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Genetics Unit Word Problems
Think Carefully and SHOW YOUR WORK! I want to be able to follow your reasoning. Be sure
to define on your paper all the symbols you use.
Simple Mendelian Crosses
1. In some cases diabetes is inherited as a recessive disease.
a. Diagram a cross between a man heterozygous for this gene and a diabetic
woman. (D = normal; d = diabetic)
b. What is the probability that their first child will be diabetic?
2. In humans, the allele for brown eyes is dominant to the allele for blue eyes. A woman
with blue eyes marries a man with brown eyes. They have two children with brown eyes
and one with blue eyes.
a) What is the genotype of the mother?
b) What is the genotype of the father?
c) What is the probability that their next child will have blue eyes?
3. The urine of people has a distinctive odor after eating asparagus . This appears to be a
trait due to a gene coding for an enzyme that breaks down one of the chemicals in
asparagus. However, only some people have the ability to smell it. The ability to detect
the smell dominates over not having the ability to detect it. If a homozygous recessive
person marries a person heterozygous for this trait, what phenotypic and genotypic
ratio will their children theoretically exhibit? Use A and a for alleles.
4. Cystic fibrosis is a recessively inherited genetic disorder, in which a recessive gene
codes for a faulty non-working membrane protein whose job is to pump chloride ions
out of lung cells. Individuals who lack these functional membrane proteins accumulate
mucus in their lungs and usually don’t survive to adulthood. You are a genetic
counselor. A worried couple (Tom and Nicole), with a family history of cystic fibrosis,
comes to your office for advice as to whether they should have children or adopt. After
charting their family history, you conclude that Tom is heterozygous (a “carrier”) for
cystic fibrosis and Nicole is homozygous dominant.
a. Create a Punnett square showing their childrens’ possible genotypes.
b. What is the probability that they will have a child with cystic fibrosis?
c. What is the probability that their child will be a carrier for the cystic fibrosis
5. Freckles are dominant to no freckles. A man with freckles and a woman with no freckles
have 3 children with freckles and one with no freckles. What are the genotypes of the
parents and children?
6. Lactose is a sugar that is found in dairy products. Lactose intolerance is inherited as a
dominant trait. People who have an allele for lactose intolerance cannot properly digest
milk products. Paul is intolerant of lactose but his wife, Laura, is not. Additionally,
Paul's dad is lactose intolerant but his mom is not. From this information about his
family, you realize that Paul is heterozygous (Aa) for the lactose intolerance trait.
a. (a) Based on the information about Paul’s parents, explain how you realized Paul
must be heterozygous.
b. Knowing Paul's genotype, what alleles can he pass on to his offspring?
c. What is the probability that Paul and Laura will have a child that is intolerant of
7. A red-haired man (Harry), both of whose parents (Laura and William) have brown hair,
marries a brown-haired woman (June) whose father (Frank) has brown hair and whose
mother (Cindy) has red hair. Harry and June have one child (Peggy) who is red-haired.
Assuming that brown hair is dominant over red hair, give the genotype of as many of
the seven people in this family as you can.
8. In certain portions of the Jewish population, there is a genetic disease called Tay Sachs
disease, which is fatal to infants within the first five years of life. This disease is caused
by a recessive allele of a single gene. Why does this disease persist, even though it is
invariably fatal long before the afflicted individual reaches reproductive age? (In other
words, why doesn’t the allele for Tay Sachs disease simply disappear?)
9. About 80% of the human population can taste the chemical phenolthiocarbamide (PTC),
while the other 20% can’t. This characteristic is governed by a single gene with two
alleles, a tasting allele and a non-tasting allele. What does this statistic tell us about
which allele (tasting or non-tasting) is dominant?
Double Mendelian Crosses
1. In Guinea pigs rough and black are dominant. A rough-coated, black guinea pig, whose
mother was smooth and white, is mated with a smooth, white guinea pig. What kinds of
offspring would they produce and in what relative numbers?
2. In humans the allele for dimples is dominant to the allele for "no dimples"; the allele for
curly hair is dominant to the allele for straight hair. A dimpled woman with straight hair
marries a non-dimpled, man with curly hair.
a. What are the possible genotypes of these two people? By assuming, when there
is doubt, that they are heterozygous, predict the possible phenotypes and their
proportions for this couple's children.
3. In cocker spaniels two genes determine coat color. Allele A results in the production of
red pigment; recessive a produces no pigment. Allele B results in the production of livercolored pigment; recessive b produces no pigment. If both pigments are present, the dog
is black; if neither pigment is present, the dog is lemon. A black female cocker spaniel is
mated to a lemon male and produces a lemon pup.
a. What is her genotype?
b. If this same black dog is mated to another of her own genotype, what
proportions of what different colors would be expected among the offspring?
4. A rough, black guinea pig bred with a rough, white one gives 28 rough, black; 31 rough,
white; 11 smooth, black, and 9 smooth white. Give the genotypes for the parents and
each offspring.
5. Two rough black guinea pigs when bred together have two offspring, one of them rough
white and the other smooth, black. If these same parents were to be bred together
further, what offspring would you expect from them.
6. A right -handed, blue-eyed man whose father was left-handed marries a left-handed,
brown-eyed woman from a family in which all the members have been brown-eyed for
several generations what offspring may be expected from this marriage as to the two
traits mentioned? Assume that brown eyes (B) are dominate over blue (b) and righthandedness (R) over left handedness(r). (Actually some modifying factors do not allow
exactly this interpretation). Assume these two people have 20 children. Give the
phenotype that you would expect in these kids.
7. A right-handed, blue-eyed man marries a right-handed, brown-eyed woman. They have
two children, one left-handed and brown-eyed and the other right-handed and blueeyed. By a later marriage with another woman who is also right-handed and browneyed, this man has nine children, all of whom are right-handed and brown-eyed. What
are the genotypes of this man and his two wives?
8. Elizabeth is married to John, and they have four children. Elizabeth has a straight nose
(recessive) and is able to roll her tongue (dominant). John is also able to roll his tongue,
but he has a convex (Roman) nose (dominant). Of their four children, Ellen is just like
her father, and Dan is just like his mother. The other children—Anne, who has a convex
nose, and Peter, who has a straight nose—are unable to roll their tongues. Please answer
the following questions about this family.
a. What are the genotypes of Elizabeth and John?
b. Elizabeth’s father was a straight-nosed roller, while her mother was a convex-nosed
non-roller. What can you figure out about their genotypes?
c. John’s father was a straight-nosed roller, while his mother was a convex-nosed roller.
What can you determine about their genotypes?
Codominant Crosses
1. In a particular family, one parent has Type A blood, the other has Type B. They have
four children. One has Type A, one has Type B, one has Type AB, and the last has Type
O. What are the genotypes of all six people in this family?
2. Mrs. Smith has blood type AB. Mr. Smith has blood type B. Their first child has blood
type AB. Their second child has blood type O. Mr. Smith suspects all is not as it seems
with child number 2. Should he be suspicious?
3. In a recent case in Spokane, Washington, a young woman accused a soldier of being the
father of her child. The soldier, of course, denied it. The soldier’s lawyer demanded that
blood types be taken to prove the innocence of his client. The following results were
obtained: Alleged father, Type O. Mother, Type A. Child, Type AB. The court found the
soldier guilty on the basis of the woman’s remarkable memory for dates and details that
apparently eliminated all other possible fathers.
a. What are the possible genotypes for these three people?
b. Do you agree with the court’s decision? Why or why not?
4. It was suspected that two babies had been exchanged in a hospital. Mr. and Mrs. Jones
received baby #1 and Mr. and Mrs. Simon received baby #2. Blood typing tests on the
parents and the babies showed the following:
Mr. Jones: Type A
Mr. Simon: Type AB
Mrs. Jones: Type O
Mrs. Simons: Type O
Baby #1: Type A
Baby #2 Type O
a. Were the babies switched? How do you know whether they were or they
5. A man with type O blood marries a woman with Type AB blood. Among their children,
what proportion would you expect to have blood types like one or the other of these
parents? What proportion would have expect to have blood types different from both
parents? Explain.
6. A woman has a daughter. There are three men whom she claims might have been the
father of the child. The judge in the paternity court orders that all three men, the child,
and the mother have blood tests. The results are: mother, Type A; Daughter, Type O;
Man #1, Type AB; Man #2, Type B; Man #3, Type O. The mother claims that this proves
that Man #3 must be the little girl’s father.
a. Is the mother correct? Why or why not?
b. The judge isn’t satisfied, so he asks for the medical records of the people
involved. He discovers that the little girl is colorblind. Men #’s 1 and 2 are also
colorblind; Man #3 has normal color vision, as does the mother. (NOTE:
Colorblindness is X-linked and recessive.) Assuming that one of these three men
must be the father, can you now determine which of the three it is?
7. Another woman has the same problem. Her blood type is A, her child’s is B. She again
has three candidates for fatherhood. Their blood types are: Man #1, B; Man #2, AB; Man
#3, O. Based on blood types, the mother says it must have been #1.
a. Do you agree? Why or why not?
b. This child, a son this time, is also colorblind. The only one of the men in question
to share this characteristic is #2. The mother is not colorblind. Can you now
determine who the father of the little boy is, assuming it must be one of these
men? Explain your answer.
Sex Linked Crossed
1. Can a human male be homozygous for a sex-linked character? From which parent does a
human male receive his sex-linked genes? To which sex among his offspring does he
transmit his sex-linked characters? If a gene were located in his Y chromosomes, to what
offspring would he transmit it?
2. Color-blindness is a sex-linked recessive. A girl of normal vision whose father was colorblind married a color blind boy. What type of vision will be expected in their children?
3. A colorblind woman marries a man with normal vision. She is pregnant. What is the
probability that her child will be
a. a girl with normal vision?
b. ) a color blind girl?
c. a boy with normal vision?
d. a color blind boy?
4. A woman of normal vision, whose father was color-blind, marries a man of normal
vision whose maternal grandfather was color-blind. What type of vision will be
expected in their children?
5. A woman whose maternal grandfather suffered from hemophilia has parents that are
normal. The woman's husband is normal. She has a hemophiliac son. What is the chance
that the next son will be normal? Will any of the daughters be hemophiliac? Will any be
carriers? Hemophilia is sex linked recessive.
6. Two normal-visioned parents produce a color-blind son. What are the genotypes of the
parents? What are the chances of their next child being a color-blind daughter?
7. In human beings, migraine (a type of sick headache) is due to a dominate factor. A
normal visioned woman who has never suffered from migraine takes her daughter to
the doctor for an examination. In the course of the examination the doctor discovers that
the girl is color-blind, and suffers from migraine. What does the doctor automatically
know about the father?
8. In man, aniridia (a type of blindness) is due to a dominate factor. Optic atrophy (another
type of blindness) is due to a recessive sex-linked factor. A man blind from optic atrophy
marries a woman blind from aniridia. Assuming that the mother is homozygous for both
characters, would any of their children be expected to be blind? Which type of blindness
would they have?
9. 4. In cats the genotype BB is black; Bb is tortoise shell; and bb is yellow. The gene is on
the X chromosome. (Gender is determined in cats the same way as in humans.) A
tortoise shell female is crossed with a black male.
a. What offspring would be expected?
b. Explain why tortoise shell males are extremely rare.
10. A color blind Turk has two wives. By the first he has a colorblind son, a colorblind
daughter, a normal son and a normal daughter. By the second he has six children, four
boys and two girls, none of whom is color blind.
a. State the genotype and phenotype of each wife.
b. Give genotypes of all the kids.
c. About whom is there some doubt?
11. Plutonian Tickle-bellies have a sex determination system just like mammals. Hairy
Snout is a holandric trait (carried on the Y chromosome). MyxRotcccc, a handsome male
Tickle-belly, has lovely orange hair on his snout. He and his mate, OrgggWny, have six
offspring, three boys and three girls. Please answer the following questions about this
a. How many of MyxRotcccc’s and OrgggWny’s offspring have hairy snouts? Can
you predict which ones?
b. Their eldest son, Bobrggg, marries and has a son. What is the chance that
Bobrggg’s son will also have a hairy snout?
c. JoKchew, MyxRotcccc’s and OrgggWny’s youngest daughter, marries a male
who has a smooth, hairless purple snout. She has eight offspring, each one
lovelier than the last, and all boys. What percentage of these offspring do you
expect to have hairy snouts? Explain.
12. A condition known as icthyosis hystrix gravior appeared in a boy in the early eighteenth
century. His skin became very thick and formed loose spines that were sloughed off at
intervals. When he grew up, this porcupine man married and had six sons, all of whom
had this condition, and several daughters, all of whom were normal. For four
generations, this condition was passed from father to son. From this evidence, what can
you postulate about the location of the gene?
13. A “normal” couple has a son with sex-linked muscular dystrophy (MD).
a. From which parent did the son receive gene(s) for MD.?
b. If their next child is a girl, what is the probability she will inherit sex-linked
muscular dystrophy?
.Lalitha and Ankit have three children, Vishnu, Vivek, and Ganesh. Vishnu and Vivek are color
blind, though Ankit is not. All three children have blue eyes, though both parents have very
dark eyes. Lalitha, Ganesh, and Vishnu have very long (dominant) eyelashes though the other
family members do not. Lalitha is expecting another baby.
a. What is the most probable phenotype of the new baby?
b. What is the probability that the three boys will have a little sister who is
colorblind with long eyelashes and blue eyes?
c. What is the probability that the three boys will have a little brother who is
colorblind with long eyelashes and blue eyes?