Extensions of Mendel's Principles

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44 Terms

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mendelian rules work well for

genotypes and inheritance of information

phenotypes are more complex

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complete recessive dominance

wild type allele → dominant

mutant allele → recessive

will only show if it passes the threshold

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complete dominance

mutant allele → dominant

wild type allele → recessive

only show if it passes the threshold

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haploinsufficiency

one copy is insufficent

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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

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novel function

dominant mutation since mutated protein is stuck in an on state whereas the wild type turns on then off (achondroplasia)

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dominant negative

reacts with normal protein in heterozygote and prevents assembly

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incomplete dominance

homozygotes and heterozygotes create all different phenotypes

heterozygote phenotypeis intermediate

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hypercholesterolemia

incomplete dominance

homozygous → wild type

heterozygous → heart disease in 40s and 50s

homozygous → mutated → heart disease in childhood

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complete dominance

heterozygote and homozygote give the same phenotype

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incomplete dominance

heterozygote and one homozygote give the same phenotype

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incomplete dominance

heterozygote has its own phenotype

(an intermediate between the 2 homozygote phenotypes → blended)

ex: red + white flowers = pink flowers

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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

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multiple alleles in a population

for one gene (like human blood types)

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IA and IB

encode a protein that differs in 4 amino acids

(both dominant alleles)

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i allele

non-functional so no sugar is added

(recessive allele)

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dominant lethal allele

causes death at an early age of development if homozygous

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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

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complete penetrance

100%, same genotype shows same phenotype

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partial penetrance

everyone has same genotype but doesn’t show the same phenotype

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expressivity

degree to which a character is expressed from the same genotype

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fully expressive allele

always expressed the same

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partial expressivity

all express the genotype but in different severities

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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

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Genetics v. Environment

Genetics → determines susceptibility to a disorder

Environment → determines if the disorder appears

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Pleiotropy

a single gene determines a number of distinct and (seemingly) unrelated characteristics

unlike Mendel who said: one gene determines one trait

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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

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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

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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

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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

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polygenic diseases

additive effect of alleles in different genes

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polygenic score

sum of risk from each known allele’s contribution

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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

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sometimes genes assort independently

BUT do not act independently in phenotypic expression

  • novel phenotypes

  • epistasis

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novel phenotypes

happens when 2 genes interact to determine 1 trait

(new phenotype showing)

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epistasis

one gene masks the effect of another gene at a different locus (not like the dominance allele at the same locus)

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epistatic gene

gene that does the masking

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hypostatic gene

gene whose effect is masked

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recessive epistasis

2 genes, homogenous recessive alleles are epistatic to the dominant allele, 3 phenotypes

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dominant epistasis

dominant allele masks recessive allele, 3 phenotypes

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duplicate epistasis

2 genes, 2 phenotypes

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sex-influenced traits

phenotypes expressed more in one sex despite being autosomal

  • dominance relationships might differ between sexes

  • expressivity can also differ between sexes

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sex-limited traits

genotypes only expressed IN ONE SEX despite being autosomal

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modifier genes

a second gene whose alleles modify the phenotypes of another gene