BSCI222: Extension of Mendelian Genetics

What is complete dominance?

Heterozygotes and homozygotes express the same phenotype

What happens when the mutant is recessive?

As long as there is one functional allele, the normal phenotype is expressed

What happens when the mutant is dominant?

Mutation in at least one allele will produce the mutated phenotype to be expressed (due to haploinsufficiency)

What is haploinsufficiency?

One functional copy of the allele is NOT enough (seen in dominant mutants)

What is incomplete dominance?

Heterozygote displays intermediate phenotype between homozygotes

• ex. heterozygote is pink flower, homozygotes are red and white

What is co-dominance?

Heterozygote displays both phenotypes

• ex. roan horse color (red and white hairs)

How does blood type inheritance function?

Multiple alleles for one gene!

I^A and I^B are codominant and i is recesssive

• A type: I^AI^A or I^Ai

• B type: I^BI^B or I^Bi

• AB type: I^AI^B

• O type: ii

What are two traits that display dominance series?

• Rabbit coat color: C > c^ch > c^h > c^a

• Tay Sachs: different mutated forms, genotypes with one functioning alleles lead to less severe forms (age of onset later)

What are dominant lethal alleles?

Alleles that produce a dominant phenotype, but lethal when homozygous

• cannot produce phenotype that are homozygous

• skews ratio to 1:2:0

• ex. agouti yellow mouse (YY leads to embyro death)

What is penetrance?

Percentage of individuals having a particular genotype AND expressing the associated phenotype!

• 100% penetrance = all individuals express the expected phenotype

What is expressivity?

The degree to which a trait is expressed (severity of phenotype varies among those who have the same genotype)

• ex. polydactyly

What is environmentally dependent phenotype?

Expression of a phenotype depends on the environment (temperature, metabolism, altitude, childhood)

What is phenylketonuria?

Inability to convert phenylalanine into tyrosine

• Phenotypes: odor, intellectual disability, etc.

Early detection and changing diet to LOW phenylalanine and HIGH tyrosine get decrease phenotypes.

What is pleiotropy?

A single gene affects multiple traits

• ex. Waardenburg Syndrome (deafness, white forelock, blue eyes, pale skin)

What is a polygenic trait?

A phenotype is determined by multiple genes.

What are heterogenous traits?

Multiple genes affect one phenotype (so any mutation is one leads to the phenotype)

• ex. deafness

Can utilize complementation test of two recessive mutations

What is complementation test?

Cross two homozygous mutants with each other:

• produces 100% normal progeny if its a mutation on different genes

• Produces only mutant phenotype if its on the same gene

What is the additive effect of multiple genes?

Multiple alleles that are partially active rather than inactive v. active; genotypes at all genes determine one phenotype

• Often a continuous trait

• ex. skin, color, height

What are novel phenotypes?

Two genes interact and produce four phenotypes!

What is epistasis?

One gene “masks” the effect of another gene at the same locus

What are the two types of genes in epistasis?

Epistatic gene - gene that does the masking

Hypostatic gene - gene that is MASKED

What is dominant epistasis?

One dominant allele @ epistatic gene will hide hypostatic gene.

Ratio: 12:3:1

What is recessive epistasis?

Homozygous recessive at genotype will hide the hypostatic gene.

Ratio: 9:3:4

What is duplicate dominant epistasis?

One copy of the dominant allele are either gene is sufficient to produce the phenotype

Ratio: 15:1

What is duplicate recessive epistasis?

A homozygous recessive genotype at either gene will mask the trait.

Need one dominant allele at each gene to EXPRESS phenotype.

Ratio: 9:7

What are sex-influenced traits?

Phenotype is dominant in one sex and recessive in the other (despite being autosomal)!

What are sex-limited traits?

Genotype only expressed in ONE sex!