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WLHS / Biology / Monson / Unit 6 – Genetics
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GENETICS PROBLEM SET #4: SEX-LINKED TRAITS
In animals and in some plants, one pair of chromosomes is different in the two sexes. For example, in
humans there are 22 similar pairs of chromosomes (autosomes) and the 23rd pair may be different (sex
chromosomes). The sex chromosomes are similar in females and both are referred to as the "X"
chromosomes. In the male, the sex chromosomes are unlike in shape; one resembles the "X"
chromosome of the female and is called the "X" chromosome. The other male chromosome is much shorter
and is called the "Y" chromosome. Many alleles are located in the "X" chromosome but very few are known to be on the "Y"
chromosome. Genes located on the sex chromosomes are said to be SEX-LINKED.
A common sex-linked human characteristic is red-green color deficiency (“color blindness”). Red-green
color deficiency is dependent upon a recessive allele we'll represent with "d;" normal vision is controlled
by a dominant allele and will be represented with "D." Examine the table below to determine the sex and
phenotype of each genotype. Use this key to answer questions 1-4.
GENOTYPE
XDXD
XDXd
XdXd
X DY
XdY
SEX
PHENOTYPE
Listed below are the genotypes of several sets of human parents. Indicate the genotypes of the possible gametes
formed by each parent and the possible genotypes of the children that would result if those parents reproduced.
GENOTYPES OF:
FATHERS
MOTHERS
X DY
XDXD
X DY
XDXd
X DY
XdXd
X dY
XDXd
X dY
XdXd
POSSIBLE GAMETES:
FATHERS
XD
Y
50%
50%
GENOTYPES OF CHILDREN:
MOTHERS
XD
XD
50%
50%
SONS
only XD Y
DAUGHTERS
only XDXD
Use the table above as a reference to answer questions #1-3:
1) If the father is XDY and the mother is XDXD, what are the possible PHENOTYPES of the
children?
2) If the father is XDY and the mother is XDXd, what are the possible PHENOTYPES of the children?
3) If the father is XDY and the mother is XdXd, what are the possible genotypes and phenotypes of the
children?
4) Hemophilia A is caused by a sex-linked recessive allele in dogs (Xh).
A) Cross a hemophiliac male dog with a homozygous non-hemophiliac female dog. Give genotypes and
phenotypes of the possible offspring.
Male parent genotype:
Female parent genotype:
Genotype ratio in offspring:
Phenotype ratio in offspring:
B) If a female produced in the cross above (A) is mated with a normal male dog, what are the genotypes
and phenotypes of the possible offspring?
Male parent genotype:
Female parent genotype:
Genotype ratio in offspring:
Phenotype ratio in offspring:
C) Why are males more likely than females to express a sex-linked trait?
5) In humans, hemophilia is also a sex-linked trait.
The blood in hemophiliacs does not clot properly.
A) If a man has hemophilia, what MUST be his genotype?
B) What is the chance that he will pass the hemophilia allele on to his children?
C) From which of HIS parents did he get the hemophilia allele? Explain.
6) A man with hemophilia marries a woman who is a “carrier” of the hemophilia allele but has normal
blood clotting.
A) What is the genotype of each?
MAN:
WOMAN:
B) Do the cross using the Punnett Square.
C) What percentage of their sons
would you expect to have hemophilia?
D) What percentage of their daughters
would you expect to have the disease?
E) What percentage of their daughters
would be “carriers” of the hemophilia allele?
7) An X-linked recessive gene (d) produces red-green colorblindness in humans.
A) A woman with normal vision, who’s father was colorblind,
marries a color blind man. What percentage of their children
will be colorblind? (Do the cross to find out)
 Answer:
B) A man with normal vision, who’s father was colorblind,
marries a woman with normal vision who is a carrier for
the color-blind allele. What percentage of their children will
be colorblind? (Do the cross to find out)
 Answer:
8) In cats, one gene for coat color is X-linked. Male cats are either black or orange, depending on which
allele they carry. Females are black, orange, or tortoise-shell (patches of black and orange), with the
tortoise-shell phenotype resulting from the heterozygous genotype. Two cats mate and have kittens. Of
the female kittens, ½ are tortoise-shell and ½ are orange. Of the male kittens, ½ are orange and ½ are
black. What are the genotypes and phenotypes of the parent cats? Support your answer by completing
the Punnett Square:
Male parent genotype:
Male parent phenotype:
Female parent genotype:
Female parent phenotype: