MENDELIAN GENETICS PROBLEMS

These problems are divided into subdivisions composed of problems that require application of a
specific genetic principle.

A. Monohybrid Crosses
1. In Coleus, some plants have shallowly crenated edges and others have deeply incised
leaves. A cross is made between homozygous deep and shallow individuals. The
shallow trait is dominant.
a. Using S and s to symbolize the genes for this trait, give the phenotypic and
genotypic ratios for the F1 generation.
b. If self pollination is allowed, what is the phenotypic ratio for the F2
generation?

2. a. In a pea plant that breeds true for tall, what possible gametes can be produced? Use
the symbol D for tall, d for dwarf.
b. In a pea plant that breeds true for dwarf, what possible gametes will be produced?
c. What will be the genotype of F1 offspring from a cross between these two types?
d. Assuming that the allele for tall is dominant, what will be the phenotype of F1
offspring from a cross between these two types?
e. What will be the probable distribution of traits in the F2 generation? (Illustrate with a
Punnett square).

3. A woman heterozygous for polydactyly (extra fingers and toes), a dominant trait, is
married to a normal man. What is the probability of producing an offspring that has
extra fingers or toes?

4. Parents who do not have Tay Sachs disease produce a child who has this terrible
affliction. What are the chances that each child born of this union will be affected?

5. In human beings, ability to curl the tongue into a U-shaped trough is a heritable trait.
"Curlers" always have at least one curler parent, but "noncurlers" may occur in
families where one or both parents are curlers. Using C and c to symbolize this trait,
what is the genotype of a noncurler?

6. In a certain plant, both purple x purple and purple x blue yield purple and blue colored
progeny, but blue x blue gives rise only to blue.
a. What does this tell you about the genotypes of blue- and purple-flowered plants?
b. Which gene is dominant?

7. Two short-haired female cats are mated to the same long-haired male. Several litters are
produced. Female No. 1 produced eight short-haired and six long-haired kittens.
Female No. 2 produced 24 short-haired ones and no long-haired. From these
observations, what deductions can be made concerning hair-length inheritance in these
animals? Assuming the allelic pair S and s, give the likely genotypes of the two female
cats and the male.

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 8. In human beings, a downward pointed frontal hairline ("widow's peak") is a heritable
trait. A person with a widow's peak always has at least one parent who also has this trait,
whereas persons with a straight frontal hairline may occur in families in which one or
even both parents have widow's peak. When both parents have a straight frontal hairline,
all children also have a straight hairline. Using W and w to symbolize genes for this
trait, what is the genotype of an individual without widow's peak?

9. Rh negative children (those not producing rhesus antigen D) may be born to either Rh
positive or Rh negative parents, but Rh positive children always have at least one Rh
positive parent. Which phenotype is due to a dominant gene?

B. Dihybrid Crosses
1. 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. Give the genotype and phenotype of F1 progeny
obtained from a cross between a vestigial-winged, hairy male and a normal, homozygous
female. If the F1 from this cross are permitted to mate randomly among themselves,
what phenotypic ratio would be expected in the F2 generation?

2. 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). The genes are not
linked. Calculate both phenotypic and genotypic ratios for the results of each of the
following crosses:
a. TtSs x TtSs b. Ttss x ttss c. ttSs x Ttss d. TtsSS x ss

3. In a particular species of flower, tall is dominant to short, and orange petals are dominant
to the recessive white color. Use T and t to symbolize the alleles for height, and F and f
to symbolize the alleles for flower color. A homozygous tall white flower is crossed
with a flower heterozygous for both traits. List the genotypes of the parents. What are
the F1 genotypic and phenotypic ratios?

4. How many phenotypic classes are produced by a dihybrid test-cross where one parent is
heterozygous for both pairs of genes?

5. In hogs, an allele that produces a white belt around the animal's body (W) is dominant
over its allele for a uniformly colored body (w). The dominant allele of another gene (F)
produces a fusion of the two hoofs on each foot. Suppose a uniformly-colored hog
homozygous for fused hoofs is mated with a normal-footed hog homozygous for the
belted character.
a. What are the genotypes of the parents?
b. What are the genotypic and phenotypic ratios of the F1?
c. If the F1 were allowed to interbreed, what are the genotypic and phenotypic ratios of
the F2?

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C. Modifications of Complete Dominance

Incomplete Dominance

1. The so-called "blue" (really gray) Andalusian variety of chicken is produced by a cross
between the black and white varieties, both of which breed true (i.e., both are
homozygous). What color chickens (and in what proportions) would you expect if you
crossed two blues? a blue and a black?

2. It has long been known in the field of human genetics that wavy hair is the expression of
a heterozygous genotype in which the allele for straight hair is paired with the allele for
curly hair. Lucinda Lovelee married Larry Legg. Both of these charmers have wavy
hair. What is the probability that their offspring, the littlest Legg, will have:
a. wavy hair?
b. curly hair?
c. straight hair?

3. If pale colored horses are crossed with chestnut-colored horses to produce "palomino",
an intermediate coat color:
a. What type of expression is suggested?
b. A number of matings between palominos produced 19 pale, 21 chestnut, and 44
palominos. Does this evidence support or contradict your answer to (a)? Why?

Codominance

1. For each of the following pairs of parental genotypes, calculate the phenotypic
ratios for the F1 generation.
a. IAIA x ii b. IAIA x IAIB c. IAIA x IBi
d. IAIA x IAi e. IAi x IAi f. IAi x IAIB g. IAi x ii

2. If Mr. and Mrs. Fecundity, both having blood type B, have 12 children, 3/4 of whom are
type B and 1/4 of whom are O, what are the genotypes of the parents?

3. A family of six includes four children, each of whom has a different blood type: A, B,
AB and O. What are the genotypes of parents for this trait?

4. A man with blood type B, with one parent of blood type O, marries a woman with blood
type AB. What will be the theoretical percentage of their children with blood type B?

5. Mortimer has type B blood. His wife Murgatroyd is unsure of her blood type. If their
first child, Magnifica, is type B, their second offspring, Maximum, is AB and the twins,
Maud and Lyn, are A, can you determine the genotypes of Mort and Murg?

6. In a well-publicized paternity case, the following facts were unearthed; the mother, a
strikingly beautiful, twice-convicted axe murderess, is blood type A, her child, Lizzie, is
type O, and the alleged father, a mild-mannered felon, is type B. Could he be the father?
Explain. Is there any chance that little Lizzie, the "Bad Seed", will grow up to be a

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Sex-linked Genes

1. Red-green color blindness is inherited as a sex-linked recessive. If a color-blind woman

2. Suppose that gene b is sex-linked, recessive, and embryonic 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?

3. 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?

4. In the mouse, the dominant sex-linked gene B results in a short, crooked tail. Its
recessive allele b produces a normal tail. If a normal-tailed female is mated with a bent-
tailed male, what phenotypic ratio should occur in the F1 generation?

5. On the X chromosome of Drosophila there may occur a recessive gene l, which is lethal
in the larval stage. A heterozygous female is crossed to a normal male; what F1 adult
sex phenotypic ratio results?

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