Genetics Problems

The best way to gain an understanding of genetics is to

work through genetics problems. The fundamental

principles discussed in lecture will become clearer to

you if you carefully work through the following problem

set which illustrates the various patterns of inheritance

treated in the genetics’ chapter of your text book.

1. In squash a gene for white color (W) is dominant over its

allele for yellow color (w). Give the genotypic and

phenotypic ratios for the results of each of following

crosses:

WW x ww

Ww x ww

Ww x Ww

2. If pollen from a heterozygous white-fruited squash plant is

placed on the female part of a yellow-fruited plant, show,

using ratios, the genotypes and phenotypes you would expect

the seeds from this cross to produce.

3. In human beings, brown eyes are usually dominant over blue

eyes. Suppose a blue-eyed man marries a brown-eyed woman

whose father was blue-eyed. What proportion of their

children would you predict will have blue eyes?

4. If a brown-eyed man marries a blue-eyed woman and they have

ten children, all brown-eyed, can you be certain that the man

is homozygous? If the eleventh child has blue eyes, what

will that show about the father’s genotype?

5. A brown-eyed man whose father was brown-eyed and whose mother

was blue-eyed married a blue-eyed woman whose father and

mother were both brown-eyed. The couple has a blue-eyed son.

For which of the individuals mentioned can you be sure of the

genotypes? What are their genotypes? What genotypes are

possible for the others?

6. If the litter resulting from the mating of two short-tailed

cats contains three kittens without tails, two with long

tails, and six with short tails, what would be the simplest

way of explaining the inheritance of tail length in these

cats? Show genotypes.

7. When Mexican hairless dogs are crossed with normally haired

dogs, about half the pups are hairless and half have hair.

When, however, two Mexican hairless dogs are mated, about a

third of the pups produced have hair, about two thirds are

hairless, and some deformed puppies are born dead. Explain

these results.

8. In peas a gene for tall plants (T) is dominant over its

allele for short plants (t). The gene for smooth peas (S) is

dominant over its allele for wrinkled peas (s). Calculate

both phenotypic ratios and genotypic ratios for the results

of each of the following crosses:

TtSs x TtSs

Ttss x ttss

ttSs x Ttss

TTss x ttSS

9. In hogs a gene that produces a white belt around the animal’s

body is dominant over its allele for a uniformly colored

body. Another independent gene produces fusion of the two

hoofs on each foot (an instance of syndactyly); it is

dominant over its allele, which produces normal hoofs.

Suppose a uniformly colored hog homozygous for syndactyly is

mated with a normal-footed hog homozygous for the belted

character. What would be the phenotype of the F1? If the F1

individuals are allowed to breed freely among themselves,

what genotypic and phenotypic ratios would you predict for

the F2?

10. In watermelons the genes for green color and for short shape

are dominant over their alleles for striped color and for

long shape. Suppose a plant with long striped fruit is

crossed with a plant heterozygous for both these characters.

What phenotypes would this cross produce and in what ratios?

11. In the fruit fly Drosophila melanogaster, vestigial wings and

hairy body are produced by two recessive genes located on

different chromosomes. The normal alleles, long wings and

hairless body, are dominant. Suppose a vestigial-winged

hairy male is crossed with a homozygous normal female. What

types of progeny would be expected? If the F1 from this

cross are permitted to mate randomly among themselves, what

progeny would be expected in the F2? Show complete

genotypes, phenotypes, and ratios for each generation.

12. A dominant gene, A, causes yellow color in rats. The

dominant allele of another independent gene, R, produces

black coat color. When the two dominants occur together

(A/-R/-), they interact to produce gray. Rats of the

genotype a/a r/r are cream-colored. If a gray male and a

yellow female, when mated, produce offspring approximately

3/8 of which are yellow, 3/8 gray, 1/8 cream, and 1/8 black,

what are the genotypes of the two parents?

 13. What are the genotypes of a yellow male rat and a black

female that, when mated, produce 46 gray and 53 yellow

offspring?

14. In Leghorn chickens colored feathers are due to a dominant

gene, C; white feathers are due to its recessive allele, c.

Another dominant gene, I, inhibits expression of color in

birds with genotypes CC or Cc. Consequently both C-I- and

cc-- are white. A colored cock is mated with a white hen and

produces many offspring, all colored. Give the genotypes of

both parents and offspring.

15. If the dominant gene K is necessary for hearing, and the

dominant gene M results in deafness no matter what other

genes are present, what percentage of the offspring produced

by the cross kkMm x Kkmm will be deaf?

16. What fraction of the offspring of parents each with the

genotype KkLlMm will be kkllmm?

17. Suppose two DdEeFfGgHh individuals are mated. What would be

the predicted frequency of ddEEFfgGHh offspring from such a

mating?

18. If a man with blood type B, one of whose parents had blood

type O, marries a woman with blood type AB, what will be the

theoretical percentage of their children with blood type B?

19. Both Mrs. Smith and Mrs. Jones had babies the same day in the

same hospital. Mrs. Smith took home a baby girl, whom she

named Shirley. Mrs. Jones took home a baby girl, whom she

named Jane. Mrs. Jones began to suspect, however, that her

child had been accidentally switched with the Smith baby in

the nursery. Blood tests were made: Mr. Smith was type A,

Mrs. Smith type B, Mr. Jones type A, Mrs. Jones type A,

Shirley type O, and Jane type B. Had a mixup occurred?

20. Suppose that gene b is sex-linked, recessive, and lethal. A

man marries a woman who is heterozygous for this gene. If

this couple had many normal children, what would be the

predicted sex ratio of these children?

21. Red-green color blindness is inherited as a sex-linked

recessive. If a color-blind woman marries a man who has

normal vision, what would be the expected phenotypes of their

children with reference to this character?

22. A man and his wife both have normal color vision, but a

daughter has red-green color blindness, a sex-linked

recessive trait. The man sues his wife for divorce on

grounds of infidelity. Can genetics provide evidence

supporting his case?

23. In Drosophila melanogaster there is a dominant gene for gray

body color and another dominant gene for normal wings. The

recessive alleles of these two genes result in black body

color and vestigial wings respectively. Flies homozygous for

gray body and normal wings were crossed with flies that had

black bodies and vestigial wings. The F1 progeny were then

test-crossed, with the following results:

Gray body, normal wings 236

Black body, vestigial wings 253

Gray body, vestigial wings 50

Black body, normal wings 61

Would you say that these two genes are linked? If so, how

many units apart are they on the chromosome?

24. In rabbits a dominant gene produces spotted body color, and

its recessive allele solid body color. Another dominant gene

produces short hair, and its recessive allele long hair.

Rabbits heterozygous for both characteristics were mated with

homozygous recessive rabbits. The results of this cross were

as follows:

Spotted, short hair 96

Solid, short hair 14

Spotted, long hair 10

Solid, long hair 80

What evidence for linkage is shown in this cross? Give the

percentage of crossing-over and the map distance between the

genes.

25. In Drosophila melanogaster the genes for normal bristles and

normal eye color are known to be about 20 units apart on the

same chromosome. Individuals homozygous dominant for these

genes were mated with homozygous recessive individuals. The

F1 progeny were then test-crossed. If there were 1,000

offspring from the test cross, how many of the offspring

would you predict would show the crossover phenotypes?

26. The crossover frequency between linked genes A and B is 40%;

20%; between D and B, 10%. What is the sequence of the genes

on the chromosome?