When you link inheritance and variation, the cross predicts probabilities, the data shows what happened, and your sentence says only what the data supports. Words such as “suggests” and “is consistent with” are exam-safe. “Proves” is rarely justified.
This lesson is part of integrated biological reasoning. It follows the enzyme reasoning lesson. Check your own syllabus year on the Cambridge subject page for the genetics terms required.
How does a cross connect to variation?
Offspring vary because each parent passes on one allele of each gene, chosen by chance, and fertilisation combines them at random. A Punnett square lists the possible combinations and gives probabilities, not fixed counts.
Some variation is also environmental. Genotype sets a range, and conditions such as light, water or diet decide where an individual lands within it.
Worked example (invented data)
In a plant, tall (T) is dominant to short (t). A student crosses two tall plants and grows 100 offspring, counting 78 tall and 22 short. The numbers are invented for this lesson.
Step 1, suggest parent genotypes: short offspring (tt) appeared, so each parent must carry a t allele. Both parents are tall, so both are Tt.
Step 2, Punnett square for Tt × Tt:
| T | t | |
|---|---|---|
| T | TT | Tt |
| t | Tt | tt |
Step 3, probabilities: three of four outcomes are tall (TT, Tt, Tt), so P(tall) = 3/4 = 75%. P(short) = 1/4 = 25%.
Step 4, expected counts for 100: 75 tall and 25 short.
Step 5, compare: observed 78 tall and 22 short differ from expected by 3 in each group. That is a small difference for 100 plants.
Step 6, conclusion: “The results are consistent with both parents being Tt, because they are close to the expected 3:1 ratio. The sample is only 100 plants, so this suggests but does not prove the genotypes.”
The mistake to watch for
Mistaken answer: “Each family of four plants will have exactly one short plant, since the probability is 1/4.”
Probability applies to each fertilisation separately. A family of four could have none, one, two or more short plants. The 1/4 is the long-run expectation, which large samples approach.
A second slip is blaming genes for everything: “The tall plants were taller because of their alleles” cannot be said without ruling out light and water differences.
Check yourself
1. Pp × pp, where P (purple) is dominant to p (white). What is the expected ratio of purple to white offspring, and how many of each in 40 offspring?
Show answer
Offspring are Pp and pp in equal numbers, so the ratio is 1:1. In 40 offspring: 20 purple and 20 white.
2. In a Tt × Tt cross, what is the probability that an offspring is short?
Show answer
Only tt is short, so 1/4, or 25%.
3. Two seeds of the same genotype grow to 20 cm and 27 cm. Give one reason that does not involve genes.
Show answer
Environmental difference, for example one plant had more light, water or nutrients than the other.
Where this leads next
Next is interpreting ecology evidence using numerical reasoning. Use the practice set for mixed questions and the scientific investigation critic to check how strongly your conclusion is worded.
Word choice in conclusions is something our teachers work on in one-to-one Co-ordinated Sciences tuition.