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Sex Linked Diseases

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by

Miss Schwinge

on 31 January 2014

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Transcript of Sex Linked Diseases

Colorblindness
Hemophilia
X Chromosome Inactivation
Sex Linked Diseases
There is a special

pattern of inheritance

for genes located on the
X and Y chromosome
.
Since these chromosomes determine

sex
,
genes

located on them are said to be

sex-linked genes.
The

human genes

associated with

color vision
are located on the

X

chromosome. In
males
,
a

defective version

of any one of these genes
produces colorblindness (an inability to distinguish certain colors).
Hemophilia
is a

sex-linked disease

that makes it so that a person's
blood is unable to clot properly.
This can lead to people

bleeding to death from minor cuts
,
and may result in them suffering

internal bleeding from a simple bump or bruise.
If just
one X

chromosome is

enough

for cells in
males
,
how does the cell "adjust" the

extra X

chromosome in
female

cells?
More than
100 sex-linked genetic disorders

have now been mapped to the
X

chromosome. The human
Y chromosome is much smaller
,

and appears to contain only a

select few genes.
The most

common
form is a

red-green colorblindness

which is found in about

1 in 10 males

in the U.S. Among
females
,
however, colorblindness is

rare (only around 1 female in every 100)
.

Why this difference?
Males have just one X

chromosome. Thus,
all X-linked alleles are expressed in males, even if they are recessive
.

In order for a
recessive allele
(like the one for colorblindness) to be expressed in

females
, there must be

two copies of the allele

(one on each of the two chromosomes).
This means that the
recessive phenotype
of a
sex-linked genetic disorder
tends to be much
more common among males

than females. In addition, because

men pass on

their

X

chromosome to their
daughters
, the woman is called a
"carrier"
because

she herself does not have the disease.
Sex-linked diseases move from fathers --> daughters --> sons of the daughters
The reason this happens is that

two important genes carried on the X
chromosome helps

control blood clotting
,
and a
recessive allele
in

either of the genes
may produce the

disorder.
About
1 in 10,000 males

is born with a form of

hemophilia
In
female

cells,

one X chromosome is randomly switched off
so there is

only one working copy of an X

chromosome. This is true for humans and other mammals.
In
cats
, for example, a

gene

that controls the

color of coat spots

is located on the

X

chromosome.
One X
chromosome may have
an allele for orange spots

and the other may have
an allele for black spots
.
In cells in
some parts of the body, one X

chromosome is
switched off
,
while in other parts of the body

the other X

chromosome is

switched off
.
As a result, the

cat's fur

will have a

mixture of orange and black spots.
Full transcript