Few Genetic Questions!

[Igal0s]

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!

[mith]

1. 22cM means that 22% of your offspring will be recombinants.
2. I think it's larger. Consider the different genotypes the parents can be.

[Igal0s]

I understand but not all 22% will produce a genotype that does not lead to the required phenotype. The likely answers (its a MPQ) to me seems either 25 or 22 (with allowing some room for crossing over) but i am highly dubious of this answer as it seems they placed it there for people who have no clue and will read figures of the question (22%).

With Question 2, you reckon it's even higher than 3/4? The way I see it, the only combination that will produce a phenotypically healthy child is hh. Hence, assuming its a HhxHh cross, it is 1-1/4. But I am really unsure if I can utilize such a assumption so simply?

Thanks in advance again!

[mith]

1. A correction to my previous post, it should say that the gametes are recombinants.

2. You can also have a HH X HH cross -- which produces no healthy children.