What it does
In humans, sex is set by a pair of sex chromosomes. A mother has two X chromosomes (XX), so every egg she makes carries an X. A father has one X and one Y (XY), so half his sperm carry X and half carry Y. If an X sperm meets the egg the child is XX, a girl. If a Y sperm does, the child is XY, a boy.
The Punnett square shows two XX and two XY outcomes, so each child has a 50 % chance of being a girl. The simulator then makes thousands of births to show that overall the share lands near half, while small families vary a lot. To see the same method with other traits, try mendel's pea garden and punnett square.
How to use it
- Read the cross at the top.
- Choose how many births to simulate and how many children per family.
- Press Simulate births and read the bar chart and the totals.
- Press it again with fewer births to see how much a small sample jumps about.
What each setting does
- Births: how many children are made. More births land closer to 50 %.
- Children per family: groups the births into families and shows how many girls each family has.
- Class 11: draws the exact chances, C(n, k) / 2^n, as black lines over the bars.
Worked example
For 3 children per family there are 8 equally likely patterns, so 3 girls has chance 1/8 = 12.5 % and 1 girl has 3/8 = 37.5 %. Simulate 10 000 births and the bars sit close to those values. The girls overall come to about 5 000.
FAQs
Does the mother decide? No. Every egg carries an X, so the sperm decides.
After three girls, is a boy “due”? No. Each child is a fresh 50 : 50 chance.
Why is it not exactly half? Chance. A sample of 10 000 is usually within about a percentage point of half, but rarely exact.
Where the numbers come from
- In humans a female has two X chromosomes (XX) and a male has one X and one Y (XY); the sperm decides the sex. The family-size chances are the binomial C(n, k) / 2^n, checked in the tests. This is standard school biology and probability.
- The simulation is a teaching model of equal numbers of X and Y sperm. Drawings are fibub's own. Nothing you do on this page is stored or sent anywhere.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-10.