BIOL 2500 - Topic 5 (part 4)

Complete penetrance

The genotype always produces the same phenotype, therefore, an organism carrying the dominant allele will always produce the dominant phenotype

Mendel’s traits have ________ penentrance

Complete

Incomplete penetrance

1.) Phenotypic variation is observed, such that the same genotype does not always produce the same phenotype

2.) This is usually due to the environment or other genetic influences

Penetrance

The percentage of individuals with a given allele that exhibits the phenotype that is associated with that allele

Variable expressivity

The same genotype produces variable phenotypes, due to varying expression of the alleles (i.e. expect purple but it does not express the full amount of purple)

Incomplete penetrance example

Polydactyly (extra fingers), an autosomal dominant allele with incomplete penetrance

Variable expressivity example

Piebald spotting in beagles, such that the brown shows varying degrees of intensity in fur colour

Gene-environment interactions

1.) Sex

2.) Age

3.) Temperature

4.) Chemicals and diet

5.) Pathogens and exposure

6.) Pleiotropy

Sex (environmental effect on gene expression)

I.e. sex-limited and sex-influenced traits, as a result of hormone levels

Age (environmental effect on gene expression)

It influences cell functioning, such as telomere shortening, Huntington’s disease, etc

Temperature (environmental effect on gene expression)

Certain enzymes are sensitive to temperatures, resulting in reduced function if not at an optimal temp, such as Himalayan rabbits

Chemicals and diets (environmental effect on gene expression)

Certain diets and exposure to specific chemicals can influence certain traits/conditions, such as PKU and human height

PKU

A defective metabolism due to the lack of breakdown of phenylalanine, causing it to build up and affect mental functioning, but it can be managed by diet

Pathogens and parasites (environmental effect on gene expression)

1.) It can affect the development of allergies

2.) It can create an educated immune system in young children, as a result of early exposure

Pleiotropy

When a single mutation in the genotype alters multiple features in the phenotype

Is pleiotropy the same as variable expressivity

No because variable expressivity involves one trait, while pleiotropy involves multiple traits

Pleiotropy example

Sickle cell disease, where a mutation in the B-globin gene affects the RBC shape, which decreases iron and oxygen intake, among other things