Inheritance questions in Combined Science tend to reward a clear, correctly scoped answer: the right terms, a genetic diagram, a probability and a link to the data. Extra molecular detail that the question did not ask for earns no extra marks and can introduce errors.
This lesson belongs to biological mechanisms in mixed tasks. Combined Science 0653 is a single-award course, so always check the current syllabus on the Cambridge page for what is in scope. The numbers here are invented.
What does a scoped answer contain?
For a single characteristic controlled by one gene, a scoped answer usually contains these parts:
- The alleles and which is dominant (capital letter) and recessive (small letter).
- The parent genotypes and the gametes each can make.
- A genetic diagram (Punnett square) showing the possible offspring.
- The ratio or probability, with a sentence saying it is a chance, not a certainty.
Worked example (invented data)
In pea plants, tall (T) is dominant to short (t). Two heterozygous tall plants, Tt × Tt, are crossed.
Step 1, gametes: each parent makes T or t.
Step 2, Punnett square:
| T | t | |
|---|---|---|
| T | TT | Tt |
| t | Tt | tt |
Step 3, read the offspring: 1 TT : 2 Tt : 1 tt. Tall plants are TT or Tt, so 3 tall : 1 short. The probability of a short plant is 1 ÷ 4 = 25%.
Step 4, link to data: in an invented count of 80 offspring, 62 are tall and 18 are short. The expected numbers are 60 tall and 20 short (80 × 3/4 = 60 and 80 × 1/4 = 20). The results are close but not identical, as chance predicts.
The mistake to watch for
Mistaken answer: “The gene is copied into mRNA, which is translated into a protein at the ribosome, and the next four offspring will include exactly one short plant.”
This has two problems.
The first half goes beyond what the question asked, so check whether your syllabus requires it before spending time on it. The second half treats a probability as a certainty. The correction is to give the genetic diagram and say “there is a 25% chance each offspring is short”.
Check yourself
1. Cross Tt × tt. What is the probability of a short offspring?
Show answer
Gametes: T or t from one parent, only t from the other. Offspring: 2 Tt : 2 tt, which is 1 Tt : 1 tt. Probability of short (tt) = 1 ÷ 2 = 50%.
2. Cross TT × tt. What are the offspring genotypes and phenotypes?
Show answer
All offspring are Tt, and all are tall, because T is dominant.
3. Two Tt plants produce 120 offspring. How many short plants do you expect?
Show answer
Probability of short is 1 ÷ 4, so 120 × 1/4 = 30 short plants. The actual number may differ slightly because inheritance is a matter of chance.
Where this leads next
Finish the module with the mixed practice set, then return to the module overview to plan revision. The mistake log and retest queue helps you track which scope errors keep recurring.
A teacher can compare your answers with the Cambridge syllabus line by line in online one-to-one Combined Science tuition.