In sex-linked inheritance the gene sits on the X chromosome, so you write the cross with X and Y. Males have only one X, so a recessive allele on it shows up more easily in males.
This lesson follows completing monohybrid crosses. The Punnett square works the same way, but now you write the alleles on the chromosomes.
How does the notation change?
Write the allele as a superscript on the X chromosome. The Y chromosome carries no allele for this gene.
- Female genotypes: XᴿXᴿ, XᴿXʳ, XʳXʳ
- Male genotypes: XᴿY, XʳY
A female with XᴿXʳ is a carrier: she has the recessive allele but does not show the trait.
Worked example with an invented insect
An invented insect has red eyes (Xᴿ, dominant) or white eyes (Xʳ, recessive). The gene is on the X chromosome.
Question: A carrier female (XᴿXʳ) is crossed with a red-eyed male (XᴿY). Find the expected offspring.
- Parents: XᴿXʳ and XᴿY.
- Gametes: the female makes Xᴿ or Xʳ. The male makes Xᴿ or Y.
- Offspring: XᴿXᴿ, XᴿXʳ, XᴿY, XʳY.
- Ratio: females all red-eyed (half carriers); males half red-eyed and half white-eyed.
A written answer: “All daughters have red eyes, and half of them are carriers. Half of the sons have red eyes and half have white eyes.”
Overall, one in four offspring is a white-eyed male, because each of the four combinations is equally likely.
The mistake that loses marks
The common slip is to write the male’s alleles as a pair: “XᴿXʳ”. That treats him like a female. The male has one X and one Y.
| Step | Wrong | Right |
|---|---|---|
| Male with white eyes | XʳXʳ | XʳY |
| Male gametes | Xʳ and Xʳ | Xʳ or Y |
| Result | Wrong ratio | Correct ratio |
Write the sex chromosomes first, then add the allele to the X only.
A routine for any sex-linked cross
- Write the key with the allele on the X.
- Write the parents’ full genotypes with X and Y.
- List gametes, remembering the Y has no allele.
- Count offspring separately by sex.
The simple inheritance cross explorer helps you check the squares.
Check yourself
Invented insect as above. A white-eyed female (XʳXʳ) is crossed with a red-eyed male (XᴿY). State the eye colour of the sons and daughters.
Answer
Gametes: the female makes only Xʳ. The male makes Xᴿ or Y. Offspring: XᴿXʳ (daughters) and XʳY (sons).
All daughters have red eyes and are carriers. All sons have white eyes.
What to study next
Test the cluster with the inheritance practice set. For working backwards from a family diagram, see reading pedigrees without making personal genetic claims.
If you want a teacher to check your sex-linked crosses, see online one-to-one Biology tuition or the one-hour trial class (from RM50).