Extending Mendelian Genetics 19 October, 2005 Text Chapter 14

Incomplete Dominance Alleles of some genes show incomplete dominance. In these cases, the heterozygote has an intermediate phenotype - a mixture of the phenotypes conferred by its alleles. Is this an example of blending? Why or why not? No. Two pink individuals can mate to produce red, white offspring.

Codominance In a set of codominant alleles, the heterozygote does not show an intermediate phenotype. The phenotypes of both alleles are individually expressed.

What is a Dominant Allele? Usually, dominant alleles are recipes for functional proteins. Recessive alleles are altered recipes for non-functional proteins. Think about flower color in pea plants. Substrate (colorless) Product (purple) The P allele is a recipe for a functional enzyme. The p allele is a recipe for a non-functional enzyme. Purple is dominant because one copy of a functional recipe is enough. Enzyme P In the analogous situation in snapdragons, one copy is not enough, And an intermediate phenotype is seen. At the molecular level, both functional and non-functional proteins are present. This is more like codominance.

Epistasis In epistasis, the alleles of one gene alter the expression of alleles of another gene.

Polyfactorial Characters Most interesting characters are influenced not by one or two genes, but by dozens or hundreds. In this example, alleles at three loci control skin color. The distribution shown here is characteristic of quantitative characters. These characters vary continuously, rather than in a few discrete states. Environmental factors can also influence these polyfactorial characters

Pleiotropy Pleiotropy is the converse of the concept of polyfactorial characters. In this case, one allele causes many different phenotypes.

Pedigree Analysis Pedigree analysis is a useful tool for studying inheritance in families.

Mendelian Inheritance in Humans In humans, many disorders follow Mendelian patterns of inheritance. Cystic fibrosis, sickle cell disease, Tay-Sachs syndrome, and many others are inherited as recessive alleles. The recessive allele codes for a non-functional protein. Usually, one functional allele is enough, so heterozygotes are asymptomatic. Why are cystic fibrosis and sickle cell alleles so common? In both cases, heterozygotes are protected from other diseases - recessive CFTR alleles protect against typhoid infection, while HbS alleles protect against malaria. Lethal dominant alleles are (for obvious reasons) less common. Huntingtons disease is caused by a dominant allele whose effect on phenotype is not obvious until after age 40.