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. In contrast, a dominant trait only requires one copy of the gene. The concept of dominant and recessive traits in genetics was introduced by Gregor Mendel, a 19th century scientist who is often considered the father of genetics, thanks to his extensive work in the field.
When organisms like humans reproduce, they produce offspring with a genetic inheritance from both parents, caused by the fusion of haploid sperm with haploid egg cells to create a diploid organism. In the case of humans, the genetic material is encoded in 46 chromosomes, of which 23 come from each parent. Each chromosome contains a series of alleles, or genes, that encode a variety of information, from the color of a child's eyes to the way the feet develop.
When the alleles of both parents are the same, someone is said to be homozygous for that allele. If a child inherits two different alleles, such as a gene for red hair and a gene for brown hair, he is said to be heterozygous for that allele. When someone inherits the same allele twice, that trait will manifest whether they are dominant or not. However, when someone is heterozygous, 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 that were homozygous with white on the allele for flower color would develop white flowers. In short, he adopted the convention of using capital letters to denote a dominant trait, and lowercase 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 show up in parents who had a gene for light hair. Dark. 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 % chance that the child is homozygous for dark hair.