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# Genetics -- map units

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EDIT to the problem below: I just realized that the various numbers of offspring add up to 9900, not 10,000, which is the supposed total. So for problem #2, I randomly added 100 to my recombinant numbers and got the answer, 30.1, which turned out to be correct.

But the same thing did not work for problem #1. Adding 100 to the recombinant numbers resulted in an answer of 1.2, which is not correct. Nor is the correct answer 1.1, which would make up the distance between 30.1 -- 29 (the distane from sp-cn minus the distance from px-cn.)

So here are my questions about this new twist: 1) Why would 100 offspring be left out of the various classes of offspring. How is this a reflection of what happens in real life experiments? 2) How do you know where to add the 100 offspring in and where not to? The distance between px-cn didn't need the extra 100. And the distance between sp-cn did. 3) And why isn't any of this working for the distance between px-sp?

Thank you!!

Hi. I can't figure out this problem:

Females heterozygous for the recessive second chromosome mutations pn, px, and sp are mated to a male homozygous for all three mutations. The offspring are:

px sp cn 1410
px sp + 3498
px + cn 1
px + + 11
+ sp cn 8
+ sp + 0 (none seen)
+ + cn 3483
+ + + 1489
-----
10,000 total

I correctly figured out that the middle gene is px.
I correctly figured out that the distance between
px-cn is 20 m.u.

But I can't seem to figure out 1) the distance between
px-sp or 2) the distance between sp-cn.

Here's what I'm doing:

First, I assigned the following types to the above listed classes, in the order that they are listed above:

SCO (single crossover)
P (parental)
DCO (double crossover)
SCO
SCO
DCO
P
SCO

1) To figure out the RF of px-sp: [(1 + 11 + 8 + 0) / 10,000] x 100 = 0.2 m.u.

1) To figure out the RF of sp-cn: [(1410 + 11 + 8 + 1489) / 10,000] x 100 = 29.18 m.u.

But neither one of those answers is correct.

Can you please explain how to do this problem?

Thank you.

##### Solution Preview

I would expect that this is a typo in the question. You should not have to guess about the "missing" 100 offspring. While a real experiment is not likely to give such a nice, round number as 10000, a (honest) researcher would never add in numbers that were not there. Finally, I think you switched the numbers of px sp cn and px sp +, as well as + + cn and + + +, in the numbers you gave me (at least based on your attempts at the problem).
<br><br>So, if px sp + is 1410, and + + cn is ...

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