A genetic ratio such as 3 : 1 or a probability such as 1/4 describes the chance of each offspring having a feature. It does not promise that any particular group of offspring will match it. The exam skill is to state the probability and then describe it in words that avoid claiming certainty.
This is the last lesson of cell division and inheritance, and it uses the grids from constructing a simple inheritance grid.
Why do real results differ from the ratio?
Each fertilisation is random. Whichever allele the last offspring received does not change the chance for the next one. A coin that has landed heads three times still has a probability of one half of heads on the next toss.
Random variation is larger in a small sample. As the number of offspring grows, the proportions tend to come closer to the expected ratio, although they rarely match it exactly.
How to write a careful answer, step by step
- Work out the probability from the grid, for example 1/4 for a white offspring.
- Say what it applies to: “for each offspring”.
- Use chance language: “expected”, “probability”, “likely”, not “will” or “must”.
- Compare with observed results if given, and note the size of the sample.
- Conclude sensibly: a small difference from the expected value is consistent with chance.
Worked example
Two Rr purple plants are crossed, so the probability of a white offspring (rr) is 1/4. The data below are invented for practice.
| Sample | Purple | White | Total |
|---|---|---|---|
| Small batch | 3 | 1 | 4 |
| Large batch | 122 | 38 | 160 |
Expected in the large batch: 1/4 × 160 = 40 white and 160 − 40 = 120 purple.
Observed: 38 white and 122 purple, which is close to 40 and 120.
Probability of no white among 4 offspring: each is purple with probability 3/4, so (3/4)⁴ = 81/256 ≈ 0.32.
Probability of at least one white among 4: 1 − 81/256 = 175/256 ≈ 0.68.
Check: 3⁴ = 81 and 4⁴ = 256. Also 175 + 81 = 256, so the two answers add to 1.
Written conclusion: “Each offspring has a 1/4 chance of being white. In a small family of four, having no white offspring is still fairly likely, about 32%. The large batch is close to the expected 3 : 1 ratio, which supports the model but does not prove that every cross will match it.”
The mistake to watch for
The most common error is to claim that the ratio must appear in the offspring.
Mistaken answer: “The parents are Rr, so exactly one of their four children will be white.”
This treats a probability as a certainty and also ignores that each offspring is an independent event.
A second error is the gambler’s idea that a white offspring is “due” after three purple ones. The chance is 1/4 for each one, whatever came before.
Check yourself
The data below are invented for practice.
1. Two Rr parents have 3 offspring. State the probability that all 3 are white.
Show answer
Each is white with probability 1/4, so (1/4)³ = 1/64.
2. A cross is expected to give 25% white offspring. A class counts 200 offspring and finds 54 white. How many white were expected, and is the result reasonable?
Show answer
Expected: 25% of 200 = 50. The observed 54 is close to 50. The difference of 4 is small for a sample of 200, so the result is consistent with chance.
3. Rewrite this sentence so that it does not claim certainty: “The offspring will be 3 purple to 1 white.”
Show answer
“Each offspring has a 3/4 probability of being purple and a 1/4 probability of being white, so the expected ratio is 3 purple to 1 white.”
Where this leads next
Put all five lessons together in the mixed practice set. The probability tree and counting board and the inheritance model board both let you compare a small sample with a large one, using fictional data only.
A teacher in online one-to-one Biology tuition can help you turn a correct calculation into a sentence that earns the mark.