Question

In some bees, thin stripes (S) are dominant over thick stripes (s) and black eyes (B) are dominant over gray eyes (b). Complete a dihybrid cross for parents with the genotypes: SsBB x ssBb and answer the following in complete sentences. (5 points)

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

1. Use a 2X2 Punnett square to solve this problem

2. It would be 1/4 SsBB, 1/4 SsBb, 1/4 ssBb and 1/4 ssbb

3. 50% chance they will be thin black, 25% chance they will be thick black, and 25% chance they will be thick gray.

Hope this helped. Have a great day!

Answer 2

1. The number of rows and columns of the Punnett square is determined by the number of allele combinations an organism produces. For example, for a single trait an organism can produce 2 allele combinations so, there would be 2 rows and 2 columns in a square.

Similarly, in a dihybrid cross or when two traits are considered in a cross, an organism can produce 4 allele combinations. Hence, there will be four rows and four columns in a square.

Maximum number of allele combination an organism can produce is determined by 2ⁿ where n is the number of traits.

Now, Parental genotype: SsBB and ssBb

Number of trait in consideration = 2

So, number of allele combinations = 2² = 4.

Hence, there will be 4 row and 4 columns in the Punnett square.

Gametes produced by SsBB = SB and sB

Gametes produced by ssBb = sB and sb

Punnett square of the cross is shown the figure attached.

2. Total four genotypes were produced:

SsBB, ssBB, SsBb, and ssBb

Each having the ratio of 4/16 = 1/4

3. Two phenotypes were observed:

Thin strips with black eyes (S_B_) and thick strips with black eyes (ssB_).

Each phenotype was found to have the ratio of 8/16 = 1/2 or 50%.