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(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.

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More examples needed?

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Doc Brown's School Biology Revision Notes

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