Cells have many chores to perform, but none are more important than synthesizing proteins. The recipe for this activity resides in an organism’s deoxyribonucleic acid, which it inherits from each parent. The cells of sexually reproducing organisms contain two matched sets of DNA-protein packages, the chromosomes. Genes are chromosome segments that code for proteins, and a pair of matching genes from the parents, known as alleles, can interact in different ways.
Genes act as templates for the synthesis of messenger ribonucleic acid (mRNA). Enzymes transcribe genetic information from the gene’s DNA onto mRNA strands that drive protein synthesis performed by the cell’s ribosomes. Humans have 23 pairs of chromosomes that contain about 20,000 gene pairs, but genes make up only about 2 percent of chromosomal real estate. Each pair member, or allele, codes for more or less the same protein, but the exact coding can differ and therefore express different versions of the protein. Some genes are so mutated that they can't be expressed as proteins.
Dominant and Recessive Alleles
In some cases, a dominant allele masks the expression of its recessive partner. For example, a plant might carry genes that code for either red or white flowers. If the red gene is dominant, then an offspring can have white flowers only if it receives two alleles for white color. A cross of red- and white-flowered parents yields about 75 percent red-flowered offspring and 25 percent white-flowered ones. The white trait might reflect a mutation that renders the flower incapable of producing pigment.
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Codominant and Semidominant Alleles
Some traits reflect the equal dominance of both alleles in a pair. In this situation, the resulting gene expression, or phenotype, is the product of the different proteins synthesized from each allele. Suppose that the flower color alleles for a species of plants are codominant. A cross between red-flowered and white-flowered parents will produce offspring with spotted red and white flowers. Had the alleles been incompletely dominant, or semidominant, the offspring would exhibit a blended phenotype, pink flowers, because the offspring would have only a single dose of the protein that produces red color.
Epistasis is an interaction among two or more different allele pairs that combine to influence the expression of a trait. Sometimes, one gene masks or modifies the expression of multiple genes. For example, researchers have identified two different genes that help determine the shape of a chicken’s comb, the rose comb gene and the pea comb gene. The combs of offspring show a mix of four different comb styles, indicating two allele pairs are at work. The relationships among the alleles in an epistatic group can give rise to many different phenotypes.