BIOL 2500 - Topic 5 (part 1)

Variantions on dominance

Roughly 90% of traits do not follow typical Medelian patterns

Looking at traits that don’t follow the typical Mendelian pattern

1.) Can look at interactions between the alleles of a single gene

2.) Can look at the interactions between two or more genes

Mendel’s traits exhibit...

Complete dominance

Complete dominance

It is when the dominant allele completely masks the phenotypic expression of the recessive allele, which is why we see the dominant phenotype

F1 and F2 of complete dominance

F1 - All exhibit the dominant phenotype

F2 - 3:1 phenotypic ratio

Is the recessive allele still being expressed in heterygotes?

Yes, but the presence of the dominant allele masks its effects

How does complete dominance work (at the molecular model)

1.) The mutation in the gene for the recessive allele causes the protein to be non-functional, therefore in the presence of the dominant allele, its effects are masked

2.) The only time the non-functional recessive allele is expressed is when it is homozygous recessive

Haplosufficient

It is when only one copy of the allele is needed to produce enough product (seen in complete dominance)

Haplosufficiency in complete dominance

1.) It is why the WT can be seen in heterozygotes, as only one copy of the allele is needed for it to be functional

2.) And it is why the WT cannot be seen in homozygous recessive, since it has no copies of the allele for it to be functional

Dominant mutations

Mutations that causes a change in the WT, therefore causing heterozygotes to become haploinsufficient

Null mutations

Produces a non-functional protein

Dominant negative

A protein is produced that inhibits the WT protein `

Complete dominance are really _________ of a phenotypic continuum

Extremes

Incomplete dominance

1.) It has two extreme phenotypic ends

2.) And then phenotypes that fall between the two extremes, like they are blending

F1 generation of incomplete dominance

It produces offspring that do not exhibit the dominant phenotype and instead produces the blended phenotype

F2 phenotypic and genotypic ratio of incomplete dominance

1:2:1 (for both the phenotypic and genotypic ratio)

Incomplete dominance in humans

We tend to not see incomplete dominance in humans, as many of our genes are the result of multiple genes (polygenic) or allelic interactions

Simplified examples of incomplete dominance in humans

1.) Dark colouration of eye

2.) Height

3.) Wavy hair

4.) Tay-Sachs disease

Tay-Sachs disease (autosomal recessive)

It cannot break down lipids due to missing/reduced enzyme activity, resulting in lipid build up in the brain and nervous system

Tay-Sachs disease (incomplete dominance)

Homozygous D: normal

Heterozygote: some dysfunction in the enzyme

Homozygous R: complete lack of enzyme