Genetic ratios & probability
AQA GCSE Biology calculation · part of the free Calculation Climb
From a Punnett square you predict ratios and chances. Crossing two carriers (Bb × Bb) gives a 3 : 1 ratio (dominant : recessive) — the same as a 75% : 25% chance.
How to do it — step by step
- Turn the ratio into a fraction or percentage.
- 3 : 1 means 3 out of every 4 (75%) and 1 out of 4 (25%).
Worked example
Bb × Bb → 3 brown : 1 blue.
chance of blue = 1 out of 4 = 25%
Now practise it — free
Science Street's Calculation Climb walks you from a worked example, through a guided step-by-step version, to questions you do yourself — with fresh examples every time and a built-in calculator and periodic table. No sign-up needed.
Practise Genetic ratios & probability on Science Street →