Biology

3.13 - Sex-Linked Inheritance

4 min

note 1 of 2

What a sex-linked characteristic is

A sex-linked characteristic is a feature whose gene is on a sex chromosome, not on an ordinary (autosomal) chromosome.
In practice this almost always means the X chromosome.
Because the gene sits on a sex chromosome, the characteristic turns up more often in one sex than the other.

Why the two sex chromosomes matter

The Y chromosome is much smaller than the X chromosome and carries far fewer genes.
Most genes on the sex chromosomes are found only on the X, with no matching version on the Y.
Males (XY) - Have one X and one Y, so for a gene found only on the X they have just a single allele. That allele is expressed even if it is recessive, because there is no second X carrying a dominant allele to mask it.
Females (XX) - Have two X chromosomes, so they have two alleles for that gene. A recessive allele on one X can be masked by a dominant allele on the other.

Why males are affected more often

A male needs only one copy of the faulty recessive allele to be affected, because his single X has no partner to mask it.
A female needs two copies, one on each X, to be affected.
A female who carries a single copy of the recessive allele is not affected herself, because the dominant allele on her other X masks it - but she can still pass the faulty allele on to her children.
Such a female is called a carrier.

Three panels: a carrier female, one X carrying the normal dominant allele; an affected female with 2 copies of faulty recessive allele; an affected male, one X plus a shorter Y.

note 2 of 2

Following a sex-linked cross

A sex-linked condition caused by a recessive allele on the X chromosome is followed with a genetic diagram (Punnett square) just like any other monohybrid cross.
The difference is that the allele is written as a superscript on the X chromosome, so the sex chromosome and the allele it carries are shown together:
- X^N - The dominant (normal) allele
- Xⁿ - The recessive (faulty) allele
- Y - Carries no allele for this gene, so it's written as a plain Y

Example - Carrier mother × unaffected father

Mother is unaffected but carries one copy of the recessive allele - genotype X^N Xⁿ.
Father is unaffected - genotype X^N Y.
The mother's eggs each carry X^N or Xⁿ; the father's sperm each carry X^N or Y.

Punnett square with the carrier mother's X^N and X^n across the top and the unaffected father's X^N and Y down the side, cells labelled unaffected daughter, carrier daughter, unaffected son and affected son.

Reading off the results

Daughters - X^N X^N (unaffected) and X^N Xⁿ (unaffected carrier).
Sons - X^N Y (unaffected) and Xⁿ Y (affected - his single X carries the recessive allele and there's no second X to mask it).
Three of the four children are unaffected and one is affected - a ratio of 3 unaffected : 1 affected - and the only affected child is a son.
Chance of any child being affected = 1 in 4 (25%), rising to 1 in 2 (50%) once you know the child is a boy.
No daughter is affected here, because every daughter inherits the father's normal X^N, which masks any recessive allele she also inherits.

key terms

TermDefinition

Sex-linked characteristicA feature whose gene is located on a sex chromosome rather than an ordinary chromosome.

CarrierA person who has one copy of a recessive allele. They do not show the characteristic it causes, but can pass the allele on to their children.

end of notes

Key points to remember

What a sex-linked characteristic is

A sex-linked characteristic is a feature whose gene is on a sex chromosome, not on an ordinary (autosomal) chromosome.

In practice this almost always means the X chromosome.

Because the gene sits on a sex chromosome, the characteristic turns up more often in one sex than the other.

Why the two sex chromosomes matter

The Y chromosome is much smaller than the X chromosome and carries far fewer genes.

Most genes on the sex chromosomes are found only on the X, with no matching version on the Y.

Males (XY) - Have one X and one Y, so for a gene found only on the X they have just a single allele. That allele is expressed even if it is recessive, because there is no second X carrying a dominant allele to mask it.

Females (XX) - Have two X chromosomes, so they have two alleles for that gene. A recessive allele on one X can be masked by a dominant allele on the other.

Why males are affected more often

A male needs only one copy of the faulty recessive allele to be affected, because his single X has no partner to mask it.

A female needs two copies, one on each X, to be affected.

A female who carries a single copy of the recessive allele is not affected herself, because the dominant allele on her other X masks it - but she can still pass the faulty allele on to her children.

Such a female is called a carrier.

Following a sex-linked cross

A sex-linked condition caused by a recessive allele on the X chromosome is followed with a genetic diagram (Punnett square) just like any other monohybrid cross.

The difference is that the allele is written as a superscript on the X chromosome, so the sex chromosome and the allele it carries are shown together:

X^N - The dominant (normal) allele
Xⁿ - The recessive (faulty) allele
Y - Carries no allele for this gene, so it's written as a plain Y

Example - Carrier mother × unaffected father

Mother is unaffected but carries one copy of the recessive allele - genotype X^N Xⁿ.

Father is unaffected - genotype X^N Y.

The mother's eggs each carry X^N or Xⁿ; the father's sperm each carry X^N or Y.