Variations are differences between individuals of the same species.
Variations can be caused by:
- Genes (eg. eye colour)
- Environment (eg. language)
Genes carry the information for an organism to develop. Different genes control different characteristics (eg hair colour.) Genes are found in long strings called chromosomes and are located inside the nucleus of every cell of the organism.
Chromosomes are made of DNA (deoxyribonucleic acid.) Each DNA molecule has two strands, which are coiled to form a double helix. The DNA forms a complete set of instructions on how an individual should be constructed and how the cells should function. Chromosomes normally come in pairs. Both of the pair have genes in the same place. Different species have different number of pairs. Humans have 23 pairs. The sex cells (egg and sperm) contain single chromosomes (a total of 23- half the number of a normal body cell.)