Human Inheritance of a Sex-linked Trait
Standards:
Grade 7 Genetics
2b. Students know sexual reproduction produces offspring that inherit half their
genes from each parent.
2c. Students know an inherited trait can be determined by one or more genes.
By analyzing the pedigree chart of Queen Victoria’s descendants, the
inheritance of a sex-linked trait can be determined. Hemophilia is a genetic
condition in which the blood platelets have great stability so that normal clotting
of blood does not occur. It is caused by a mutation of a gene on the X
chromosome that can be passed on to future generations. Since males have only
one X chromosome, there are no male carriers, only normal or hemophiliac
males. Females, on the other hand, have 2 X chromosomes, so can be
homozygous normal, homozygous hemophiliac or heterozygous, a condition in
which the blood clots normally, but carries a hemophilia gene that can be passed
on to offspring.
Analysis Questions:
1. From the pedigree chart list the mothers who must have been carriers.
2. Queen Victoria's daughter Alice mated with Louis IV and produced
seven children.
a. What percentage of their daughters were carriers?
b. What percentage of their sons had hemophilia?
c. How does this compare with what would be expected for male and
female offspring of a carrier mating with a normal male.
3. Consider the marriage of a hemophilia man and a woman homozygous
for normal blood clotting. What percentage of their male offspring do
you expect to be hemophiliacs?
4. Tatiana, a daughter of Nicholas II and Alexandria, was assassinated at a
young age. What do you expect to be her probability for being a carrier
of hemophilia?
5. What is the probability that Prince Harry is a hemophiliac?
CST- like Question
1. Hemophilia in humans is due to an X-chromosome mutation. What will
be the results of mating between a normal (non-carrier) female and a
hemophilic male?
a. half of daughters are normal and half of sons are hemophilic.
b. all sons are normal and half are hemophilic; all daughters are
carriers.
c. all daughters are normal and all sons are carriers.
d. Half of daughters are hemophilic and half of daughters are carriers;
all sons are normal.
Sex-linked Traits tutorial
http://www.biology.arizona.edu/mendelian_genetics/problem_sets/sex_linked
_inheritance/06t.html