Genetic

Ascospores from a cross leu-2 X ad-7 met-3 (all auxotrophic markers) are plated on minimal medium (no adenine, leucine, or methionine). If the three genes are unlinked, what proportion of ascospores is expected to grow into a colony on this medium?

1 strain is leu-2 ad+ met+, the other is leu+ ad-7 met-3

3 unlinked genes suggests that they're each on separate chromosomes.

The two strains are mated, making a diploid

Remember that in fungi, the haploid products of meiosis are spores that can germinate and grow into adult, haploid progeny.

Only spores that can synthesize their own leucine, adenine and methionine can grow on minimal medium, so we're looking for leu+ ad+ met+ progeny.

The question can be distilled to what is the likelihood of getting a haploid leu+, ad+ met+ meiotic product ?

For each locus on a different chromosome, there is a 0.5 chance of that ending up in a spore. Thus for 3 genes on different chromosomes, the chance of one particular combination of 3 alleles is 0.5 x 0.5 x0.5 = 0.125.

The F, G, and H loci are linked in the order written. There are 30 map units between F and G, and 30 map units between G and H. If a plant F G H/f g h is testcrossed, what proportion of progeny plants will be f g h/f g h if there is no interference?

We need only consider the trihybrid parent's gametes. 30 map units between F & G means that these two genes are 70% linked. This means that 70% of the gametes will be F G or f g (equal proportions of each). Similarly, G & H are 70 % linked, so 70% of the gametes will be GH or gh (equal proportions of each). So, 70% of the gametes with the parental F & G alleles (FG and fg) will also carry the respective parental H or h allele. Multiplying these together, there is a 0.7 x 0.7 = 0.49 chance of gametes getting the parental (FGH and fgh) combinations. These will be in equal proportions so the chance of either one is 0.49 ÷ 2 = 0.245