In fruit flies, curly wings (c) is recessive to straight wings (C). Ebony body (
ID: 95338 • Letter: I
Question
In fruit flies, curly wings (c) is recessive to straight wings (C). Ebony body (e) is recessive to tan body (E). Suppose you had a male fly that had curly wings and was heterozygous for body color and a female fly that was heterozygous for wing type and had had an ebony body.
a)What is the genotype for the male fly?
b)What possible allele combinations could the male fly produce?
c)What is the genotype for the female fly?
d)What possible allele combinations could the female fly produce?
e)Construct a Punnett Square to determine the PHENOTYPIC ratio that you would expect to find in the offspring of these two flies.
Explanation / Answer
ANSWER:
Recessive traits are only expressed if BOTH alleles are recessive. Therefore, for the wings, it must have (cc) as its genotype to have curly wings. The genotypes (CC) and (Cc) will both yield straight wings because the dominant allele (C, or straight) trumps the recessive allele (c).
The dominant allele (C) will mask the expression of the recessive allele (c). This is why the genotype (Cc) yields straight wings, because the dominant (C) will mask the expression of the recessive (c). However, the genotype (cc) has no dominant allele (C) to mask the expression of the recessive trait. Therefore, this fly will express the recessive trait, curly wings.
So, let's look at the flies.
a) The male fly has curly wings and is heterozygous for color, which gives it the genotype:
(cc) and (Ee). The curly wings are recessive, therefore BOTH alleles must be recessive; otherwise, a dominant allele would mask the recessive trait. Heterozygous means it has both alleles, hence its body color is tan, but it has the recessive allele (e, for ebony). The recessive trait is not shown because the dominant (E, tan) allele hides it.
b) Because there are two traits being looked at here, it will produce gametes with two alleles, one for wing type and the other for color. Its gametes will be: (cE), (ce). This combines one allele from the wingtype and one allele from the color per gamete. If you make all the cross-comparisons, you get (cE), (ce), (cE), and (ce), but obviously some of those are the same. ****Even though the male fly has tan wings, it has the genotype (Ee), which means it can pass along the recessive allele even though he does not express the recessive trait.
c) The female fly is heterozygous for wing type and has an ebony body, which gives it the genotype:
(Cc) and (ee). It has both wing type alleles, but has straight wings because the dominant (C, straight) allele masks the recessive (c, curly) allele. Because ebony is a recessive trait, both alleles must be recessive (ee). Otherwise, the dominant allele (E, for tan) would hide the expression of the recessive trait (ebony).
d) The female fly will make gametes with the possible combinations (Ce) and (ce), when you cross-compare the alleles in the same manner you did for the male fly.
e) The punnett square will show the possible offspring produced by the gametes above.
------- cE | ce
Ce| CcEe | Ccee
ce| ccEe | ccee
The phenotypes (going clockwise starting with CcEe) will be straight wings, tan body; straight wings, ebony body; curly wings, ebony body; curly wings, tan body. Remember, dominant allele trumps recessive allele, so when both alleles are recessive, the recessive trait will be expressed. If there is at least one dominant allele, the dominant trait will be expressed.
The phenotypic ratio, then, 25% Straight and Tan, 25% Straight and Ebony, 25% Curly and Tan, and 25% Curly and Ebony.