Brainlag PlusEdexcel (Biology only, Higher): genes on the X chromosome, carriers, crosses and family trees for haemophilia and colour blindness; and for every board, deciding from a family tree whether a condition is dominant or recessive.
Genes on the X chromosome (Edexcel Higher, Biology only)
This section is for Edexcel 1BI0, Biology only, Higher tier.
AQA and OCR stop at how the X and Y chromosomes decide sex.
A sex-linked characteristic is controlled by a gene on a sex chromosome, almost always the X. The Y chromosome is small and carries very few genes, so it has no allele of most X genes.
- Females are XX: two copies of each X gene. With one recessive allele a female is a carrier: she does not have the disorder but can pass it on.
- Males are XY: one copy. A single recessive allele is enough to cause the disorder, because nothing on the Y masks it.
That is why haemophilia (blood does not clot properly) and red-green colour blindness are far more common in males. We write the alleles on the X: XH and Xh.
| Genotype | Phenotype |
|---|---|
| XHY | male, unaffected |
| XhY | male with haemophilia |
| XHXH | female, unaffected |
| XHXh | female carrier |
| XhXh | female with haemophilia |
A father gives his X to every daughter and his Y to every son. A son's X always comes from his mother.
The rest of this lesson and its 10 practice questions are in Plus.
Screen 1 of 4, free to read.
- 10 exam-style questions, marked as you go, each with a worked solution
- Anything you get wrong is saved to your flashcards