Variation among people
Variation among humans is easy to observe: no two people, except identical twins, look exactly alike. These differences arise from our genes and from the environment we grow up in. Human variation is classified as continuous or discontinuous, just like in other organisms.
Continuous variation in humans
Traits such as height, body mass, hand span and skin colour vary continuously across a range of values. They are polygenic (controlled by many genes) and strongly shaped by the environment — for example diet, exercise and health. In a large group these traits form a bell-shaped distribution, with most people near the average and few at the extremes. This is why a class of students shows a smooth spread of heights rather than just two or three fixed values.
Example
If you measure the body mass of 200 adults, most cluster around a middle value and few are very light or very heavy. Two people with the same genes for height may still differ if one had a poorer diet during childhood.
Discontinuous variation in humans
Traits such as ABO blood group, the ability to roll the tongue, and free or attached earlobes fall into clear-cut categories. They are controlled by one or a few genes and are little affected by the environment. You either can or cannot roll your tongue; there is no in-between. Because these categories are so clear, they are useful in forensic science and in tracing family relationships, since a child's blood group must be compatible with the possible combinations from the parents.
Key idea
Human continuous variation (height, mass): many genes + environment, bell curve. Human discontinuous variation (blood group, tongue rolling): few genes, distinct groups, little environmental effect.
Remember
The same phenotype can hide different genotypes, and the environment can change a phenotype without changing the genes.