BIO - Heredity
Practice genetics problems
1.) A student recently told me that both she and her
other have blue eyes (
), and that both of her
parents are
own-eyed. Using a Punnett square, list the genotypes and phenotypes showing how it’s
possible for this to happen.
2.) In fruit flies, L = long wings and l = short wings. When a long-winged fly is crossed with a
short-winged fly, the offspring exhibit a 50:50 phenotypic ratio. What is the genotype of the parent flies?
Prove how this is possible. Develop a Punnett square, and then list the genotypes and phenotypes.
3.) In tomatoes, red fruit (R) is dominant over yellow fruit (r). Tallness within plants (T) is dominant over
shortness (t). A male tomato plant whose genotype is RrTT is crossed with a female plant whose genotype
is
Tt. What are the chances of their offspring being heterozygous for both traits (RrTt)? Develop a
Punnett square, and then list the genotypes and phenotypes prior to answering the question.
4.) In dogs, there is a hereditary deafness caused by a recessive gene (d). A kennel owner has a male dog
that she wants to use for
eeding purposes if possible. The dog can hear, so the owner knows his
genotype is either DD or Dd. If the dog’s genotype is Dd, the owner does not wish to use him for
eeding
so that the deafness gene will not be passed on. This can be tested by
eeding the dog to a deaf female
(dd). Draw the Punnett squares to illustrate these two possible crosses. List all genotypes and phenotypes
showing how many of the offspring would be normal and deaf.
Background information for problem #5
In humans there is a gene that controls formation of hemoglobin (the protein in red blood cells {RBC’s}
that is responsible for ca
ying oxygen to the tissues of the body). The normal allele of this gene codes for
normal hemoglobin. However, there is another allele for this gene that contains a mutation in the DNA
and the resulting hemoglobin protein is defective. An RBC with this defective hemoglobin will, under
stress, crystallize within the cell and cause a characteristic sickle shape. While the letters S and s are often
used to represent these alleles, in reality neither is dominant to the other (codominant). Someone who is
SS makes all normal hemoglobin, someone who is ss makes all abnormal hemoglobin (someone who has
sickle-cell anemia), and someone who is Ss essentially has two sets of instructions (and so makes some of
each kind of hemoglobin; often refe
ed to as sickle-cell trait). Because the RBC’s of a person who is ss
contain all abnormal hemoglobin, they will sickle very easily, with very little stress required to provoke
that activity. Those abnormally shaped cells will then get lodged in small capillaries as they move through
them, leading to strokes, heart attacks, and pulmonary embolisms that can ultimately lead to death.
Because only some of the RBC’s of a person who is Ss contain abnormal hemoglobin, that person usually
only has trouble with a lot of cells sickling if they are under a lot of stress trying to meet a higher than
normal oxygen demand, and so the chances of a person dying from sickle-cell trait are much lower than
for full blown sickle-cell anemia.
Now, malaria is a parasitic disease that is prevalent in tropical areas. When a mosquito that is ca
ying the
parasites bites someone, the parasites enter the person’s bloodstream, and invade and live in the person’s
RBC’s. However, if a person has sickle-cell anemia (ss), the presence of a parasite in an RBC is so
stressful, it causes the RBC to crinkle up and when that happens the parasite is killed before it can
multiply and spread to other RBC’s. As a result, a person who is ss is immune to malaria. If a person is Ss
and a malaria parasite tries to invade an RBC with abnormal hemoglobin, again the RBC will sickle,
killing the parasite before it has a chance to reproduce. If a parasite invades am RBC with normal
hemoglobin, it will be able to live and multiply, but if its offspring invade other RBC’s with abnormal
hemoglobin, they too, will be killed. As a result, a person who is Ss is resistant (although not totally
immune) to malaria. If a person is SS and has all normal hemoglobin, the malaria parasites do just fine.
Thus, an SS person usually dies, eventually, from causes related to the malaria.
5.) A man and a woman living in a tropical area where malaria is prevalent and health care is not
accessible have seven children during their lifetime. The genotypes of their children are: ss, Ss, SS, ss, Ss,
Ss, and SS. What must the genotypes of both parents be? Your answer should include a Punnett square to
illustrate your work, and list all the genotypes and phenotypes.
6.) In the previous problem, which of their children would you expect to live to adulthood and reproduce?
Explain.
7.) In rural northeast Kansas domesticated cats are becoming wild, and biologists are observing that
wildcats occur in three colors. Fur color is controlled by a single gene that exhibits incomplete
dominance. A homozygous dominant cat is blue (BB), a homozygous recessive cat is red (
), and a
heterozygous wildcat will be purple (Bb). What would be the expected phenotypes of the offspring if a
lue wildcat mates with a red wildcat? Develop a Punnett square and list the genotypes as well.
8.) A naturalist is visiting an island in the middle of Lake Superior and observes a new bird species with
three distinct types of beaks. Those with short crushing beaks (BB) feed on oak nuts; those with long
delicate beaks (
) pick the seeds from pine cones; and those with intermediate beaks (Bb) feed on both
oak nuts and pine cones. (Note: because their beaks are structurally & functionally inadequate, few Bb
offspring successfully reach adulthood). Let’s assume that this difference in beak morphology is the result
of incomplete dominance in a single gene. In a year in which most of the food available on the island is in
the form of hard oak nuts, which of the mated pairs below will produce the most offspring that will be
able to survive to adulthood? Use Punnett squares to back up your answer. List all phenotypes and
genotypes.
a.) Bb x Bb
.) Bb x
c.) BB x Bb
d.) BB x
9.) In humans, when a dark-skinned person reproduces with a very light-skinned person, their children
may range in color from dark to
own. This occurs because skin color is controlled by more than one
gene (the exact # of genes is not known). For your convenience, a table is presented below that shows
several possible genotypes & phenotypes:
Genotypes Phenotypes
AABB Very dark skin
AABb or AaBB Dark skin
AaBb, AA
, or aaBB Medium
own skin
Aa
or aaBb Light skin
aa
Very light skin
If a man with dark skin whose genotype is AaBB reproduces with a woman who has light skin (aaBb),
what are the possible skin colors that their children will have? Develop a Punnett square and list the
possible genotypes & phenotypes.
10.) What is the probability that the following cross will produce an organism homozygous recessive for
all 5 traits? AaBbCcDdEe x AaBbCcDdEe Show your work including the rule(s) of probability you use to
answer the question.