Topic 17.4 · Supplement
Sex-Linked Inheritance
Sex determination and sex linkage sound similar and get confused for exactly that reason, but they answer different questions. Sex determination asks whether a zygote is XX or XY. Sex linkage asks where the gene for some other characteristic sits — and specifically, what it means for that gene to sit on a sex chromosome rather than an ordinary one.
What makes a characteristic sex-linked
A sex-linked characteristic is one in which the gene responsible is located on a sex chromosome. Because males and females don’t carry the same number of copies of a sex chromosome, that location alone can make the characteristic noticeably more common in one sex than the other. Cambridge’s required example is red-green colour blindness, with the relevant gene carried on the X chromosome.
A standard notation for this gene uses XN for the dominant allele giving normal colour vision, and Xn for the recessive allele giving red-green colour blindness. The Y chromosome is written simply as Y, since the gene being followed sits on the X, not the Y.
Why the recessive phenotype is more common in males
A typical female carries two X chromosomes; a typical male carries one X and one Y. That difference in copy number is the entire explanation. A male with genotype XnY has only one copy of this gene — there is no second X chromosome carrying a possible dominant allele to mask it — so a single recessive allele is enough for the colour-blind phenotype to be expressed.
A female with genotype XNXn also carries the recessive allele, but her second X chromosome carries the dominant allele, so she does not show the recessive phenotype. She is described as a carrier: unaffected herself, but able to pass the recessive allele on to her offspring. A female would only show the colour-blind phenotype if she carried the recessive allele on both X chromosomes, XnXn — which needs two copies of a comparatively rare allele rather than one, and is correspondingly less likely. That asymmetry in how many recessive copies each sex needs is the whole reason an X-linked recessive characteristic such as red-green colour blindness turns up more often in males than in females.
A worked sex-linked cross
Cross a carrier female, XNXn, with a male who has normal colour vision, XNY. The female’s gametes carry XN or Xn; the male’s gametes carry XN or Y.
| XN | Xn | |
|---|---|---|
| XN | XNXN | XNXn |
| Y | XNY | XnY |
The four possible offspring are: a female with normal colour vision and no recessive allele (XNXN); a carrier female with normal colour vision (XNXn); a male with normal colour vision (XNY); and a colour-blind male (XnY). Each of the four outcomes has an expected probability of 1/4 across all offspring. Looking at sons specifically, the expected ratio is 1 normal : 1 colour-blind — a narrower, more specific probability than “1/4 of all offspring,” and exam questions often ask for exactly that narrower figure.
This is why a sex-linked genetic diagram has to keep the allele attached to its X chromosome throughout — writing XN and Xn rather than a bare N and n — instead of treating the allele as an ordinary, unlinked characteristic the way monohybrid inheritance does. Losing the X notation loses the entire biological point of the question: that this characteristic’s inheritance depends on sex in the first place.
Don’t confuse the two ideas
Sex determination — whether a zygote ends up XX or XY — is covered on the chromosomes, genes and proteins page, and it is a separate question from sex linkage. Sex linkage is about where the gene for some other characteristic happens to be located, and what consequence that location has for inheritance. A cross can involve both ideas at once — as the worked example above does — but they are answering different questions, and an answer that only addresses one when the question asks about the other will lose marks.