1.You are looking at a strain of rats that is doubly heterozygous for 2 genes th

Question

1.You are looking at a strain of rats that is doubly heterozygous for 2 genes that are passed on through a dominant mode of inheritance. You mate two of these rats. What are the expected phenotypes of their offspring? Be sure to include ratios!
2 A black guinea pig (BB) and a white guinea pig (bb) produce offspring. The pattern of inheritance for fur color in guinea pigs is codominance. Indicate the genotype and phenotype of all possible offspring.
3. You are breeding a new type of plant and studying traits for height, color, and seed shape. The alleles are as follow: H=tall, h=short; C=green, c=white; S=round, s=oval. In this plant the dominant alleles for height and color are linked, and the recessive alleles for height and color are linked. You cross a plant that is heterozygous at all gene loci with a plant that is homozygous recessive for height and color and heterozygous for seed shape. What will be the phenotypic ratios of their offspring?

Explanation / Answer

2)
You have to set up a Punnett Square that looks roughly like this: 

   b  b
B Bb Bb
B Bb Bb

What you can see is that all of the offspring are heterozygous (mixed), and because the species is codominant (http://en.wikipedia.org/wiki/Codominance) meaning that both genes are active in the offspring, the offspring will be black and white -- so it'll turn out grey.

 

3) 

We are lucky in problem three that it is asking us, in essence, to do the same problem three times. If we determine what happens with one of the genes, we know what happens to all three. So, here's the Punnett Square in this case:

   h    h 
H Hh  Hh
h hh    hh

genotype: the offspring are half heterozygous, half homozygous recessive. phenotype: the offspring are half dominant, half recessive.

 

It gets a bit hairier though because we're dealing with three of these. We take, for example, the top column's Hh Hh: those will get paired with Cc half the time and cc the other half. They will also get paired with Ss half the time and ss the other half.

 

Eventual result is this:

Genotype ratios:

1 -- Hh, Ss, Cc
1 -- Hh, Ss, cc
1 -- Hh, ss, Cc
1--  Hh, ss, cc
1 -- hh, Ss, Cc
1 -- hh, Ss, cc
1 -- hh, ss, Cc
1 -- hh, ss, cc 

Phenotype ratios:

1 -- Tall, green, round
1 -- Tall, green, oval
1 -- Tall, white, round
1 -- Tall, white, oval
1 -- short, green, round
1 -- short, green, oval
1 -- short, white, round
1 -- short, white, oval 

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