Genetics Problems 3
Answers

1. An individual male fruit fly was discovered that had a bluish cast to its body. When it was bred with a scarlet-eyed female, all the F1 females had the blue body color, and all of the F1 males were entirely normal. Remember that only the mutant (= unusual) traits are mentioned; if a trait is normal, it is not specified.

a. Are the mutations (blue body and scarlet eyes) autosomal or X-linked?

blue body - X-linked

scarlet eyes - autosomal

 

b. Are the mutations dominant or recessive?

blue body - dominant

scarlet eyes - recessive

 

c. What are the genotype of the parents?

blue-bodied male - SSX^BY

scarlet-eyed female - ssX^bX^b

 

d. What would be the expected phenotypic ratio in the male and female F2 offspring if the F1 progeny were intercrossed?

SsX^BX^b x SsX^bY

for both sexes:

3/8 normal:

3/8 blue-bodied:

1/8 scarlet-eyed:

1/8 scarlet-eyed and blue-bodied

2. A child with type O blood questions whether she was adopted. Her father's blood is type A, her mother's is type B. What can she conclude from this information?

She can't conclude anything. Her parents could produce a type O child, but she still could have been adopted.

3. A true-breeding female fruit fly with a yellow body (rather than the normal grey body) was crossed with a true-breeding male with wrinkled wings. The F1 female progeny all had wrinkled wings and the F1 male progeny all had yellow bodies and wrinkled wings.

a. Are yellow body and wrinkled wings dominant or recessive?

yellow body - recessive

wrinkled wings - dominant

 

b. Are yellow body and wrinkled wings autosomal or X-linked?

yellow body - X-linked

wrinkled wings - autosomal

 

c. What are the genotypes of the parents?

yellow-bodied female - X^bX^bww

wrinkled-winged male - X^BYWW

 

d. If you intercrossed the F1 progeny, what phenotypes and ratio would you expect among the F2's?

X^bX^BWw x X^bYWw:

Their progeny would be:

females:	3/8 yellow wrinkled :	3/8 normal wrinkled :	1/8 yellow normal:	1/8 normal
males:	same as females

 

4. A rare human trait is phenylketonuria, or PKU. People with PKU cannot properly metabolize the amino acid phenylalanine. The phenylalanine builds up in their tissues and causes mental retardation and physical developmental abnormalities. Fortunately, the symptoms can be prevented by maintaining phenylketonurics on a low phenylalanine diet starting within a few days of birth. That's why there is a national PKU screening program for all newborns. Given that a phenylketonuric can be born to two normal parents, what is the mode of transmission? Consider autosomal vs. X-linked and dominant vs. recessive.

The most likely mode of transmission of this common trait is autosomal recessive. In fact, it is the only possible mode of transmission. You know it must be recessive because affected kids show up from unaffected parents. It must be autosomal because an affected daughter comes from a normal father. If it were sex-linked, the daughter would have had to get an Xa from her dad, and he would then have to have been affected.

 

5. The black and yellow pigments in the coats of cats is controlled by an X-linked pair of alleles. Females heterozygous for these alleles have areas of black and areas of yellow in their coat (called "calico").

a. A calico cat has a litter of eight kittens -- one yellow male; two black males; two yellow females; and three calico females. Assuming there is a single father for the litter, what is his probable color?

The father had to be yellow. To get a yellow female, both parents had to give the kitten a yellow allele. Therefore, dad had to have a yellow allele, and since he's hemizygous, that was his only allele.

b. A yellow cat has a litter of four kittens -- one yellow and three calico. Assuming there is a single father for the litter, what is the probable sex of the yellow kitten?

The yellow kitten would have to be male. Since the mother cat gave all the kittens a yellow allele, the calico kittens would have to have gotten their black allele from their dad. Because the black allele would be the only X the dad had to give, he must not have given it to the yellow kitten…he must have given that kitten his Y instead.

 

6. A true-breeding purple-eyed female fruit fly was crossed with a true-breeding black-bodied male. The F1 progeny were all normal for both traits. The F2 phenotypic ratios differed between males and females as shown below.

females: 3/4 normal : 1/4 purple-eyed

males: 3/8 normal : 3/8 yellow-bodied : 1/8 purple-eyed : 1/8 purple-eyed and yellow bodied

Diagram this cross, showing the genotypes for the parents, the F1's and each of the F2 phenotypes.

When F1 progeny are intercrossed, traits that show up only in male progeny are sex-linked. Therefore, in this cross, yellow body is sex-linked and purple eyes is autosomal. Given that, the purple-eyed female must be X^BX^B pp, and the yellow-bodied male must be X^bY PP. The F1 females are dihybrids (X^BX^bPp) and the F1 males are heterozygous for purple eyes (but have normal eyes) and are yellow-bodied (X^bY Pp).

The observed F2 ratios can be calculated by:

females: (3/4 normal : 1/4 purple)(1/2 X^BX^B : 1/2 X^BX^b) (all normal body color)

males : (3/4 wild-type : 1/4 purple)(1/2 X^BY : 1/2 X^bY)

 

7. Pedigree problems

What is the most likely mode of inheritance for each of the following RARE traits?

 

a. sex-linked recessive	b. autosomal recessive	c. autosomal dominant
d. sex-linked dominant	e. Y-linked