sex-linked diseases
These are that they are passed from parent to child through one of the X or Y chromosomes. X
for males and Y for females. the best known in men are fragile X syndrome, hemophilia A and
Duchenne muscular dystrophy. there are also others that are more common in women, such as
Rett syndrome, which are also due to a mutation on the X chromosome.
The questions on our topic are
In whom is the x chromosome mostly present and what causes it?
Can women have the x chromosome and not have a disease? And because?
Dominant inheritance occurs when an abnormal gene from one parent causes the disease, even
though the matching gene from the other parent is normal. The abnormal gene dominates.
However, in recessive inheritance, both compatible genes must be abnormal to cause the disease.
If only one gene of the pair is abnormal, the disease is absent or mild. Someone who has an
abnormal gene but no symptoms is called a carrier. Carriers can pass the abnormal genes on to
their children.
The term "sex-linked recessive" often refers to X-linked recessive.
Information
X-linked recessive diseases almost always occur in males. They only have one X chromosome.
A single recessive gene on that X chromosome will cause the disease.
The Y chromosome is the other half of the XY gene pair in males. However, the Y chromosome
does not contain most of the genes on the X chromosome.
Because of this, it does not protect the male. Diseases such as hemophilia and Duchenne
muscular dystrophy occur as a result of a recessive gene on the X chromosome.
TYPICAL SCENARIOS
In each pregnancy, if the mother is a carrier of a certain disease (she has only one abnormal X
chromosome) and the father is not a carrier of the disease, the expected outcome is:
● 25% chance of a healthy male
● 25% chance of a male with the disease
● 25% chance of a healthy girl
● 25% chance of a carrier girl without the disease
If the father has the disease and the mother is not a carrier, the expected results are:
● 50% chance of having a healthy male
● 50% chance of having a girl without the disease, who is a carrier
This means that none of the children would actually show signs of the disease, but the trait can
be passed on to grandchildren.
X-LINKED RECESSIVE DISORDERS IN FEMALES
Females can get an X-linked recessive disorder, but this is very rare. An abnormal gene on the X
chromosome from each parent would be required, since a female has two X chromosomes. This
could happen in the following two scenarios:
In each pregnancy, if the mother is a carrier of the gene and the father has the disease, the
expected outcomes are:
●
●
●
●
25% chance of a healthy male
25% chance of a male with the disease
25% chance of a carrier girl
25% chance of a girl with the disease
If both the father and the mother have the disease, the expected results are:
● 100% probability that the child will have the disease, be it a boy or a girl
The chances of either of these two scenarios are so low that X-linked recessive diseases are
sometimes called male-only diseases. However, this is not technically correct.
Female carriers may have a normal X chromosome that is abnormally inactivated. This is called
"biased X chromosome inactivation". These women may have similar symptoms to men or they
may have only mild symptoms.
In conclusion, sex-linked diseases are inherited through one of the sex chromosomes. Dominant
inheritance occurs when an abnormal gene from one parent can cause the disease, although a
compatible gene from the other father is normal.
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