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Genetic Linkage and Crossover

Hi. Here's the problem:

In Zebrafish, 3 genes have been mapped on the third chromosome, in the order

fiz--6 m.u.---myob----14 m.u.-------sufi

The wild-type allele of each gene is dominant.

A strain that is
fiz myob sufi/+++
is testcrossed, and there are 1000 offspring fish.

1. How many fiz + + offspring would you expect to see in these 1000 offspring?

2. How many + + sufi offspring would you expect to see in these 1000 offspring?

I got two other questions regarding this problem correct: that there would be 400 wild-type offspring (a parental class) and only 4 "+ myob +" offspring (a double crossover class). But I can't seem to figure these two out.

My approach: If there are 6 map units between fiz and myob, then there's a 6% chance of a crossover between them. That is, a 6% chance of getting either "fiz + +" or "+ myob sufi". 6% x 1000 = 60. And the "fiz + +" class should be approximately half of that, which is 30.

But that's the wrong answer, and I'm not sure why.

Same thing with question number 2.

There are 14 m.u.'s between myob and sufi, so there's a 14% chance of a crossover between them, a crossover that would result in either "fiz myob +" or "+ + sufi." 14% x 1000 = 140. 140/2 = 70.

But again, that's not the right answer.

Can you explain to me how to do this problem, and the correct answers?

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Solution Preview

The reason you were getting the wrong answer is that you were not taking into account the double cross-overs. That is, the progeny that are: f++ AND +ms AND .5(f+s AND +m+) equals 60.
<br><br>You halve the double cross-overs ...