|MadSci Network: Genetics|
Good question, Lauren. For the sake of simplicity I have assumed we are discussing a Mendelian trait.
A gene generally refers to a stretch of DNA which encodes a single protein. We inherit two copies of each gene, one from our mother and one from our father. Many genes have multiple variants which influence a given trait (a phenotype). For example, there are four phenotypes at the ABO blood group: A, B, AB and O. Each variant of a gene is referred to as an allele. The ABO blood group has three alleles: A, B, and O. The combination of two alleles (one maternal and one paternal) in an individual is referred to as the individual's genotype. Genotypes at the ABO locus include AA, BB, AB, AO, BO, and OO.
Strictly speaking, genes are not dominant or recessive; only alleles are dominant or recessive. To determine whether an allele is dominant or recessive with respect to another allele, you need to know which homozygote the heterozygote resembles. In our example, the AO heterozygote is more similar to the AA homozygote than the OO homozygote. Therefore the A allele is dominant over the O allele. B is also dominant over O. The dominance relationship between A and B is more complicated: the heterozygote AB resembles neither the AA nor the BB homozygote. This is referred to as codominance.
On an experimental level there are several ways to determine whether a trait is dominant or recessive. The easiest is to compare a known heterozygote with known homozygotes. For example, if you cross two purebred strains of mouse which are yellow and black respectively, and the offspring (obligate heterozygotes) are black, then the black allele is dominant. However, in some species it is difficult if not impossible to obtain purebred strains (for example, it is rather difficult to arrange marriages in humans), so sometimes you need to resort to more complicated studies. By observing the offspring from many different crosses, it is possible to deduce the pattern of inheritance for a trait. If parents with yellow feathers never have chicks with green feathers, but parents with green feathers sometimes have chicks with yellow feathers, then the green allele is probably dominant over the yellow allele.
Of course, as I stated at the outset, all of this assumes Mendelian inheritance: one gene affecting one trait. Some traits are influenced by many genes, most traits don't have two clear cut variants, and some are strongly influenced by environment. For example, in humans height, hair color, skin color and eye color don't have any clear cut variants, but exist in a continuous spectrum. All of these traits are likely to be influenced by many genes, and probably have environmental factors as well (sun bleaches hair and tans skin, diet influences height, etc.) So for some traits you simply cannot determine dominance because the system is much mor complicated than a single gene with a few variants.
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