Genetics: Dominance, Multiple Alleles, Epistasis & Cytoplasmic Inheritance

Complete Dominance

A relationship in which one allele is completely dominant over another

Incomplete Dominance

Phenotype of the heterozygote is in between the phenotypes of the two homozygotes; results when alleles have an additive effect.

Co-dominance

Phenotype of the heterozygote includes the phenotypes of both homozygotes.

Lethal Alleles

The homozygous YY condition is LETHAL in early development.

Y= dominat for fur color

Y= recessive for lethality

Multiple Alleles

3 or more possible alleles exist in the population for a particular genetic locus

Genotype Calculation for Multiple Alleles

General rule - the number of possible genotypes: n = number of different alleles [n(n+1)] / 2.

Important fact about multiple alleles

Each individual's diploid GENOTYPE still contains 2 alleles

ABO Blood Groups

3 alleles; 4 phenotypes; 6 genotypes; demonstrates Complete & Co-dominance & Multiple Alleles.

Gene Interaction

The products of genes at different loci combine to produce new phenotypes that are not predictable from the single-locus effects alone.

Novel Phenotypes

trait affected by 2 or more different genes; genes cooperate to produce phenotype neither could produce alone

Agouti (A)

Determines distribution of pigment in each hair.

Black (B)

Determines whether pigment is black or brown.

Extension (E)

Determines whether pigment is deposited.

Spotting (S)

Determines whether spots will be present.

Epistasis

A gene that masks the effect of another gene at a different locus

Hypostatic gene

A gene whose expression is masked by the action of a gene at a different locus

Recessive Epistasis

9:3:4 ratio; a lab will be yellow if E locus is ee, regardless of genotype at B locus because NO pigment deposited.

Cytoplasmic Inheritance

Inheritance of characteristics encoded by genes located in the cytoplasm. Because the cytoplasm is usually contributed entirely by only one parent, most characteristics are inherited from a single parent.

Leber's hereditary optic neuropathy (LHON)

Results in sudden blindness in both eyes due to death of optic nerve.

Genetic Maternal Effect

The phenotype of the offspring is determined by the nuclear genotype of the mother.

Direction of Shell Coiling in Snails

Is Maternally Inherited; s+s and s+s+ mothers make a protein that results in dextral (right-handed) coiling.

Dextral coiling

Result of s+s and s+s+ mothers depositing a protein into the egg.

Sinistral coiling

Result of ss mothers not depositing the protein into the egg.

F1 offspring coiling

Are sinistral even though their genotype is s+s, because their mother lacks an s+ allele.

F2 ss offspring coiling

Are dextral because their F1 s+s mother deposited the s+ gene product into the egg.

Cytoplasmic genes

Passed from the mother only, in the large egg.

Random segregation of mitochondria

Carrying a defect in a gene required for cellular respiration results in reduced ATP production.