Name__________________KEY____________________Date____________Period___
Inheritance of Sex-linked Traits
1. Explain what a sex-linked trait is.
A trait for a gene on one of the sex chromosomes (X or Y) –most commonly
the X.
2. The X and Y chromosome pair in males is the only non-homologous pair
of chromosomes. Explain what this means.
The X and Y pair in males is not the same length and does not have the
genes for the same traits.
3. Red-green colorblindness is caused by a recessive sex-linked allele (Xb). The
allele for normal red/green color vision is dominant (XB). Determine the
genotypes for the following individuals. (Hint: remember that men are XY
and women have 2 X chromosomes.)
What is the genotype of a colorblind man? ____XbY___________
What is the genotype of a man with normal color vision? ____XBY_______
What is the genotype of a colorblind woman? _______ Xb Xb_______
What is the genotype of a woman who is a carrier? ____ XB Xb _________
4. Fill out the Punnett square below for the offspring of a colorblind mother
and a father with normal color vision have children.
Genotype of mother _______ Xb Xb ______ Possible Gametes _ Xb _, _ Xb
Genotype of father _____ XB Y__ Possible Gametes ___ XB __, __ Y___
Egg
X
Xb
B
X X
X X
Y
Xb Y
Xb Y
Sperm
Sperm
Xb
b
B
b
B
Probability of a color blind daughter ___O%___ (out of all children)
Probability of a daughter with normal color vision __50%____
5. Explain
traits are more
Probability
of awhy
colorsex-linked
blind son __50%____
common
in males
than females.
Probability
of a son
with normal
color vision __0%_____
6. In order for a son to be colorblind, which parent(s) have to possess a
colorblindness allele? Does either parent have to be colorblind? Explain.
Mother has to be at least a carrier. Doesn’t matter what father’s
phenotype is for trait, because will pass down the Y to any sons.
7. In order for a daughter to be colorblind, which parent(s) have to possess a
colorblindness allele? Does either parent have to be colorblind? Explain.
Father must be colorblind- passes down his only X to all daughters.
Mother must be at least a carrier.
8. Hemophilia is also caused by a recessive sex-linked allele (Xh). Fill out the
Punnett square below for a mother who is a carrier of the hemophilia
allele, and the father does not have hemophilia.
Genotype of mother _______XHXh_______ Possible Gametes XH, Xh
Genotype of father ________XHY________ Possible Gametes XH, Y
Fill out the Punnett square to determine the probabilities of the different
phenotypes in their offspring?
Egg
Sperm
Sperm
XH
Xh
XH XHXH XHXh
Y
XHY
X hY
Probability of a hemophiliac daughter __O%____ (out of all children)
Probability of a daughter with the normal phenotype_50%__
Probability of a hemophiliac son _25%_
Probability of a son with the normal phenotype 25%
9. If a father is a hemophiliac and a mother is homozygous dominant for the
trait (does not have the disease and is not a carrier), what are the
probabilities of the different outcomes for their offspring.
XH
Y
X X
H
Sperm
X
h
XH
h
XHY
X X
H
h
XHY
Probability of a hemophiliac daughter __0%____ (out of all children)
Probability of a daughter with the normal phenotype__50%____
Probability of a hemophiliac son ___0%___
Probability of a son with the normal phenotype ___50%____
10. If a trait is sex-linked (on the X chromosome), fathers pass the sex-linked
allele to
a. all of their daughters and none of their sons
b. all of their sons and none of their daughters
c. half of their daughters and half of their sons
d. half of their daughters and none of their sons
e. half of their sons and none of their daughters
11. Two parents have normal color vision, and have a color-blind son. What
are the genotypes of the parents?
A. XBXB and XbY D. XbXb and XBY
B. XBXB and XbY
E. Not enough information to determine
B b
B
C. X X and X Y
Colorblind son must have inherited the allele from his mother so she must
have the colorblindness allele Xb but she is not colorblind so she must also
have the dominant, so mother must be XBXb. Dad is not colorblind so must
be XBY.
12. Genes found on the Y chromosome are called Y-linked. One example of a
Y-linked trait is the hairy ear allele. There are very few Y-linked traits
compared to X-linked traits and unlike some genes on the X chromosome,
none of the Y-linked genes result in non-gender related genetic conditions
(such as red-green color-blindness on the X chromosome). Explain why.
Females do not have a Y chromosome, so it can not contain any critical
genes. Whereas, the X chromosome has some genes that are not related to
gender since both males and females have at least one X chromosome.
13. A man’s Y chromosome can come from which grandparents?
a. mother’s father
b. father’s father
c. father’s mother
D. A and B
E. A, B and C
14. A woman’s X chromosome can come from which parents?
a. mother’s mother
D. A and B
b. father’s mother
E. A, B and C
c. father’s father
15. Recently, geneticists were trying to determine whether Thomas Jefferson
was the father of the children of his slave Sally Hemings. In order to
determine the paternity over 200 years later, they compared the genetic
sequence of the Y chromosome in male descendents of Sally Hemings’
children, following a male lineage (following sons through each generation)
with male descendents that were known to be Thomas Jefferson’s children
(again following a male lineage). Explain why the Y chromosome is useful
for studying heredity after many generations. Why would the X
chromosome or any of the autosomal chromosomes not be possible to
compare in this same way?
For all other chromosomes, one of each pair is passed down and you can
not predict which chromosome will be inherited (including the X for
females since they inherit one of either mother’s X chromosomes.) For
males, must have inherited the Y chromosome from each parent.