Download WS sex-linked genetics problems 2011-2012

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Bio 300 – WS Inheritance of Traits on Sex Chromosomes
Being male or female is determined by the inheritance of sex chromosomes. Sex chromosomes are
labeled “X” and “Y.” Girls inherit an X chromosome from both parents. Boys inherit an X
chromosome from their mother and a Y chromosome from their father.
Another way of viewing this would be to use a Punnett square:
Mom’s gametes
gametes
X
X
X
XX
XX
Y
XY
XY
Dad’s gametes
Sample problem: In sex-linked crosses, the sex chromosomes of each parent must be included in
the cross.
Red-green color blindness is a sex-linked trait in humans, located on the X chromosome.
XN = normal color vision
Xn = red-green color blindness
A woman who is heterozygous for this trait (XN Xn) marries
Predict the fraction of their children that will be color blind.
Answer = _________ of the children
a man with normal vision.
X-Linked Trait Problems
1) A color-blind woman and a man with normal vision start a family.
a) What the chance their first child is a boy? How do you know?
b) What percent of their sons would be expected to have normal vision?
c) What percent of their daughters would be expected to be color blind?
2) The gene for hemophilia is a recessive mutation found on the X-chromosome.
a) If a male child who is a hemophiliac is born to two normal parents, what must be the genotypes
of the parents? Explain your answer.
b) If these parents have a female child, what is the probability that she will be a hemophiliac?
c) What is the probability that she will be a carrier?
3) In fruit flies, red eye color (R) is dominant over white eyes (r). The trait is sex-linked and sex in
fruit flies is determined the same way as in humans. What genotypes and phenotypes would be
expected from a cross between a red-eyed male and a heterozygous red-eyed female?
4) In humans, color blindness (n) is recessive to normal vision (N) and is sex-linked. Two parents
with normal vision have a color-blind son.
a) What are the genotypes of the parents?
b) What is the probability that their next child will be a color-blind daughter?
5) A man and his wife both have normal color vision, but a daughter has color blindness. The man
sues his wife for divorce on grounds of infidelity. Can genetics provide evidence supporting the
man’s case?