0.3 Genetics in clasical understanding  (Page 4/6)

A classic example of pleiotropy is the human disease PKU ( phenylketonuria ). This disease can cause mental retardation and reduced hair and skin pigmentation , and can be caused by any of a large number of mutations in a single gene that codes for an enzyme ( phenylalanine hydroxylase ) that converts the amino acid phenylalanine to tyrosine , another amino acid. Depending on the mutation involved, this results in reduced or zero conversion of phenylalanine to tyrosine, and phenylalanine concentrations increase to toxic levels, causing damage at several locations in the body. PKU is totally benign if a diet free from phenylalanine is maintained.

Antagonistic pleiotropy refers to the expression of a gene resulting in multiple competing effects, some beneficial but others detrimental to the organism.

This is central to a theory of aging first developed by G. C. Williams in 1957. [1] Williams suggested that some genes responsible for increased fitness in the younger, fertile organism contribute to decreased fitness later in life. One such example in male humans is the gene for the hormone testosterone . In youth, testosterone has positive effects including reproductive fitness but, later in life, there are negative effects such as increased susceptibility to prostate cancer. Another example is the p53 gene which suppresses cancer, but also suppresses stem cells which replenish worn-out tissue [2] .

Whether or not pleiotropy is antagonistic may depend upon the environment. For instance, a bacterial gene that enhances glucose utilization efficiency at the expense of the ability to use other energy sources (such as lactose) has positive effects when there is plenty of glucose, but it can be lethal if lactose is the only available food source.

Polygenic inheritance

Height in humans is a polygenic trait, as is color in wheat kernels. Height in humans is NOT discontinuous . If you line up the entire class, a continuum of variation is evident, with an average height and extremes in variation [very short (vertically challenged) and very tall (vertically enhanced)]. Traits showing continuous variation are usually controlled by the additive effects of two or more separate gene pairs. This is an example of polygenic inheritance. The inheritance of EACH gene follows Mendelian rules.

Usually polygenic traits are distinguished by

1. 1. Traits are usually quantified by measurement rather than counting.
2. 2. Two or more gene pairs contribute to the phenotype.
3. 3. Phenotypic expression of polygenic traits varies over a wide range.