Lecture 3. Extension of Mendelian Principles

I. Importance

Organisms represent complex interactions among traits and genes

To Fully understand the relationship between genotype and phenotype, we need a complete understanding of the

mode of gene expression

II. Dominance Revisited

A. Incomplete dominance

Hybrid resembles neither parent and often expresses an intermediate trait

B. Codominance

The traits of both parents show up equally in the heterozygote

Note that dominance relations among alleles at a locus does not effect the transmission of alleles

e.g., color pattern on peas and blood type

C. Relative Dominance

Alleles are dominant relative to other alleles (often many alleles)

III. Mutations

Source of new alleles is mutation

Mutation rates are 1/10,000- 1/1,000,000 per locus

Wild type= > 1% frequency; Mutant = < 1% frequency

Monomorhpic – only 1 wild type allele

Polymorphic – more than 1 wild type allele

IV. Frequency dependent selection- a force to maintain high polymorphism – will discuss later

V. Pleiotropy- 1 gene/ locus effects more than 1 trait

VI. Recessive lethal alleles- For some loci, recessive homozygotes are lethal, thus 2:1 ratios

VII. Multiple genes contribute to a trait

A. Mendelian examination of 2 loci determining the expression of 1 trait, 9:3:3:1 ratio

B. Complementary gene action; 9:7 ratio (Complementation TEST)

C. Epistasis: the genotype at one locus is masked by the expression of alleles at another locus

1. Recessive epistasis; 9:4:3 ratio

2. Dominant epistasis; 13:3 ratio

VIII. Genetic plus environmental effects

No gene interactions, many loci, environmental effects = many phenotypic classes that are barely

discernable for others. Cannot predict genotype from phenotype. Will discuss later

Combination of gene and environment

Penetrance- % of population of a given genotype that expresses a phenotype

Expressivity- degree to which a trait is expresses

Modifiers- change the expression of other genes

IX. Summary: Single genes or simple 2 locus models only describe a small subset of the way genotypes

determine phenotypes

X. Terms and Concepts to Know: Dominance, Codominance, incomplete dominance, wild type, pleiotropy, complementation test, recognize different forms of epistasis, modifier genes, environmental effects including penetrance and expressivity. Figs from chapter 3: 2, 3, 4, 6, 8, 9, 11, 12, 13, 14, 15, 21. Table 2