17.9 - Sex-Linked Inheritance

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

• Red-green colour blindness is a sex-linked condition caused by a recessive allele carried on the X chromosome.
• Because it's sex-linked, the chromosome and the allele are written together:
  • X^N - The normal colour-vision allele
  • Xⁿ - The faulty colour-blindness allele
  • Y - The Y chromosome carries no allele for colour vision, so it's written as a plain Y

Cross - Carrier mother × unaffected father

• Mother - Normal colour vision but carries one faulty allele, so her genotype is X^N Xⁿ.
• Father - Normal colour vision, so his genotype is X^N Y.
• The mother's gametes each carry X^N or Xⁿ; the father's each carry X^N or Y.

Reading the results

• Daughters - X^N X^N (normal vision) and X^N Xⁿ (a carrier with normal vision).
• Sons - X^N Y (normal vision) and Xⁿ Y (colour blind - his single X carries the faulty allele with no second X to mask it).
• Ratio is 3 unaffected : 1 colour blind, or 2 : 1 : 1 for unaffected : carrier : colour blind, since the two unaffected daughters differ in carrier status.
• Chance of any child being colour blind is 1 in 4 (25%), rising to 1 in 2 (50%) once you know the child is a boy.
• No daughter is colour blind in this cross, because each inherits the father's normal X^N.

This is why colour blindness is far rarer in females than in males: a female needs two copies of the faulty allele to be colour blind, whereas a male needs only one.