Sex Linkage
Recall:
Sex in humans is controlled by two chromosomes X and Y. Males have XY and females have
XX. Mendel looked at the inheritance of many traits. These traits were controlled by things
he called factors (we now know them as genes). As a result of his experiment Mendel
formulated the Law of independent assortment. That is that factors behave independently of
other when gametes are formed.
Background: Males have a single X chromosome that they receive from their mother. The Y
chromosome from their father carries very few genes. One gene it does carry causes
masculinity by activating the testes which results in the production of male hormones.
X chromosomes carry a variety of genes which affect many functions, but not many are
related to sex.
Characteristics determined by genes on the X chromosome are said to be sex-linked. Because
females have two X chromosomes they can be homozygous or heterozygous for genes found
on the X chromosome.
Think – Males are XY. What implication does this have on their genotype regarding sex
linked genes? (i.e why are men more likely to exhibit sex linked diseases?)
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Punnett Squares and Sex Linkage
You can use Punnett Squares to predict the outcomes from a sex linked cross. It is not hard
but you do need to change the way you represent the alleles. For example haemophilia is a
sex linked recessive trait. We can represent a woman who doesn’t have haemophilia as XHXH
or XHXh . A man with haemophilia would be XhY, and a normal male would be XHY. So a
cross between a woman with haemophilia and a normal male would be:
XH
Y
Xh
XH Xh
XhY
Xh
XH Xh
XhY
Problems
1. Can a colour blind female have a son that has normal vision? Colour blindness is caused by
a sex-linked recessive allele.
2. Baldness is a sex-linked recessive trait. What parental genotypes could produce a bald
woman?
3. Muscular dystrophy in humans is not one disease, but a group of related diseases. One
form of the disease is caused by an X-linked recessive gene and is usually strikes in the first
five years of life and pursues a relentless course that may end in death within 20 years.
Diagram the cross between a healthy male and a wife that unsuspectedly a “carrier” of
muscular dystrophy. Use the symbol xm for muscular dystrophy. What are the chances that
they will have a child with muscular dystrophy?
Sex Linkage - Haemophilia
Question Haemophilia is an example of a sex linked disease. What are
the symptoms of Haemophilia?
Punnett Squares and Sex Linkage
A cross between a woman who carries the allele for haemophilia and a normal male would be
(complete the Punnett square)
XH
Xh
XH
Y
You can see from the Punnett square that there is a ….…..% chance of the sons having
haemophilia and ………% chance that her daughters would have the disease. ………% of the
daughters would be termed ………… because they carry the gene for haemophilia but don’t
have the disease.
Consider the pedigree chart for the European Royal Family which shows incidences of
haemophilia:
Problems
1. How are the current Queen of England and her husband the Duke of Edinburgh related?
2. How many Haemophiliacs are there in the European Royal Family?
3. Given the pedigree diagram on the previous page:
a. What were the genotypes for Queen Victoria and Prince Albert?
b. What was the probability that one of their sons would have haemophilia? (Draw a
Punnett square if it helps)
c. What was the probability of them having a haemophiliac female offspring?
4. Do you think it is a good thing that Prince William has married Kate Middleton from a
scientific point of view? Explain.
Now watch this Youtube video!
https://www.youtube.com/watch?v=bmQwMllhCUM