Cambridge O Level Biology · Syllabus 5090 · Inheritance
Expected Ratio
What is Expected Ratio?
The ratio of offspring phenotypes or genotypes predicted by a genetic diagram, understood as a set of probabilities that apply independently to each offspring rather than as a guaranteed count. The expected number of offspring of a given type is the probability of that type multiplied by the total number of offspring. Because each fertilisation is an independent event, small numbers of offspring frequently depart from the expected ratio by chance, while larger numbers tend to approach it more closely.
This definition is part of the Inheritance chapter in Cambridge O Level Biology.
Common mistakes with Expected Ratio
- “A 3:1 ratio means three out of every four offspring.” It is an expected probability applying to each offspring independently. Four offspring can easily come out 4:0. Write “the expected ratio is 3 : 1”, and add that observed numbers vary by chance, especially in small samples.
Questions students ask about Expected Ratio
If a cross gives an expected 3:1 ratio, why did my four seedlings come out 4:0?
Because the ratio is a probability that applies to each offspring independently, not a rule about how a group of four must turn out. With a \(\tfrac{3}{4}\) chance of the dominant phenotype each time, four dominant offspring in a row happens roughly three times in every ten sets of four. Larger numbers tend to approach the expected ratio, which is why genetic experiments count hundreds or thousands of offspring rather than four.