Children usually look a little like both their mother and their father, but they will not be identical to either one of them. This is because they get half of their inherited features from each parent.
Each sperm cell and each egg cell contains half of the genetic information needed for an individual (each one is haploid - it has half the normal number of chromosomes). When these join at fertilisation, a new cell is formed. This zygote has all the genetic information needed for an individual (it is diploid - it has the normal number of chromosomes).
Examples of genetic variation in humans include blood group, skin colour and eye colour.
Gender is also an inherited variation - whether you are male or female is a result of the genes you inherited from your parents.
Characteristics of animal and plant species can be affected by factors such as climate, diet, accidents, culture and lifestyle. For example, if you eat too much you will become heavier, and if you eat too little you will become lighter. A plant in the shade of a big tree will grow taller to reach more light.
Other examples of features that show environmental variation include:
Some features vary because of a combination of genetic and environmental causes. For example, identical twins inherit exactly the same features from their parents. But if twin A eats more than twin B (and all other conditions stay the same), then twin A is likely to end up heavier.