Exam 2 part 3 Flashcards
multiple alleles
most genes have >2 alleles in a population
a single diploid can have up to 2 alleles in genome
Arise through changes in DNA sequence by mutation
calculating number of possible genotypes in a population
Based on number of alleles n=alleles
n(n+1)/2
Hierarchy of dominance
ex. fly eye color
eye color gene has more than 3 alleles with a decreasing order of dominance
incomplete dominance
phenotpe of heterozygous is intermediate of homozygotes
molecular explanation of incomplete dominance
gene dosage!
homozygosity for functional allele cause 2 doses of gene products
Heterozygosity causes 1 doe
homozygosity for nonfunctional allele causes no dose
Haplosufficiency
one does of gene product supports life
Due to threshold effect classified as autosomal recessive
ex. Cystinosis
Haploinsufficiency
one does of gene product is not enough to support life
Homozyous wt is healthy
hets and homozygous mutants are dead/sick
ex. BRCA mutations
essential genes
gene that encodes a product that is required for life
when mutated, causes a lethal phenotype
no complementary gene
Lethal allele
presence results in death
Recessive lethal
need 2 copies of lethal allele to exhibit lethal phenotype
most truly recessive lethals are lethal in utero
ex. cystic fibrosis
Dominant lethal
both homozygotes for dominate lethal allele and heterozygotes display lethal phenotype
causes by a gain of function mutation or dominant negative mutation
dominant negative mutation
loss of function mutation in the mutant allele interferes with function of wt allele
codominance
heterozygote shows the phenotypes of both homozygotes simultaneously
ex. AB blood type
molecular explanation for codominance
equally strong, expressed gene products that don’t block or interfere with the cellular functions of each other
Pleiotropy
one gene is responsible for multiple phenotypes
ex. PKU - low IQ, seizures, light skin pigments
linked alleles
when alleles are on the same chromosome, they tend to be inherited together unless separated by recombination
Selective sweeps
when 2 or more genes show strong linkage and one gene has an allele that gives a selective advntage, the other allele tends to be inherited with the advantageous allele
Genetic hitchhiking
ex. Crohn’s disease
Polygenetic trait
most traits are not monogenic
most traits result from the action of multiple loci
Epistatsis
Form of gene interaction in which one gene interferes with the expression of another gene
–one gene (epistatic) masks another gene (hypostatic)
modifier gene
alters the expression of a second gene
transcription factors
recessive epistasis
homozygous recessive genotype at locus A masks phenotype expression at locus B
aa epistatic to BB, Bb, bb
aa, B- and aabb have same phenotype
Dominant epistasis
Presence of dominant allele at locus A masks phenotype expression at locus B
A- is epistatic to BB, Bb, bb
A-,B- and A-bb same phenotype
synthetic lethal -negative epistasis
mutation A-alive mutation B-alive mutation A and B -dead B is masking a and A is masking b gene products of A and B wt are involved in complementary pathways
Gene expression and the environment
not all individuals with a particular genotype display the expected phenotype
