Genetics Practice: Sex-linkage, Incomplete Dominance, Co-dominance, Multiple Alleles

and Gene Linkage –YOU MUST SHOW YOUR WORK TO GET CREDIT!
Background Information: Autosomal traits are carried on chromosomes 1-22 in humans. This
means the trait will appear in both sexes equally. Sex-linked traits have recessive alleles on the
X chromosome. Because males on have one X chromosome, if they have the recessive allele
they will have the condition. Two types of different alleles are incomplete dominant alleles and
codominant alleles. With incomplete dominance, the heterozygous genotype will express a
phenotype somewhere in between the dominant and the recessive. With codominance, the
heterozygous genotype will express both phenotypes. Please take notes about each of the
genetic disorders found in humans as well. You will be asked questions about the disorders on
your test.
Sex-linkage:
1. In fruit flies, white-eyes is an X-linked recessive form of the trait for eye color (red-eyes being the dominant form). A
heterozygous female for eye color is crossed with a white-eyed male. Make a Punnett square to show your work.
a. How many of their offspring will be white-eyed females?

b. How many white-eyed males?

c. How many of the males will be red-eyed?

2. Hemophilia is an X-linked recessive gene that prevents the blood from clotting. The effects of this X chromosome
disorder develop almost entirely in males even though the gene is inherited from one of the mother’s X chromosome. A
normal woman whose father had hemophilia marries a normal man. What are the chances of hemophilia in their children?
Make a Punnett square to show your work and use percentages for your answer.

3. Red-green color blindness is a recessive X-linked genetic disorder where middle (green) or long (red-yellow)
wavelengths cones in the eye have a partial or complete loss of function. A human female "carrier" who is heterozygous
for the recessive, sex-linked trait causing red-green color blindness, marries a color blind male. What proportion (use a
fraction) of their male progeny will have red-green color blindness? Make a Punnett square to show your work.
Incomplete Dominance
4. Incomplete dominance is seen in the inheritance of hypercholesterolemia. Mack and Toni are both heterozygous for this
characteristic, and both have elevated levels of cholesterol. Their daughter Zoe has a cholesterol level six times normal;
she is apparently homozygous, hh. What fraction of Mack and Toni’s children are likely to have elevated but not extreme
levels of cholesterol, like parents? If Mack and Toni have one more child, what is the probability (use the rules of
probability) that the child will suffer from the more serious form of hypercholesterolemia seen in Zoe?

5. Red (CR) and white (CW) carnations show incomplete dominance. Show what happens when a red carnation is crossed
with a white carnation. Make a Punnett square to show your work. What would be the color of the carnation offspring?

6. What is the phenotypic ratio of a cross between two heterozygous carnations (see question above)? Make a Punnett
square to show your work.

7. Roan is a name for cattle that show a heterozygous incomplete dominant genotype. A roan calf's parents were a white
cow and a red bull. Make a Punnett square to show your work.
a. What is a roan's genotype?

b. Can two roans mate and produce all roans? Explain.

8. Coat color in one breed of mice is controlled by incompletely dominant alleles so that yellow and white are
homozygous, while cream is heterozygous. What phenotypes would the cross of two cream individuals will produce?
Make a Punnett square to show your work.
Codominance and Multiple Alleles
9. One of the most firmly established series of multiple alleles in humans involves the genetic locus controlling the blood
types: A, B, AB, and O. The ABO locus has three common alleles: IA, IB, and i. IA and IB are codominant (IA IB
heterozygotes have both A and B antigens on their red blood cells) and i is recessive (ii homozygotes have no AB antigens
on their red blood cells; IA i and IB i heterozygotes have A and B antigens, respectively, on their red blood cells.)

What are the possible blood types of children in the following families? Make Punnett squares to show your work.
a. Type A mother x type A father
Possible blood types of offspring: _______________________________

b. Type A mother x type O father

Possible blood types of offspring: _______________________________

c. Type B mother x type AB father

Possible blood types of offspring: _______________________________

10. In paternity lawsuits, blood typing often is used to provide genetic evidence that the alleged father could not be related
to the child. For the following mother-child combinations, complete the table and indicate which blood types could NOT
have been the father’s.

Mother Possible Child Possible Impossible Impossible

Genotypes for Genotypes for Genotypes for blood types
Mother Child Father for Father
O B

B A

AB B

11. Color pattern in a species of duck is determined by a single pair of genes with three alleles. Alleles H and I are
codominant, and allele j is recessive to both. How many phenotypes are possible in a flock of ducks that contains all the
possible combinations of these three alleles? Make a Punnett square to show your work.

12. Mary has blood type A and she marries John, whose blood type is B. They have three children: Joan. James and Pete.
Joan has blood type O, James has blood type A, and Pete has blood type B. Explain how this is possible.

13. In a legal case, a man is accused of fathering a child of blood type AB. He is blood type O and the child's mother is
blood type AB. Can this man be the father? Explain your answer?
14. A man with type AB blood marries a woman with type O blood, but whose father was type A blood. What genotype
would you expect their first child to have?

Other Problems:
15. A black guinea pig crossed with an albino guinea pig produced 12 black offspring. When the albino was crossed with
a second black one, 7 blacks and 5 albinos were obtained. What is the best explanation for this genetic situation? Write
genotypes for the parents and offspring. See #18 for help.

16. Phenylketonuria (PKU) is an inherited disease caused by a recessive allele that causes the body to not be able to break
down the amino acid phenylalanine. If a woman and her husband are both carriers, what is the probability of each of the
following? Use the rules of probability to show your work.
a) all three of their children will be of normal phenotype

b) one or more of the three children will have the disease

c) all three children will have the disease

d) at least one child will be phenotypically normal

17. Cystic fibrosis is an autosomal recessive disease where mucus develops in the lungs, liver and the pancreas. Make a
Punnett square and determine the expected genotypic and phenotypic ratios from crossing homozygous dominant parent
with a carrier of cystic fibrosis.

Genotypic ratio: ________________________

Phenotypic ratio: _______________________

18. Albinism (Albino) causes deficiency of pigmentation in skin, hair and eyes. Albinism is recessive and autosomal.
When a single gene affects many traits, like albinism, it is called pleiotropy. Make a Punnett square and determine the
expected genotypic and phenotypic ratios from crossing a pure bred recessive parent with a heterozygous parent.

Genotypic ratio: _________________________

Phenotypic ratio: ________________________

19. Sickle-cell anemia is an autosomal recessive genetic disorder that causes red blood cells to change shape and can
cause the red blood cells to become stuck in blood vessels. This blocking can deprive tissues of oxygen and cause organ
damage like strokes. One benefit of sickle-cell anemia is that people who has one or two alleles of the sickle-cell anemia
also exhibits incomplete dominance at which the individual who does not have the disease retains immunity to malaria.
Make a Punnett and determine the expected genotypic and phenotypic ratios form crossing pure bred recessive and
homozygous dominant parents.

Genotypic ratio: __________________________

Phenotypic ratio: _________________________

Genetic Mapping:
20. Use the recombination frequencies between the following genes to assign the genes their position on the chromosome
map. In other words, put the genes in the order in which they’d be found on the chromosome. Show your work!
Loci Genes Recombination Frequency
b/vg Body color/vestigial wings 17%
b/cn Body color/cinnabar eye color 9%
Cn/vg Cinnabar eye color/vestigial wings 9.5%

Order of genes on chromosome: _________________________

21. A fruit fly with a gray body and red eyes (genotype BbPp) is mated with a fly having a black body and purple eyes
(genotype bbpp).
a. What offspring, in what proportions, would you expects if the body color and eye color genes are on different
chromosomes (unlinked)?

b. When this mating is actually carried out, most of the offspring look like the parents, but 3% have gray body and
purple eyes, and 3% have black body and red eyes. What is the recombination frequency?

c. Are these genes linked or unlinked? Explain.

22. A series of matings shows that the recombination frequency between the black-body gene and the gene for dumpy
(shortened) wings is 36%. The recombination frequency between purple eyes and dumpy wings is 41%. What is the
sequence of these three genes on the chromosome? (Hint: You will need to use part of your answer in the previous
question to solve this problem).