Variation: genetic and environmental causes
Biology · Key Stage 3 · The School
Two sources of variation
GENETIC variation comes from the genes a person inherits from their parents. Eye colour and blood group are almost entirely genetic. ENVIRONMENTAL variation comes from a person's surroundings and experiences. A scar, a suntan or a learned skill is almost entirely environmental. None of these three passes on to offspring.
When both act together
Many traits come from BOTH genes and environment, acting together. Height is strongly shaped by inherited genes. It is also shaped by diet, illness and general health in childhood. Two people with very similar genes for height can still end up different heights, if one grew up with much better food than the other.
Continuous and discontinuous variation
DISCONTINUOUS variation falls into a few clear groups, with nothing in between. Blood group is A, B, AB or O — never part-way between two of them. CONTINUOUS variation can take any value across a range. Height and body mass vary smoothly; every value between the extremes is possible. A trait controlled mainly by one gene tends to be discontinuous. A trait shaped by many genes plus environment tends to be continuous.
How can you tell if a trait is mainly genetic, mainly environmental, or a real mix of both? One classic method compares identical twins raised apart. Identical twins share all their genes. If they almost always match on a trait, such as blood group, that trait is almost entirely genetic. If they often differ, on exact height or on a skill like a second language, then environment is doing real, measurable work too. This is exactly why height counts as both genetic and environmental. Twin studies show identical twins usually end up close in height, but not exactly equal — and that small gap is down to environment.