Transcript Sex Linked Genetic Conditions

Sex Linked Genetic
Conditions
Biological Science 4
2011
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What decides whether an individual is
male or female?
The male parent’s gametes determine the
sex of the offspring.
This occurs during the production of
secondary spermatocytes- half the receive
an X chromosome while half receive a Y
chromosome.
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The Y chromosome is a
truncated X
chromosome.
Because the Y
chromosome is
truncated, there are
many genes with an
allele on the X
chromosome but no
corresponding allele on
the Y chromosome.
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There are two variants of sex-linked genetic
conditions- either X linked dominant or X
linked recessive.
X linked dominant genes only require one
copy of the allele for expression, whether in
males or females.
X linked recessive genes:
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In males, one allele gives expression
In females, two alleles are required for
expression.
Colour Blindness
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7% of males are colour blind, but only 0.5% of
females.
Colour blindness doesn’t mean that the
affected individual doesn’t see colours at all
(that is Daltonism, and is very rare).
Colour blind individuals cannot distinguish
between either red & green or red & violet (less
common).
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The human eye has two types of
receptors that detect light:
 Rods detect differences in the
intensity of light
 Cones detect colours
 There are 3 types of cones,
which detect red, blue & green
wavelengths of light
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For example,
someone with
red/green colour
blindness will have
problems
distinguishing
between these
colours, which all
have the same
intensity.
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Normal subjects see
an 8
Colour blind subjects
see a 3
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Normal subjects
see a 6
Colour blind
subjects can’t see
any number
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Normal subjects see
a7
Colour blind subjects
can’t see any
number
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Normal subjects see
a 35
Colour blind subjects
see either a 3 or a 5
depending on the
type of colour
blindness
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Colour blind subjects
can see numbers, as
they detect
differences in
intensity
Normal subjects
can’t see any
numbers!
I don’t know what
the numbers actually
are!
What are the possible advantages of colour
blindness?
 Colour blind males may be able to see
camouflaged animals more easily, which
would give them an advantage when
hunting.
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The allele for colour blindness is sometimes
called Xc.
It is a recessive condition.
In carrier females, the normal allele cancels
out the affect of the Xc.
In affected males, there is no normal X to
cancel out the Xc.
What if the father is colour blind?
-all daughters are carriers
-all sons are normal
Parents
X
X
Xc
X Xc
X Xc
Y
XY
XY
What if the mother is a carrier?
-1/2 the daughters are carriers
-1/2 the sons are colour blind
Parents
X
Xc
X
XX
X Xc
Y
XY
XcY
Other X-linked recessive conditions
include:
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Haemophilia
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Lack of clotting factor VIII
Increased bleeding & bruising
Gout
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Metabolic problem leading to increased amounts
of uric acid in the body
This causes joint inflammation & kidney stones
X linked Dominant Conditions
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These are much rarer.
Examples include inherited rickets, a form of
kidney disease and a skin disease called
Incontinentia pigmentia.
The frequency is similar in males and
females. However, the absence of male-tomale transmission distinguishes X linked
dominant diseases from autosomal dominant
diseases, in which both sexes are also
equally affected.
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If the father has
the condition,
then none of his
sons will be
affected, but all
daughters will
be affected.
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If the mother has
the condition,
then all of her
offspring have a
½ chance of
having the
condition.
Queen Victoria’s legacy
Queen Victoria was a carrier for haemophilia. Her
daughters married into royal families throughout
Europe.
Rules for X-linked conditions
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X-linked recessive
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Males have the condition
Females are carriers
If a male has the allele
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If a female has the allele
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All daughters are carriers
All sons are normal
½ daughters are carriers
½ sons have the condition
X-linked dominant
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If a male has the allele
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All daughters have the condition
All sons are normal
If a female has the allele
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½ offspring have the condition (whether sons or daughters)