17.2: Inheritance of traits at different gene loci

Bateson and Punnett

In 1905, they showed that two different genes may influence the same trait and can show ratios consistent with independent assortment if unlinked.

Chicken Comb Phenotypes example

cross between rose (RRpp) and pea (rrPP)

• F1 hybrid was walnut comb (RrPp)

• F2 dihybrid shows rose, pea, walnut and novel single comb (rrpp) in 9:3:3:1 ratio

Polygenic Trait

The result of multiple genes collaborating to influence a phenotype

• e.g. height is thought to be influenced by hundreds of loci

Other Examples of Polygenic Traits

Weight, blood pressure, and ridges in fingerprints.

Multiple Alleles

Multiple alleles of the same gene interact to influence phenotypes.

Multiple Alleles example - red blood cells

Haemoglobin protein has a component encoded by beta-globin gene with several different alleles.

• mutant sickle allele homozygotes have stiff crescent-shaped red blood cells that clog vessels

• normal alleles have flexible disc-like cells that travel easily

• heterozygotes may display mildly curved cells which are exacerbated under low oxygen = incomplete dominance and codominance