GENE3200 - Mendelian Extensions (class 8)

penetrance

-percentage of a population with a particular genotype that shows the expected phenotype

-can be complete (100%) or incomplete (ex: 60%)

expressivity

-degree or intensity with which a particular genotype is expressed in a phenotype

-can be variable or invariable

a genotype doesn’t always produce the expected phenotype

both incomplete penetrance and variable expressivity occur with dominant and recessive phenotypes, but are easiest to observe with dominant phenotypes

complete dominance

heterozygous phenotype is the same as the phenotype for one of the homozygotes

incomplete dominance (partial dominance)

heterozygous phenotype is intermediate between the two homozygous phenotypes

codominance

heterozygous phenotype includes both of the homozygous phenotypes

with incomplete dominance and codominance

neither allele is dominant or recessive to the other

dominance def

dominance is always relative to another allele of the same gene

dominance

-more than one pattern of dominance may exist between different alleles of a gene

-can reveal functional differences between gene products encoded by different alleles

different levels for observations of phenotype

-anatomical, physiological, molecular

-type of dominance depends on the level of phenotypic analysis

-ex: cystic fibrosis is caused by mutations in CFTR gene (disease phenotype is recessive (physiological level) + alleles of CFTR (normal and mutant) are codominant (molecular level))

four blood types of the ABO system

-three alleles of the I gene (I^A, I^B, and i)

-I^A, and I^B alleles are codominant but each is completely dominant to the i allele

blood type: A

I^AI^A or I^Ai

blood type: B

I^BI^B or I^Bi

blood type: AB

I^AI^B

blood type: O

ii

allelic series def

multiple alleles of a gene

allelic series

-can exist in natural populations, in domesticated animals, and in lab strains

-each allele has a specific nucleotide alteration or mutation in the gene that makes it different from other alleles of that gene

different alleles may have different defects on gene function

null mutation, hypomorphic mutation, hypermorphic mutation

null mutation

function of gene product is completely abolished

hypomorphic mutation

function of gene product is partially abolished

hypermorphic mutation

function of gene product is increased or expression of gene occurs in ectopic sites in the body

some genes have more than two alleles more

-phenotypes for different alleles can have different severity

-dominance relations are unique to a pair of alleles

lethal alleles cause either prenatal or postnatal death to the individual

-almost all lethal mutations have recessive inheritance (only the homozygotes are inviable; viability is dominant to lethality)

-lethal alleles can be detected as distortions in segregation ratios caused by one or more missing classes or progeny

pleiotropy

one gene affects multiple characteristics that may or may not be obviously related to each other