A gene can come in different versions. These different versions of the same gene are called alleles. For example, the gene for flower colour in a pea plant might have one allele for purple flowers and another allele for white flowers.
Alleles are found at the same position on a pair of chromosomes. In a body cell, there are two copies of each chromosome, one from each parent, so an organism has two alleles for each gene. The two alleles may be the same or different.
Different alleles code for slightly different forms of the same protein, or for different amounts of it. This leads to different characteristics. Because offspring inherit a different mixture of alleles from their parents, there are differences in the inherited characteristics of individuals. This is why brothers and sisters can look different from each other and from their parents.
