Gene Linkage Problems

Genetics as it applies to evolution, molecular biology, and medical aspects.

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Gene Linkage Problems

Post by kkisamazing » Sun Jan 22, 2012 1:07 pm


I've been doing some revision on gene linkage questions and really can't answer the following two questions:

1. A rat was taken from a group that was known to breed true for black and straight fur. It was then bred with a rat of an unknown genotype and the offspring were:

Black, straight fur 1315
Red, curly fur 1370
Red, straight fur 21
Black, curly fur 19

Determine the genotype of the unknown parent and give a genetic explanation of the results.

From my research I'm presuming:
- The black, straight fur rat has dominant alleles
- The red straight fur and black curly fur is the result of crossing over
- If the black, straight rat is dominant, then all the offspring would have black, straight fur
- So the black straight fur rat and the red curly fur rat's alleles are codominant?

I think from there I can figure out whether its homozygous/heterozygous, etc. But am I also right to presume that one of the rats is heterozygous?

2. A breed of goat has yellow legs and large eyes as dominant characteristics. White legs and small eyes are recessive. The genes are linked and crossing over occurs in both sexes. Two goats with the dominant phenotypes were test crossed. The results were:

Cross 1 Cross 2
Yellow legs, large eyes 275 9
Yellow legs, small eyes 6 260
White legs, large eyes 8 282
White legs, small eyes 290 7

Explain genetically how these results were produced.

So for this question, I'm really confused. I've tried crossing all the different dominant versions of the alleles and still can't get the same ratio as above. I've crossed YYLL with YYLL, YyLl with YyLl and YYLL with YyLl. Am I missing something or just completely doing the wrong technique?

I would really appreciate any help because I'm completely confused. Thanks!

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Post by RLBallantyne » Sun Jan 22, 2012 8:07 pm

In your linked gene rat hair problem the black, straight haired genotype has bred true for multiple generations. So it must be homozygous for both traits, either homozygous recessive or homoszygous dominant for both traits.

The fact that the unknown genotype produces 50 % red/curly haired phenotypes indicates that those traits are dominant, but that that rat was heterozygous for both traits.

The linked genes on that unknown heterozygote would be

R| |b
C| |s

When this is mated with the b-s homozygote 50% would be R-C/b-s (red/curly) and 50 % would be b-s/b-s (black straight). Which is result listed.

One type of cross-over chromosome would have R-s linked, and when mated with the normal b-s recessive linked chromosome would produce a R-s/b-s genotype which would be phenotypically red with straight fur.

The other cross over would be a b-C link, and when crossed with the normal b-s linked chromosome would produce a b-b/C-s genotype which wo ould be phenotypically black with curlly fur.

I think!

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