The passing on of something from one generation to the next. In biology it refers to the passing on of genetic and physical characteristics such as the colour of the coat. These characteristics are known as heritable traits. Heritable traits are determined by genes, which are located on chromosomes. There are two copies of each chromosome; one from the mother, one from the father. There are therefore also (almost) always two copies of a gene. These copies are also called alleles, which may contain slightly different information (e.g. black coat or chestnut coat). If both alleles in a gene are the same (i.e. two black genes or two red alleles), the gene is homozygous. If there are two different alleles (i.e. a black one and a red one), the gene is heterozygous.
There are four different ways in which the genetic make-up influences the physical trait: there are dominant, recessive and incomplete dominant and codominant traits.
If a trait or allele is dominant, the same physical outcome (e.g. coat colour) results regardless of whether an individual has one or two copies of the particular allele (black is dominant over red, therefore a horse will appear black both if it has two black alleles or one black and one red allele).
If a trait or allele is recessive, the physical property will only manifest if the horse carries two copies of the same, recessive gene (e.g. two red alleles).
If the trait shows incomplete dominance, the dominent allele will produce more pronounced physical characteristics if it is homozygous than if it is heteozygous (e.g. the Creme colour gene in horses will produce a palomino if one Cr allele is present, and a cremello if two Cr alleles are present).
In a codominant inheritance, a heterozygous state will show attributes of both alleles in equal measure (e.g. blood type AB in humans).