Question

In one type of dog, black spots (B) are dominant over brown spots (b) and long tails (L) are dominant over short tails (l). Complete a dihybrid cross for parents with the genotypes: BbLL × BBLl and answer the following in complete sentences:

1.Describe how you would set up a Punnett square for this cross.
2.List the likelihood of each possible offspring genotype.
3.List the likelihood of each possible offspring phenotype.

Answer 1

You have two chromosomes. One chromosome has the coat color allele on it. The other chromosome has the tail length allele on it. These two chromosomes sort independently.

That means that the possible combinations in which the BbL1L2 (where L1 indicates the tail length allele on the first chromosome and L2 indicates the tail length allele on the second chromosome) parent can sort themselves are B L1, B L2, b L1 and b L2.

Similarly, the possible combinations in which the B1 B2 L l (where B1 indicates the dominant allele on the first chromosome and B2 indicates the dominant allele on the second chromosome) can sort are B1 L, B1 l, B2 L and B2 l.

You put those combinations on either side of the Punnett Square and then you carry out your cross to see what genotypes are possible for the offspring. Then you interpret those genotypes to see what phenotype is associated with each one.

Is that enough help for you to set up and do the problem yourself?

Answer 2

I just don't understand how to do the Punnett Square. I've read the section on it & i've tried to do it myself from the directions, & I still don't get how it works.

Answer 3

Because you're doing a dihybrid cross, you have four possible chromsome assortment combinations for each parent. That means you start with a four by four square like this:

Answer 4

Along the top, you list the possible assortment combinations for the first parent. Along the left hand side, you list the possible assortment combinations for the second parent.