To distinguish inherited and environmental influences, ask two questions: is the trait set by alleles passed on in the gametes, and does a condition during development change the outcome? Most characteristics involve both, so a strong answer names the inherited part and the environmental part separately.
This lesson belongs to human reproduction and development and connects to cell division and inheritance.
What does each influence look like?
Inherited features are set at fertilisation by the combination of alleles in the zygote. Examples in a simple model are a fictional single-gene trait, or the sex chromosomes: XX gives a girl and XY gives a boy, with a 1 in 2 chance for each child.
Environmental features are changed by conditions: the mother’s diet, substances that cross the placenta, and later nutrition and activity. Adult height is a classic blend: alleles set a range, and nutrition influences where within that range a person ends up.
Worked example
Invented data, for teaching only.
A fictional allele a is recessive. Two parents are both Aa.
Step 1. Gametes. Each parent makes A and a gametes in equal numbers.
Step 2. Combine. Offspring genotypes: AA, Aa, Aa, aa.
Step 3. Probability. Chance of aa = 1/4.
Step 4. Two children. The chance that two children are both aa = 1/4 × 1/4 = 1/16. (Each birth is independent. The probability tree board shows this with a tree.)
Now an environmental comparison, with invented figures. In a fictional study of 200 babies in each group, the mean birth mass was 3.3 kg where the mothers did not smoke and 3.0 kg where they did.
Step 5. Difference. 3.3 − 3.0 = 0.3 kg. Step 6. Percentage. 0.3 ÷ 3.3 = 0.0909, so about 9.1% lower. Step 7. Interpret carefully. Smoke chemicals can enter maternal blood and cross the placenta, so a mechanism exists. The data still show an association. Diet, age or other factors could differ between the groups unless the comparison was controlled.
The mistake to watch for
Mistaken answer: “The smokers’ babies weigh less, so smoking is what makes them 9.1% lighter and the environment is the only cause.”
There are two slips here. The data show an association and not proven cause, and birth mass also has an inherited part. The correction is: “Birth mass was lower in this group, which suggests an environmental effect, but inherited factors and other conditions may also contribute.”
Another slip is thinking that environment cannot change inherited traits at all. Alleles do not change because of diet, but the expression of a characteristic can.
Check yourself
1. An Aa × Aa cross gives what percentage of offspring with the genotype aa?
Show answer
25% (1 in 4).
2. Give one inherited and one environmental factor affecting adult height.
Show answer
Inherited: the alleles passed on from parents. Environmental: nutrition during growth (other valid answers exist).
3. Mean birth mass is 3.4 kg in group X and 3.1 kg in group Y. Calculate the difference as a percentage of group X.
Show answer
Difference = 0.3 kg. 0.3 ÷ 3.4 = 0.0882, so about 8.8% lower in group Y.
Where this leads next
Put these ideas into full sentences with precise terminology, and check yourself on the mixed practice set. The inheritance model board lets you vary a fictional cross.
If you find yourself unsure whether to say “because” or “is associated with”, our teachers can practise that distinction in online one-to-one Biology tuition.