In humans, red-green color blindness is sex-linked and is recessive (c by zbk75252



X& Y chromosome

Humans have 46 chromosomes/ cell (23 pairs)

Genes on the 23rd or Sex chromosome pair work differently from those on
autosomal chromosomes (the non-sex chromosomes)

An X chromosome has many genes; a Y chromosome (shorter) has just a few.

Many of the genes on the X chromosome do not have counterparts on the Y

           Humans with 1 X and 1 Y are male (XY)

          Humans with 2 X chromosomes are female (XX)

Females have two copies of X chromosome genes (XA XA)

Males have only 1 copy of X chromosome genes (XA Y)

Traits due to genes on X-chromosome are called X-linked

These traits will be inherited differently for males vs. females.

Sex-linked traits

       In principle, can have X-linked (only on X) and Y-linked (only on Y) genes

       X-linked most common (since there are more genes on the X)

       Color blindness , hemophilia, Duchenne muscular dystrophy: all X-linked
                      Sex-Linked Genetics Problems
Directions: You may use this sheet for scratch paper, but please write all answers and
show all your work (when applicable) on a separate sheet of paper. Any problems not
answered in class will be finished for homework and due tomorrow in class.

1. Color blindness is a sex-linked recessive characteristic carried on the X chromosome.
(a)What is the probability that a normal male and a heterozygous (“carrier”) female will
have a color-blind daughter? (b) If they have a son, what is the probability that the son
will be color-blind?

2.(a) If a normal male marries a female that is heterozygous (a carrier) for color
blindness, what is the probability that they will have a color-blind child? (b) If this couple
had four girls, how many of these girls would you predict (based on probability) to pass
the color blindness gene to their offspring?

3. A man whose mother is color-blind married a woman with normal vision, but her
father was color-blind. They have four sons and four daughters. (a) What is the genotype
of husband and wife? (b) What are the probable genotypes and phenotypes of the

4. (a) Why are men never heterozygous for an X-linked trait? (b) Why must
men always inherit an X-linked trait from their mothers? (c) Can a color-blind father
pass this allele on to his sons? Explain. (d) Can a normal male ever have a daughter that
is color-blind? Explain.

5. The gene for white eye color in fruit flies is sex-linked and recessive to normal red eye
color. Give the results of mating a heterozygous red-eyed female with a white-eyed male.

6. Hemophilia is a sex-linked recessive condition in which individuals bleed excessively
(sometimes to death) from even the most minor cuts. This is due to the fact that
hemophiliacs possess a gene that codes for a faulty protein involved in blood clotting and
thus their blood fails to clot properly when they get a cut.
(a) If a “normal” (unaffected) daughter of a man with hemophilia marries a man with
hemophilia, what is the probability that a daughter will be a hemophiliac? (b) What is
the probability that a son will be a hemophiliac? (c) If the couple has four sons, what is
the probability that all four will be born with hemophilia?

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