3.18B Explain how sex-linked genetic disorders are inherited

The idea

Sex-linked disorders are caused by alleles carried on a sex chromosome, usually the X chromosome. The X chromosome is larger and carries many genes the small Y chromosome lacks, so a recessive allele on the X can cause a disorder in males who have only one X. Females have two X chromosomes, so a recessive sex-linked disorder shows only when both X chromosomes carry the allele; a female with one affected and one normal allele is a carrier and is usually unaffected. Inheritance is traced through genetic crosses. A carrier mother (XᴿXʳ) and unaffected father (XᴿY) can produce carrier daughters, unaffected daughters, unaffected sons and affected sons (XʳY). An affected father cannot pass the allele to his sons because he gives them his Y, but all his daughters receive his affected X.

What earns the marks

• States that the allele causing the disorder is carried on a sex chromosome, typically the X chromosome.
• Explains that males have one X and one Y, so a single recessive allele on the X causes the disorder in males.
• Explains that females have two X chromosomes, so a recessive disorder appears only when both carry the allele, and one affected allele makes a carrier.
• Uses a genetic cross with X and Y chromosomes to show how the disorder passes from parents to offspring.
• Explains that an affected father passes his affected X to daughters but his Y to sons, so sons are not affected by his X allele.

Common mistakes and how to put them right

• Assuming sex-linked alleles are carried on the Y chromosome.

  Put it right by explaining most sex-linked disorders involve genes on the X chromosome.

• Thinking a carrier female shows the disorder.

  Put it right by explaining she has one normal dominant allele so is usually unaffected but can pass the allele on.

• Believing an affected father passes the disorder to his sons.

  Put it right by showing he gives sons his Y chromosome, not his affected X.

Exam tips

• Write genotypes using X and Y superscripts so it is clear which chromosome carries the allele.
• Show the gametes each parent produces before combining them in a cross.
• Distinguish clearly between affected, unaffected and carrier individuals in the offspring.