Few Genetic Questions!

Hey, hope I could get some help with those basic questions for my upcoming test. Thanks

1. When dealing with a dihybrid cross, say I am told that two genes are 22cM apart and that a cross of oossXOoSs (testcross) was done. In such cross, if I am right, I should predict an equal, 25% proportion of the 4 different phenotypes. My question is, If i am asked what number, out of the 100 from the cross will exhibit a particular phenotype (aka 1 of the four, 25%), how do I account for the 22cM distance of the two genes (O for round/oval shape and S for red/yellow)?

2. What is the probability that an autosomal dominant condition will be inherited to the child from two affected parents? Please show workings. (I got 3/4?)

3. I have 4 gene loci on a single chromosome, A,B,C,D. Looks like this:
A <-15mu-> B <-5mu-> C <-40mu-> D
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The question states, using this information, an organism hetrozygous for A and B alleles arranged in trans would produce what pair of gametes in low frequency?

A.aB and Ab
B.AB and aB
C.AB and ab
D.aB and ab

Thanks heaps in advance!

Question 1:

You would expect equal proportion of 25% IF the genes are unlinked (50cM apart). With genes linked, that will not be the case. Distance between 2 genes tells you what percentage of the progeny is a result of recombination between parental alleles. In this case, 22cM means that 22% will be recombinant and 78% (100% - 22%) will be parental genotypes. Obviously you need to know what were genotypes of the parent of the heterozygote (OoSs) to answer ‘which is which?’ (are OS on the same chromatid or Os on the same chromatid etc.). To answer that you need to know the genotypes of the prior cross that was not given here. However, the standard procedure in producing double mutant is to cross two different purebred parents. In this case that would mean crossing OOss x ooSS and the parental arrangement of OoSs would be Os on one chromatid and oS on another.
Now you can see that progeny with these parental genotypes will account for 78% (or 39% each Ooss and ooSs) while progeny carrying recombinant chromatids with OS and os will account for 22% (or 11% each OoSs and ooss).

Question 2 you answered correctly.

Question 3:

If I read the question correctly, heterozygous for A and B arranged in trans would mean that Ab are on one chromatid while aB are on the other. Considering that they are only 15cM apart, most gametes would be of parental genotype and only a few will be AB and ab.

Good luck!

Alla, thank you so much for your help

Could you possibly show workings for questions 2?

Thanks!

There are two possibilities actually.

1. Both parents are heterozygous for it - Aa x Aa - would give you 3/4 of affected children.

2. One or both parents are homozygous - AA x Aa or AA x AA - either way all children will be affected.

However, I think that 'condition' most likely refers to a medical condition and, if that is the case, homozygous individuals are not likely to reproduce (not live to reproductive age). But if it refers to a trait such as hair color, both would be possible.

thanks