Lesson 6: Heredity
Many human traits are influenced by more than one gene. In addition, environmental factors such as lifestyle, infectious diseases, and accidents influence traits. Traits that are the result of multiple genetic and environmental factors are called complex traits. The inheritance of complex traits cannot be predicted according to the rules of Mendelian genetics. As outlined below, geneticists do not yet understand all of the variables involved in complex traits.
The number of different genes that contribute to complex traits is difficult to determine. For example, at least four genes are likely to be involved in skin color, but there may be more.
The relative contribution of each gene and each allelic variant to the expression of a trait is usually unknown.
Environmental factors regulate the activity of some genes. For example, exposure to sunlight causes skin pigment-producing cells to increase gene expression, leading to increased pigment production and darker skin.
Some environmental factors have a greater influence on trait development than genes do. For example, although there is a genetic component to obesity, amount of food intake is the most important factor that controls the development of obesity.
