BIOLOGY: Genetics
Sex-linked Inheritance
Name:__________________________
Date_________________Hr________
Sex-linked traits are controlled by genes that are found on the X chromosome, but not on the Y chromosome in
humans. The X chromosome is larger and has more genes than the Y chromosome. This complicates the
inheritance of traits on the X chromosome since we expect that a normal diploid cell will have two alleles for
any gene. Such is not the case for males since they are XY and will only have one copy of the genes that are
located on the X chromosome.
Females (XX) will have two alleles for a gene located on the X chromosome. Males (XY) will only have one
allele for that trait. Thus, females with one dominant and one recessive allele for a gene on the X chromosome
would display the dominant phenotype. However, a male will always display the phenotype of the one allele
that they have for the trait that is carried on the X chromosome.
A new way of writing alleles:
XR—normal eyesight
Genotypes:
Phenotypes:
XRXR
XRXr
Xr—colorblind
XRY
Normal Eyesight
XrXr
and
XrY
Colorblind
Remember to follow the same steps in order to solve problems:
1.
2.
3.
4.
5.
Assign letters to represent the genes
Determine the parental phenotypes and genotypes.
Predict the outcome of a cross between the possible gametes from the two parents.
State phenotypic and genotypic ratios present in the offspring.
Don’t forget to answer the question within the problem.
Example 1: In fruit flies, the gene for red eyes is dominant over the gene for white eyes. The trait is sex-linked
on the X chromosome. Cross a homozygous, red-eyed female with a white-eyed male. What is the genotypic
and phenotypic ratio of the offspring?
Step 1:
XR— Red Eyes
Xr— White Eyes
Step 2: Parents’ phenotype: Red-eyed female x
Parents’ genotype: XRXR
x
Step 3:
Step 4
R
R
X
X
Xr
XRXr
XRXr
Y
XRY
XRY
Step 5: See the ratios in the table in step 4.
White Eyed Male
XrY
Genotypic
Ratio
Phenotypic Ratio
½ XRXr
½ XRY
½ red eyed female
1/2 red eyed male
Example 2: Using the information above, cross a heterozygous female with a white-eyed male. What is the
probability of getting red-eyed males? (You must include the probability of getting males AND red eyes).
Step 1:
XR— Red Eyes
Step 2:
Red-eyed female
x
XRXr
x
Step 3:
Xr— White Eyes
White-eyed male
XrY
Step 4:
XR
Xr
Xr
XRXr
XrXr
Y
R
r
X Y
Genotypic
Ratio
Phenotypic Ratio
¼ XRXr
r r
1/4 X X
¼ XRY
¼ XrY
¼ red-eyed female
¼ white-eyed female
¼ red-eyed male
¼ white-eyed male
XY
Step 5: One out of four (25% of) offspring will be both male, and have red eyes.
Example 3: Steps 1 and 4 are given to you- check to see if you get the right ratio.
“Bent” is a dominant X-linked gene in mice. It results in a short, crooked tail. It’s a recessive allele that
produces normal tails. If a normal-tailed female is mated to a bent-tailed male, what are the phenotypic and
genotypic ratios for the next generation of baby mice?
Step 1:
XB= Bent
Xb= Normal
Step 2: Parents: _________ x ____________
_________ x_____________
Step 3:
Step 5:
Step 4:
Genotypic
Ratio
Phenotypic
Ratio
½ XBXb
½ XbY
½ Bent tail
female
½ normal tail
male
BIOLOGY: Genetics
Sex-linked Inheritance Practice 1
Name:__________________________
Date_________________Hr________
1. In humans the gene for normal blood clotting is dominant to the gene for hemophilia. This gene is on the X
chromosome. A woman heterozygous for the trait (a carrier) marries a man with hemophilia. What is the probability of
the couple having:
a. a normal (non carrier) daughter? b. a normal son? c. a hemophilic son?
2. Suppose that a normal woman marries a man with hemophilia. What is the probability of the couple having a
hemophilic son, and a hemophilic daughter?
3. A certain form of muscular dystrophy is inherited as a sex-linked, recessive gene. Jack has muscular
dystrophy. (Neither of his parents has this disease.) Jane, Jack’s wife, does not have muscular dystrophy, but her
father does. What fraction of their daughters would you expect to have muscular dystrophy? What fraction of
their sons would you expect to have muscular dystrophy?
BIOLOGY: Genetics
Sex-linked Inheritance Practice 2
Name:__________________________
Date_________________Hr________
1. In humans, the gene for normal blood clotting is dominant to the gene for hemophilia. Persons with
hemophilia do not have the correct blood proteins for the normal clotting of blood. This gene is on the X
chromosome. Suppose that a woman heterozygous for the trait (but with normal blood clotting) marries a man
with normal blood clotting. Determine the following probabilities:
a) The probability of getting a hemophilic child_________
b) The probability of getting a “carrier” (heterozygous) child___________
c) The probability of getting a normal (neither hemophilic nor carrier) child_____
d) The probability of having a son that is normal________
e) The probability of having a son that is hemophilic________
f) The probability of having a son that is a carrier__________
2. Clouded leopards are a medium sized, endangered species of cat, living in the
very wet cloud forests of Central America. Assume that the normal spots, pictured
here, are a dominant, sex-linked trait and that dark spots are recessive. What would
be the genotypic ratio if a Normal male leopard was crossed with a dark spotted
female leopard?
3. Using the information in the question above, suppose a male and female leopard have four cubs and,
conveniently, two are male and two female. One each of the male and female cubs has normal spots and one
each has dark spots. What is the genotype and phenotypes of the leopard parents?
BIOLOGY: Genetics
Sex-linked Inheritance Practice 3
Name:__________________________
Date_________________Hr________
1. The bison herd on Konza Prairie has begun to show a genetic defect. Some of the
males have a condition known as "rabbit hock" in which the knee of the back leg is
malformed slightly. Assume the genes controlling this trait are sex-linked and that
the “rabbit hock” is recessive. Now, suppose that the herd bull is normal and mates
with a cow that is a carrier for rabbit hock. What are the chances of producing a
normal son?
2. If this bull mates with this cow every year, what percentage of their daughters will have normal knees?
3. What percentage of their daughters will be carriers of rabbit hock?
4. A rancher owns a bull with many desirable characteristics. Unfortunately, he also has a sex-linked trait that in
the recessive form leads to no pigment formation in the iris of the eye. This makes the bull very sensitive to
sunlight and could lead to blindness. The rancher wishes to breed him to a cow that will minimize the chances
of any offspring showing this trait. She would especially like to produce another bull with most of his sire's
desirable qualities but without the nonpigmented eye.
a) Two cows with the dominant normal colored eyes (XN) are available that have been genetically typed
for this particular trait. Cow 1 has a genotype of XN XN and cow 2 is XNXn. Which of these two cows
should the rancher choose as a mate to her bull if she wishes to minimize the occurrence of the
nonpigmented eye in his offspring?
b) What percentage of the male offspring from the preferred cross will have nonpigmented eyes?
BIOLOGY: Genetics
Sex-linked Inheritance Practice 4
Name:__________________________
Date_________________Hr________
1. Muscular dystrophy is a group of disorders that involve muscle weakness and loss of muscle tissue that get worse
over time. Muscular dystrophy is expressed in the recessive with the dominant gene being “normal” or without
muscular dystrophy. A normal man who has a brother with M.D. marries a homozygous normal woman. What is the
probability that any of their children will have M.D.?
2. A heterozygous female has children with a male who has M.D. What are the odds that they will have a child
who is both male AND has M.D.?
3. A homozygous female with MD and her husband, also with MD, want to know if their children will be
affected. Write a detailed explanation to them of how sex-linked traits work and what phenotypic ratios they can
expect.
BIOLOGY: Genetics
Sex-linked Inheritance Practice 5
Name:__________________________
Date_________________Hr________
On this sheet you’ll find a higher density of problems for you to practice with. You’ll likely need to use your
own paper to work them out. Write step five below the questions. Don’t forget to use all five steps and to show
your work so you can practice getting all the points on your quiz!
1. In a cross between a white-eyed female fruit fly and red-eyed male, what percent of the female offspring will
have white eyes? (White eyes are X-linked, recessive)
2. In a cross between a pure bred, red-eyed female fruit fly and a white-eyed male, what percent of the male
offspring will have white eyes?
3. A white-eyed female fruit fly is crossed with a red-eyed male. What are the expected genotypes and
phenotypes of the offspring?
4. A human female “carrier” who is heterozygous for the recessive, sex-linked trait causing red-green color
blindness marries a normal male. What proportion of their male children will have red-green color blindness?
5. A human female “carrier” who is heterozygous for the recessive, sex-linked trait red color blindness, marries
a normal male. What are the odds that they would have female offspring that will show the trait?
Try these for some more challenging problems! These are more difficult questions that will test your
overall understanding. You may need to refer to information within problems 1-5 in order to answer
these questions.
6. A female Drosophila of unknown genotype was crossed with a white-eyed male fly. Half of the male and
half of the female offspring were red-eyed, and half of the male and half of the female offspring were whiteeyed. What was the genotype of the female fly?
7. Women have sex chromosomes of XX, and men have sex chromosomes of XY. Which of a man’s
grandparents could not be the source of any of the genes on his Y-chromosome?
8. Which of a woman’s grandparents could not be the source of any of the genes on either of her Xchromosomes?
9. A boy, whose parents and grandparents had normal vision, is color-blind. What are the genotypes for his
mother and his maternal grandparents?