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mendelian rules work well for
genotypes and inheritance of information
phenotypes are more complex
complete recessive dominance
wild type allele → dominant
mutant allele → recessive
will only show if it passes the threshold
complete dominance
mutant allele → dominant
wild type allele → recessive
only show if it passes the threshold
haploinsufficiency
one copy is insufficent
achondroplasia
inhibited bone growth in early development → failure to turn off results in little people
mutated protein is stuck in an “on” state while wild type gene turns on then off
novel function
dominant mutation since mutated protein is stuck in an on state whereas the wild type turns on then off (achondroplasia)
dominant negative
reacts with normal protein in heterozygote and prevents assembly
incomplete dominance
homozygotes and heterozygotes create all different phenotypes
heterozygote phenotypeis intermediate
hypercholesterolemia
incomplete dominance
homozygous → wild type
heterozygous → heart disease in 40s and 50s
homozygous → mutated → heart disease in childhood
complete dominance
heterozygote and homozygote give the same phenotype
incomplete dominance
heterozygote and one homozygote give the same phenotype
incomplete dominance
heterozygote has its own phenotype
(an intermediate between the 2 homozygote phenotypes → blended)
ex: red + white flowers = pink flowers
codominance
heterozygotes have phenotypes of both homozygotes
(50% of 1 allele expressed and 50% of the other allele)
ex: horses with 2 types of hairs
multiple alleles in a population
for one gene (like human blood types)
IA and IB
encode a protein that differs in 4 amino acids
(both dominant alleles)
i allele
non-functional so no sugar is added
(recessive allele)
dominant lethal allele
causes death at an early age of development if homozygous
penetrance
percentage of individuals having a particular genotype that expresses the expected phenotype
environment, timing, and other gene’s alleles can influence where or not some show the phenotype or not for the same genotype
complete penetrance
100%, same genotype shows same phenotype
partial penetrance
everyone has same genotype but doesn’t show the same phenotype
expressivity
degree to which a character is expressed from the same genotype
fully expressive allele
always expressed the same
partial expressivity
all express the genotype but in different severities
environmentally-dependent “conditional” phenotype
allele = susceptibility to developing a phenotype
expression of the phenotype is dependent on the environmental factors that are present
metabolism
temperature
altitude
childhood experience
Genetics v. Environment
Genetics → determines susceptibility to a disorder
Environment → determines if the disorder appears
Pleiotropy
a single gene determines a number of distinct and (seemingly) unrelated characteristics
unlike Mendel who said: one gene determines one trait
Extension to Mendel
traits are complex and determined by multiple proteins encoded by different genes
only look simple if just one of the proteins/genes is mutated
inheritance is complicated if variations occur in more than one geen
heterogenous traits
determined by more than 1 gene
mutation in any 1 gene that causes it lose function gives the same phenotype
ex: deafness → mutation in any 1 of 70 genes causes deafness
complementation test
determine if mutation is in same or different gene for 2 parents with same phenotype
if in same genes
then progeny will show phenotype due to 2 same alleles
if in different genes
then progeny would receive 2 different alleles and may not phenotype
additive polygenic inheritance
traits are encoded by the addition of many genes at many loci
wide distribution of phenotype due to multiple partially functional alleles that add to each other
height, skin color, IQ
polygenic diseases
additive effect of alleles in different genes
polygenic score
sum of risk from each known allele’s contribution
complex traits
due to multiple genes acting in different ways
some genes
rare mutant allele causing a large effect
other genes
additive nature of mutant alleles build phenotype
sometimes genes assort independently
BUT do not act independently in phenotypic expression
novel phenotypes
epistasis
novel phenotypes
happens when 2 genes interact to determine 1 trait
(new phenotype showing)
epistasis
one gene masks the effect of another gene at a different locus (not like the dominance allele at the same locus)
epistatic gene
gene that does the masking
hypostatic gene
gene whose effect is masked
recessive epistasis
2 genes, homogenous recessive alleles are epistatic to the dominant allele, 3 phenotypes
dominant epistasis
dominant allele masks recessive allele, 3 phenotypes
duplicate epistasis
2 genes, 2 phenotypes
sex-influenced traits
phenotypes expressed more in one sex despite being autosomal
dominance relationships might differ between sexes
expressivity can also differ between sexes
sex-limited traits
genotypes only expressed IN ONE SEX despite being autosomal
modifier genes
a second gene whose alleles modify the phenotypes of another gene