Sex determination and sex-linked genes sim
Drag XX and XY cards onto the parents, drag their eggs and sperm into a Punnett square, then add the colour-vision gene to see why sons are more often colour-blind.
Sex determination in humans (XX and XY, the sperm decides, a 1/2 chance of a son) and a sex-linked gene: red-green colour blindness on the X chromosome, carriers, and why sons are affected more often · covers the O-Level, IGCSE, GCSE topic
What the sim shows
Sex determination and a sex-linked gene: a female has two X chromosomes (XX) and a male an X and a Y (XY). Each egg carries an X; each sperm carries an X or a Y, so each child has a 1/2 chance of being a son, and the sperm decides. The gene for red-green colour vision is on the X chromosome, and the Y has no allele of it. A son has one X, from his mother, so one recessive b makes him colour-blind; a daughter is colour-blind only with b on both X chromosomes, and with one b she is a carrier with normal colour vision. That is why sons are affected more often. Sex determination is in the O-Level, IGCSE and GCSE syllabuses; the colour-vision (sex-linked) part is IGCSE Supplement content and extension for O-Level and GCSE. Simplified on purpose: a teaching model: each chromosome is one rod, drawn far larger than real chromosomes (a few micrometres long) and not to scale; real chromosomes are colourless until stained, so the colours are a key; only the sex chromosomes are shown; the other 22 pairs are left out; one gene for red-green colour vision, on the X, with two alleles: B (normal, dominant) and b (colour-blind, recessive); real red-green colour blindness comes from changes in two neighbouring genes on the X; each parent is drawn with two gametes, one for each chromosome of the pair; real people make very many, half of each kind; the four boxes of a square are four equally likely children, not a real family; the share of X chromosomes that carry b is illustrative and set by you; each X is taken as carrying b by chance, separately, which is the simplest model; the real figures (about 1 in 12 men and about 1 in 200 women in the UK, mostly red-green) are given only for comparison, as commonly quoted and not re-checked; the model's daughters' figure comes out a little higher than the real one, partly because the two genes are treated as one.
- Lit: the Y chromosome, which a son gets from his father
- Close-up: the parents, their gametes, your Punnett square and the chances
- Dashed outline: an empty place to drop something
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