Download Sex-Linked Problem Set

Sex-Linked Problem Set
Name____Key____________________________________
Red genotypes indicate affected; yellow boxes indicate carriers.
1. If a normal vision female marries a colorblind male and have children. What are the
chances that any of the children will be colorblind? Show the work!!!!
It depends on whether she is homozygous or heterozygous. If she is homozygous, none of
the children will be colorblind (girls will be carriers).
Xc
Y
XC
XCXc
XCY
XC
XCXc
XCY
If she is heterozygous, her children have a 50% chance of being colorblind (boys and
girls).
Xc
Y
XC
XCXc
XCY
Xc
XcXc
XcY
2. If a colorblind female marries a normal vision male, what are the chances that their
children will be colorblind? Show the work!!!! All the sons will be colorblind.
XC
Y
Xc
XCXc
XcY
Xc
XCXc
XcY
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? If so, how?
A colorblind daughter must have two recessive genes, one from the mother and one
from the father. But the man is not colorblind, so he does not have the recessive
gene to pass on to his daughter. Someone else must be the father.
4. A normal sighted woman whose father has red-green colorblindness married a man with
normal vision. What are the expected results of their children? Show the work!!!!
The woman must have inherited the recessive gene from her father. But since she is not
colorblind, she must be heterozygous. Her husband must have one good gene (otherwise
he would be colorblind). Daughters have a 50% chance of being carriers, and sons have a
50% chance of being colorblind.
XC
Y
XC
XCXC
XCY
Xc
XCXc
XcY
5. Why is it impossible for a man to pass on a sex-linked gene to his son? A man passes an
X chromosome to a daughter and a Y chromosome to a son. If the gene is on the X
chromosome, he cannot pass it to a son.
6. What would be the chances if a hemophiliac man marries a carrier woman and they have
children that have hemophilia? 50% for both sons and daughters.
Xh
Y
XH
XHXh
XHY
Xh
XhXh
XhY
7. Queen Victoria has a hemophiliac son, Leopold, Duke of Albany, and three normal sons.
Her husband, Prince Albert was not a hemophiliac.
 What was the Queen’s genotype with respect to this gene?
 Could she have any hemophiliac daughters?
Since the queen was not hemophiliac but had a hemophiliac son, she must have been a
carrier (there was no previous family history of the disease, so it is thought that she had a
new mutation). Her genotype would be XHXh. Her husband must have had a normal gene
since he was not a hemophiliac. Therefore she could not have any hemophiliac daughters
(they would have a 50% chance of being carriers).
XH
Y
XH
XHXH
XHY
Xh
XHXh
XhY
8. Hemophilia is a sex-linked genetic disease that has plagued the royal houses of Europe
since the time of Queen Victoria, who was a carrier. Her Granddaughter, Alexandria
married Nicholas II , the last tsar of Imperial Russia. Nicholas was normal. Their son,
Alexis, was afflicted with the disease. Alexis and his four sisters are all thought to have
been killed at the outbreak of the Revolution of 1917. Based on probability, what
conclusions can be made regarding the children of Alexandria and Nicholas II?
The situation is the same as in the previous question. Alexandria was a carrier,
Nicholas was normal. Their daughters each had a 50% chance of being carriers.
For a detailed pedigree of Victoria’s descendants and a discussion of hemophilia in
the royal families of Europe, see:
http://www.sciencecases.org/hemo/hemo.asp
9. A woman carries the gene for hemophilia and her husband is normal. Assuming that the
woman does not have hemophilia (is a carrier): the situation is the same as in question 7.
A. Mother's genotype: ____ XHXh _____________
B. Father's genotype: ____ XHY____________
C. What are the chances of them having a normal daughter?__100% of daughters (50% of all children)
if “normal” means not hemophiliac; 50% of daughters (25% of all children) if normal means two normal
genes.________________
D. What are the chances of them having a daughter that is a carrier? _____50% of daughters (25% of
all children)____
E. What are the chances of them having an affected daughter?_____0%____________
F. What are the chances of their son being normal? ____50%__________
G. What are the chances of their son being affected? ___50%___________
10. A hemophiliac woman marries a normal man and all of their children are normal. Is this
possible? Why or why not? Since she is a hemophiliac, she must be homozygous for the
recessive allele. All their daughters will be carriers and all their sons will have
hemophilia. Assuming “normal” means not hemophiliac, it is possible for all their
children to be normal if they only have girls.
XH
Y
Xh
XHXh
XhY
Xh
XHXh
XhY
11. If a man has muscular dystrophy, what conclusions can be made about his mother and any of
the man’s children?
The man must have an X chromosome with the recessive allele. Since a man inherits
his X chromosome from his mother, his mother must have the recessive allele, which
means she was either a carrier or had muscular dystrophy.
The man must pass the gene to all of his daughters, so they will be carriers (or could
have muscular dystrophy if their mother also carries the recessive allele).
The man will not pass the gene to his sons.