametes are BDG and 6% are bdg.
But, there will also be double crossovers where crossing over occurs between both the B and D loci as well as between the D and G loci. These double crossovers don’t produce the gametes you’re looking for, so you have to subtract them from the total 12%. There’s 24% crossing over between D and G, so the double crossovers would be 12% x 24%, or 2.88%. So you’re not working with 12%, but 12% - 2.88%, or 9.12%. That means that for each parent, 4.56% of the gametes produced will be bdg.
Now, you have to cross the two parents, so the % offspring that will get both bdg gametes from each parent is 4.56% x 4.56%, or 0.2079%. For 1000 offspring, this means only 2 should be bbddgg.
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