Studying the entire genome of an organism—even a bacteria or virus—is an enormous undertaking. Basic genomic research involves determining how many genes an organism has, the functions of those genes, and how expression or suppression of the organism’s genes is influenced by environmental and other factors.
Genomic research also includes a more general field of study known as intragenomic phenomena—interactions between various alleles and loci within the genome. This includes such genomic phenomena as epistasis, heterosis, and plieotropy.
Epistasis occurs when the action of one gene affects the expression of other genes. For example, a gene may code for a protein which, when expressed, prevents the expression of one or more other genes. Epistasis can also occur at the phenotypic level: for example, if an individual has the genetic mutation that causes albinism, the phenotypic effects of genes that control their skin, hair, and eye color will be masked by the lack of pigment caused by albinism.
Heterosis is also known as hybrid vigor, and describes the way in which hybrids of two genetically different parents have increased strength in certain characteristics. This concept is most often applied to the practice of breeding plants and livestock for characteristics such as improved yield or pest resistance.
Plieotropy occurs when one gene influences several phenotypic traits at once. Typically, this phenomenon occurs because the gene codes for a protein which is used by many different types of cells, or is involved in a specific cell-signaling pathway. Mutations in plieotropic genes often have wide-ranging effects. An example is the genetic disorder called phenylketonuria, which can reduce skin and hair pigmentation and cause mental retardation due to a mutation in a gene that converts phenylalanine into tyrosine.
