The inheritance of complex traits

Additive model for polygenic inheritance

Because complex traits involve the interaction of multiple genes, the genetic contribution of such traits can be difficult to study. To simplify, we assume that each gene involved has an equal effect on the phenotype and the effects of each gene are additive. - If a trait is controlled by two genes with two alleles: A, a and B, b - Assume that each dominant allele contributes the same amount and each recessive allele makes no contribution.

Punnett square: two genes, two alleles

For this example, assume that A and B both add 5 cm to a base height of 160 cm a and b add nothing

Additive model for polygenic inheritance III

*As the number of genes controlling a trait increases:* • The number of phenotypic classes increases. • The differences between each phenotypic class decreases. • As the number of phenotypic classes increases, the distribution of phenotypes in a population becomes a bell-shaped curve. *For multifactorial traits, where environment also has an impact:* • As the differences between phenotypic classes decreases, there is a greater chance for environmental factors to override small differences, blending phenotypes together. - E.g. exposure to sunlight can alter skin color and obscure genetic differences.

Phenotypes can be discontinuous or continuous

*Discontinuous variation: phenotypes that fall into distinct, non-overlapping classes.* • Height and color in peas: tall or short, green or yellow • ABO blood groups in humans • Diseases: Marfan's syndrome, cystic fibrosis: affected or not → Traits are usually quantified by counting *Continuous variation: a series of overlapping phenotypic classes distributed from one extreme to another.* • Height in humans • Skin color → Traits are often measured Note: Understanding the genetics underlying discontinuous and continuous traits led to the realization that some traits are controlled by single genes (simple, monogenic) and others are controlled by multiple factors (complex traits).

Additive model for polygenic inheritance II

*For traits controlled by two or more genes, in a population:* • Most individuals have an intermediate phenotype. • Fewer individuals have a phenotype at the extremes.

Monogenic vs. complex traits

*Monogenic (simple) traits*: controlled by a single gene *Complex traits*: controlled by more than one factor • Multiple genes • The interaction of genes with each other • Effects of environment *Polygenic traits*: controlled by two or more genes *Multifactorial traits*: controlled by two or more genes and environmental interactions. Each gene is inherited according to Mendelian rules... ...BUT, phenotypes do not necessarily show Mendelian ratios

Learning Objectives

Compare and contrast phenotypes that exhibit discontinuous variation and those that exhibit continuous variation. Define what causes a trait to be classified as a complex trait. Compare and contrast monogenic (simple) and complex traits. Distinguish between complex traits that are polygenic and those that are multifactorial. Describe the additive model for polygenic inheritance. Determine the genotypes and phenotypes for a polygenic trait controlled by two genes using the additive model for polygenic inheritance.

How do multiple genes control a polygenic trait?

How many genes? Could be 2, 3,...100 or more Each gene (loci) contributes to the phenotype, but they do not necessarily all contribute to the same extent *A "simple" example of polygenic inheritance:* Two genes, each with one dominant and one recessive allele. AaBb x AaBb *Simplifications and assumptions:* • Each gene contributes equally to the phenotype. • The dominant alleles (A and B) contribute to the phenotype, whereas the recessive alleles (a and b) do not. • For this example, assume that A and B both add 5 cm to a base height of 160 cm; a and b add nothing.