For Dominance in Animal behavior see Dominance (animal behavior)
Dominance in genetics describes a type of relationship between an organism's two versions (alleles) of any one gene in which the traits of one allele (the dominant one) are expressed and the traits of the other (the recessive) are repressed. Since most animals are diploid, one allele of each gene comes from the dam (mother) and one allele comes from the sire (father). If either parent gives the offspring a completely dominant gene, that trait will be found in all the offspring.
Autosomal dominance simply refers to genes on an autosome, which is any chromosome other than a sex chromosome. Any such genes (more accurately, alleles) and their associated traits are called either autosomal dominant or autosomal recessive.
Generally, in any autosomal dominant trait a gene exists in at least two versions (designated A and a). Four combinations of these two alleles are possible: AA, Aa, aA, and aa (Aa and aA are functionally the same and are both referred to as Aa). AA and aa individuals will show different forms of some trait (phenotypes), and Aa individuals will show the same phenotype as AA individuals. In that case the allele A is said to be completely dominant to allele a, and a is said to be recessive to A (this use of upper case for dominant alleles and lower case for recessive alleles is a widely followed convention).
Black color in cattle, for example, is mainly controlled by an autosomal dominant allele of the Melanocortin 1 Receptor gene (MC1R) called ED. Another version of this gene, called e, is autosomal recessive. Although there are other factors related to color caused by other genes, generally an animal ED/ED, with two copies of the dominant ED alleles, will be black and an animal e/e with two e alleles will be red while an animal ED/e with one of each allele, will appear black, but carries the gene for red color.
It's important to realize that dominance simply refers to a relationship between alleles; one allele can be dominant over a second allele, recessive to a third allele, and codominant to a fourth. For example, in the black color trait scenario with cattle above, another MC1R allele, E+, exists which is recessive to both the e (red color) allele and ED . In the E+/e combination the e allele will dominate and the animal will be red and an ED/E+ animal will be black (an E+/E+ combination can result in many different colors since other genes seem to get involved.) Dominance differs from epistasis, a relationship in which an allele of one gene affects the expression of another allele at a different gene.
Homozygous vs heterozygous
Haploid, diploid and triploid
A classic example of dominance is the inheritance of seed shape (pea shape) in peas. Peas may be round (associated with allele R) or wrinkled (associated with allele r). In this case, three combinations of alleles (genotypes) are possible: RR, Rr, and rr. The RR individuals have round peas and the rr individuals have wrinkled peas. In Rr individuals the R allele masks the presence of the r allele, so these individuals also have round peas. Thus, allele R is dominant to allele r, and allele r is recessive to allele R.