A recessive trait is a trait caused by a recessive gene, which means that someone must inherit two copies of the gene for the trait to manifest. By contrast, a dominant trait only requires one copy of the gene. The concept of dominant and recessive traits in genetics was put forward by Gregor Mendel, a 19th century scientist who is often regarded as the father of genetics, thanks to his extensive work in the field.
When organisms such as humans reproduce, they produce young with a genetic inheritance from both parents, caused by the fusing of haploid sperm with haploid eggs to create a diploid organism. In the case of humans, genetic material is encoded in 46 chromosomes, with 23 coming from each parent. Each chromosome contains a series of alleles or genes which code various information, from the color of the child's eyes to the way in which the feet develop.
When the alleles from both parents are the same, someone is said to be homozygous at that allele. If a child inherits two different alleles, such as a gene for red hair and a gene for brown hair, he or she is said to be heterozygous at that allele. When someone inherits the same allele twice, that trait will manifest whether or not it is dominant. When someone is heterozygous, however, one of the alleles will remain inactive, and this allele represents a recessive trait.
Gregor Mendel did much of his work with peas. In the course of his research, he learned that purple was a dominant trait for flower color, meaning that a flower only had to inherit one purple gene to produce purple flowers. White, on the other hand, was a recessive trait, so only peas which were homozygous with white at the allele concerning flower color would develop white flowers. For shorthand, he adopted the convention of using capital letters to denote a dominant trait, and lower case versions to indicate a recessive, in this case P and p to denote flower color.
The presence of recessives explains why two dark haired parents can have a light haired child, because the gene for light hair is a recessive trait, and therefore would not manifest in parents who had a gene for dark hair. If both parents of a child carry the dominant and recessive genes, there is a 25% chance that the child will develop light hair, a 50% chance that the child will have dark hair while carrying the allele for light hair, and a 25% chance that the child will be homozygous for dark hair.