Fruit flies can either have a brown body or a black body. The allele for a brown body (B) is dominant to the allele for a black body (b).

Question

A brown fruit fly was crossed with a black fruit fly as shown below. What is the probability of a resulting offspring being black?
B b
b ? ?
b ? ?
A.
75%
B.
100%
C.
25%
D.
50%

Answer 1

To determine the probability of a resulting offspring being black, we need to consider the genotypes of the parent fruit flies in the cross.

1. The brown fruit fly has the genotype Bb (heterozygous, since brown is dominant).
2. The black fruit fly has the genotype bb (homozygous recessive).

When we set up the Punnett square for this cross, we have:

          B       b
        -----------------
      b |   Bb   |   bb   |
        -----------------
      b |   Bb   |   bb   |
        -----------------

From the Punnett square, we can see the possible offspring genotypes:

• Bb (brown body) - 2 instances
• bb (black body) - 2 instances

Now we can summarize the probabilities:

• There are a total of 4 offspring:
  • 2 Bb (brown)
  • 2 bb (black)

To find the probability of getting a black body (bb), we count the number of bb offspring and divide it by the total number of offspring:

Probability of offspring being black (bb) = Number of bb / Total Offspring = 2 / 4 = 0.5 = 50%

Thus, the answer is:

D. 50%