A Fruit Fly Cross

Background information

• A recessive mutant allele, black, causes a very dark body in fruit flies, Drosophila, when homozygous. • The wild-type color is described

as gray.

So the Question is…•What F1 phenotypic ratio is

predicted when a black female is crossed to a gray male whose father was black?

• Hint: Use B for the dominant trait

Time to Think…5 Steps

• Review the data• Set up the problem• Make your punnett square• Look at your results• Do they make sense?

And the Answer is…

• Since the female parent is black, she must be ________ for the mutant, recessive color (bb).

That was the answer because…• The male parent is gray and thus he

must have at least one dominant allele (B). • Since his father was black (bb), and

since he received one of the chromosomes from his father, the male parent must be heterozygous (Bb).

Look at the facts…

• So the female is bb (black) • And the male is Bb (gray)• Put them into a punnett square

bb x Bb

And the results are…

_______ % Gray _______ % Black

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