Sex-linked genes
Definition
Sex-linked inheritance
The inheritance of a characteristic controlled by a gene on a sex chromosome, usually the X chromosome.
Definition
Sex chromosomes
The pair of chromosomes that determine the sex of an individual, XX in a human female and XY in a human male.
- A sex-linked gene is carried on a sex chromosome. At this level, sex-linked inheritance usually means a gene on the X chromosome.
- Human females have XXXXXX sex chromosomes, while human males have XYXYXY. The Y chromosome does not usually carry a matching allele for an X-linked gene.
- A male therefore has only one allele for most X-linked genes. If that allele is recessive and causes a disorder, it is expressed because there is no second allele on the Y chromosome to mask it.
- A female usually needs two copies of a recessive X-linked allele to have the disorder. A heterozygous female has one normal allele and one faulty allele, so she is usually an unaffected carrier.
Genotypes and inheritance
Definition
Carrier
A person who is heterozygous for a recessive disorder, so they do not have the disorder themselves but can pass the faulty allele to their children.
- Write X-linked alleles as superscripts on the X chromosome. For example, XNX^NXN can represent the normal allele and XnX^nXn the recessive disorder allele.
- A carrier female is XNXnX^N X^nXNXn, an affected female is XnXnX^n X^nXnXn, an unaffected male is XNYX^N YXNY and an affected male is XnYX^n YXnY.
- A father passes his X chromosome to every daughter and his Y chromosome to every son. He therefore cannot pass an X-linked allele directly to a son.
Common Mistake
Keep each allele attached to its X chromosome. Writing bare NNN and nnn loses the information about both the allele and the sex of the offspring.
Example
Worked cross: carrier mother and unaffected father
- Parents: XNXn×XNYX^N X^n \times X^N YXNXn×XNY.
- Gametes: the mother produces XNX^NXN or XnX^nXn eggs; the father produces XNX^NXN or YYY sperm.
- Possible daughters: XNXNX^N X^NXNXN unaffected and XNXnX^N X^nXNXn unaffected carrier.
- Possible sons: XNYX^N YXNY unaffected and XnYX^n YXnY affected.
- Across all children, the probability of an affected child is 14=25%\dfrac{1}{4}=25\%41=25%. Among sons only, the probability of being affected is 12=50%\dfrac{1}{2}=50\%21=50%.
- No daughter from this cross is affected because every daughter receives the father's normal XNX^NXN chromosome.
Exam technique
- Begin with the parents' full sex-linked genotypes, including the Y chromosome.
- List the gametes before filling the Punnett square, then identify daughters and sons separately.
- State clearly whether a probability refers to all children or only to sons or daughters.
Self review
- Why are recessive X-linked disorders more common in males?
- Write the genotype of a carrier female and an affected male.
- Why can a father not pass an X-linked allele to his son?
- For XNXn×XNYX^N X^n \times X^N YXNXn×XNY, what fraction of all children is expected to be affected?
