Complicated genetics: Sex-linked genetic disorders - colour blindness &
haemophilia plus inheritance of blood groups
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school biology revision notes: GCSE biology, IGCSE biology, O level
biology, ~US grades 8, 9 and 10 school science courses or equivalent for ~14-16 year old
students of biology
This page will help you answer questions
such as ... Explain what a sex-linked genetic
disorder is? What is the main difference between an
X and Y chromosomes? What causes colour blindness? How many alleles control your inherited
blood group? Can you draw a genetic diagram to show
how a sex-linked disorder is inherited?
Sub-index for this page
(a)
Introduction to sex-linked genetic disorders
(b)
Colour
blindness - a sex linked genetic disorder
(c)
Haemophilia - a sex linked genetic disorder
(d)
The inheritance of blood groups
(not sex-linked)
(a) Introduction to sex-linked genetic disorders
If you are male, there are certain
genetic disorders you are more likely to suffer from.
This is due to alleles linked to the male X
and Y chromosomes.
A characteristic will be sex-linked if the
allele that codes for it is located on the X or Y sex chromosomes.
The Y sex chromosome is smaller than the X
chromosome and so carries fewer genes.
Therefore most genes on the sex chromosomes
are located on the X chromosome.
Since male men have only one X sex chromosome, it
often only carries one allele for a sex-linked gene.
Since men only have one allele, the
characteristic of this allele is shown even if it is recessive.
Which means that men are more likely
than women to show recessive characteristics for genes that
are sex-linked (on the X chromosome).
Therefore sex-linked genetic disorders are
caused by faulty alleles on the sex chromosomes, and usually due to
faulty alleles on the X chromosome (male or female carriers).
e.g. colour blindness, haemophilia and
muscular dystrophy are all caused by a recessive gene carried on
the male or female X chromosome.
Some genetic sex-linked disorders are due
to faulty alleles on the male Y chromosome too e.g. male
infertility.
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(b) Colour
blindness
A faulty recessive allele on the X chromosome causes
colour blindness.
Colour blindness occurs when you are
unable to see colours in a normal way based on red-green-blue. It
often happens when someone cannot distinguish between certain
colours. This usually happens between greens and reds, and
occasionally blues.
Because colour blindness disorder is
sex-linked, both the chromosome and the allele must included in the
genetic diagram showing the possible offspring genotypes and
phenotypes.
Women need two copies of the recessive allele
to be colour blind, whereas men need only one copy.
A woman with only one copy of the recessive
allele is a carrier - in other words, she is not colour blind
herself but can pass on the recessive allele for colour blindness to
her offspring.
Example 1
of inheriting colour blindness
In the diagram below crossing an unaffected
female carrier of the faulty allele with an unaffected non-carrier
male, superscripts N represents the normal allele and n
the recessive faulty allele.
In this particular cross the ratio of
unaffected to colour blind is 3 : 1 (25% chance of the
offspring being colour blind).
The ratio using a
unaffected non-carrier (male + female) :
unaffected female carrier : colour blind male is 2 : 1 : 1
Genetic Punnett square table for crossing an
unaffected female carrier of the faulty recessive gene for
colour blindness and an unaffected male (non-carrier) |
Parent genotypes cross: XNXn x
XNY |
Gametes: XN, Xn , XN
and Y |
Genotypes of
parents - alleles |
XN |
Xn |
XN |
XNXN |
XNXn |
Y |
XNY |
XnY |
Example 2
of inheriting colour blindness
A woman with one copy of the recessive
allele is a carrier of colour blindness, but not colour blind
herself.
She is still a carrier though, whereas a
man only needs one copy of the faulty allele to be colour blind.
However, a woman needs two copies of the
recessive allele to be colour blind herself, so colour blindness
is less common (less chance) in females.
Genetic Punnett table for crossing an
unaffected female carrier of the faulty recessive gene for
colour blindness and an affected male (carrier) |
Parent genotypes cross: XNXn x
XnY |
Gametes: XN, Xn , XN
and Y |
Genotypes of
parents - alleles |
XN |
Xn |
Xn |
XNXn |
XnXn |
Y |
XNY |
XnY |
Here 3 of 4 chance (75%) of the offspring
children are likely to be carriers of the recessive allele.
There is a 2 in 4 (50%) chance the
offspring will be affected by colour blindness, but of these,
half are likely to be male and the other half female.
i.e. there for there is a 1 in 4 chance
(25%) that the offspring will be male and affected OR the
offspring will be female and affected.
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(c)
Haemophilia
Again, as with colour blindness, haemophilia
is due to a faulty allele on the X chromosome.
Haemophilia a medical condition in which
the ability of the blood to clot is severely reduced, causing
the sufferer to bleed severely from even a slight injury.
The
condition is typically caused by a hereditary lack of a
coagulation factor.
A person who suffers from this disorder is
called a haemophiliac.
Because haemophilia disorder is sex-linked,
both the chromosome and the allele must included in the genetic
diagram showing the possible offspring genotypes and phenotypes.
Example 1
of inheriting haemophilia
In the diagram below crossing an unaffected
female carrier of the faulty allele with an unaffected non-carrier
male, superscripts H represents the normal allele and h
the recessive faulty allele.
From the genetic diagram, in this particular cross the ratio of
unaffected to haemophilic is 3 : 1 (25% chance of the
offspring being a haemophiliac and male).
BUT, the ratio in more details is:
unaffected non-carrier (male + female) :
unaffected female carrier : haemophilic male is 2 : 1 : 1
Genetic Punnett square table for crossing an
unaffected female carrier of the faulty recessive gene for
haemophilia and an unaffected male (non-carrier) |
Parent genotypes cross: XHXh x
XHY |
Gametes: XH, Xh , XH
and Y |
Genotypes of
parents - alleles |
XH |
Xh |
XH |
XHXH |
XHXh |
Y |
XHY |
XhY |
If a boy, there is a 50% chance he will be a
haemophiliac.
Example
2 of inheriting haemophilia
Suppose a woman is unaffected by haemophilia
and not a carrier of the defective allele.
BUT, suppose she has a son by a male
haemophiliac.
What is the chance that their son will be a
haemophiliac?
Genetic Punnett square table for crossing an
unaffected female non-carrier of the faulty recessive gene for
haemophilia and an affected male (carrier) |
Parent genotypes cross: XHXH x
XhY |
Gametes: XH, XH , Xh
and Y |
Genotypes of
parents - alleles |
XH |
XH |
Xh |
XHXh |
XHXh |
Y |
XHY |
XHY |
The answer is zero, their son will have a 100%
chance of not suffering haemophilia or being a carrier.
If the child is a girl, there is a 100% chance
she will be a carrier of the recessive allele.
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(d) The inheritance of blood groups (not
sex-linked)
Generally speaking
(i) most characteristics are determined by
several genes,
(ii) most genes have two possible alleles -
dominant, recessive or codominant with each other.
However, as in the case of blood groups, you
sometimes get more than two alleles for a single gene.
In the case of human blood, two from three
possible alleles control which blood group you inherit.
The three alleles controlling blood type are
denoted by codominant A and B and
recessive O.
Codominant means that when an individual has
both dominant alleles, they have the blood type AB, NOT A or B,
because one cannot dominate the other. Allele O is recessive.
Therefore you can potentially have one of four
blood types: AB (from AB), A (from AA/AO), B (from
BB/BO) and
O from (double recessive OO).
(allele pairings).
Possible genotypes and phenotypes of blood groups (simplified
e.g. there are subgroups of A alleles)
Phenotype
blood group |
Possible genotypes |
A |
AA or AO |
B |
BB or BO |
AB |
AB |
O |
OO |
A worked example of the genetic inheritance of
blood groups
Parent of blood group AO crossed with a parent
blood of group BO.
Genetic Punnett square table for crossing a parent
of blood group A with a parent of blood group B |
Parent genotypes cross: AO x BO |
Gamete alleles:
A, O , B and O |
Genotypes of
parents - alleles |
A |
O |
B |
AB |
BO |
O |
AO |
OO |
Since O is a recessive allele and A and B are
dominant alleles,
this parental cross will give the ratio of the
phenotype blood groups as:
AB : A : A : O or 1 : 1 : 1 : 1, in other
words a 1 in 4 chance (25%) of any of the offspring
having ANY of the four possible blood groups.
-
More examples needed?
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