Question

Can anyone help me with Genetics? I have some problems and answers and need them explained to me.
Here are some of the questions and answers:

1. Assume that a dihybrid cross is made in which the genes' loci are autosomal, independantly assorting, and incompletely dominant. How many different phenotypes are expected in offspring?
Answer: 9

2. Assume that a dihybrid F2 ratio, resulting from epistasis was 9:3:4. If a double heterozygote was crossed to the fully recessive type, what phenotypic ratio is expected among the offspring?
Answer: 1:1:2

3. Assume that a dihybrid F2 ratio, resulting from epistasis, was 15:1. If a double heterozygote was crossed to the fully recessive type, what phenotypic ratio is expected among offspring?
Answer:3:1

If anyone can explain how these are done I would really appreciate it!

Answer 1

for #3. This is a testcross situation that would result in a 3:1 ratio because an ordinary dihybrid testcross would produce a 1:1:1:1, but 15:1 is duplicate dominant epistasis. So three of the genotypes/phenotypes have a dominant allele at one or both 10ci; therefore, 3/4 will show the dominant epistasis phenotype. Only the aabb double homozygote will be different.