Genetics 3301

Chapter 6: From Gene to Phenotype

Genetic control of metabolism:

Ä Relationship between metabolic pathways and genes; Beadle and Tatum experiments (6-3, 6-4, T6-1); The one-gene-one-enzyme hypothesis; PKU results from phenylalanine accumulation due to mutations in phenylalanine hydroxylase (6-5); Different mutant alleles lead to different protein activities and phenotypes (6-7).

Interactions between alleles:

Ä Recessive mutants are haplosufficient (6-8); Haploinsufficient alleles are completely dominant.

Ä Incomplete dominance (6-9); intermediate phenotype.

Ä Codominance; blood type and sickle cell anemia (6-11).

Ä Recessive lethal alleles; skewing of phenotypic ratios (2:1 ratio instead of 1:2:1); a dominant allele can cause lethality in homozygotes and skew the phenotypic ratio (6-14).

Genes and phenotypes:

Ä There is a one to many relationship of genes to phenotypes (pleiotropic effects).

Ä There is a one to many relationship of phenotypes to genes.

Ä Genetic screens for genes that affect a process.

Ä Determining whether two mutants with the same phenotype are in the same or different genes using a complementation test (6-16); complementation in haploid fungi (6-17).

Interactions between genes:

Ä Interacting genes in the same pathway generate a 9:7 ratio when either or both genes are homozygous mutant; Genes that regulate the expression of a structural gene that produces a phenotype as a homozygous mutant also produce a 9:7 ratio when either or both genes are homozygous mutant (6-18).

Ä Interacting genes in different pathways; 9:3:3:1 phenotypic ratio; color of camouflage in corn snakes (6-19).

Ä Interacting genes in the same pathway; 9:4:3 ratio for mutations with different phenotypes where mutations in one gene are epistatic to another gene in the pathway (6-20).

Ä Suppressors of mutant phenotypes; 13:3 ratio ≠ recessive suppressor of a recessive phenotype; suppression via protein interaction (6-22) ≠ 10:6 ratio.

Ä Modifiers altering gene regulation; Synthetic lethals due to defective protein interactions (6-23)

Penetrance and Expressivity (6-25):

Ä penetrance is the % individuals with a given genotype that expressed the expected phenotype; The environment, influence of other genes and subtlety of the mutant phenotype may lead to variable penetrance.

Ä Expressivity is the extent to which a genotype is expressed at the phenotypic level; The environment and influence of other genes may influence expressivity.

Chi-square test:

Ä Determining whether your hypothesis (based on phenotypic ratios) is upheld via statistical analysis.

Key terms: Know all of these except allelic series, functional RNAs and temperature sensitive mutants.

Problems: 2, 4, 5, 6, 7, 9, 10, 11, 12, 14, 16, 18, 19, 21, 23, 25, 28, 32, 35, 39, 44, 51.