Differences between individuals of the smae species are variations. Genetic variations occour because individuals inherit different combinations of genes. They can be caused by mutations, differences between individual gemetes (i.e. egg and sperm) or the random nature of fertilisation.
Individuals develop in different conditions, so some variations are due to environmental causes. Some charactaristics are determined by a combination of genetic and environmental factors.
All the instructions to make an individual are held on chromosomes kept in the nucleus of al body cels. A section of chromosome which codes for an ingerited characteristic or protein is called a gene. A person has 23 pairs of chromosomes; different species have different numbers of chromosome pairs. Gemetes have half the number of chromosomes of normal body cells.