mendel exceptions

7/29/2019 Mendel Exceptions

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Exceptions to Mendels Rules

Mendel chose traits in peas that showed two distinct forms one dominant,one recessive. Not all genes exhibit such simple inheritance.

Many populations contain more than two alleles for a given gene. Dominance and recessiveness do not always describe allele relationships.

incomplete dominance codominance

Alleles interact Some pairings are lethal.

Some traits are polygenic, i.e. caused by multiple genes Some genes epistatically regulate other genes. Multiple genes can lead to continuous traits.

Some genes are pleiotropic, i.e. determine multiple traits. Not all genes segregate randomly or sort independently.

Some genes are linked. Some genes are not nuclear.


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Multiple Alleles

An individual can carry no more than twoalleles, but a population can have manyalleles for any given gene within its genepool.

Genes can mutate in many ways at anynucleotide in their DNA sequence.


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Multiple Alleles: coat color in rabbits

GreyCGCG or CGcch or CGch or CGca

Albinocaca

Light graycchch orcchca

Himalayanchca or chch

Chinchillacchcch


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Incomplete dominance

Incomplete dominance is seenwith alleles that arehaploinsufficient, meaning thatone copy of the allele isinsufficient to produce thephenotype observed in thehomozygote.

The heterozygous phenotype istypically, but not always,intermediate to thehomozygous phenotypes.

indicates the heterozygous phenotype is distinct

from either homozygous phenotype.


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Codominant alleles are observedsimultaneously

The ABO gene encodes an enzyme that builds acell surface polysaccharide on red blood cells,which acts as an antigen.

Allele IAadds an A antigen

AlleleIB

adds a B antigen Allele ionever finishes building the

polysaccharide

Antigenic A and B products can be present on thecell surface at the same time.

Alleles IAand IBare codominant. This isanother way of saying that each allele ishaplosufficient.

Allele io is recessive to both IAand IB alleles.


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Codominant Alleles


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Epistasiswhen one gene affects the

expression of a secondgene.

The Hgene is epistatic to the ABOgene (I). The Hgene codes for

an enzyme that initiatesconstruction of the cell surfaceantigen.

The hhgenotype never beginsconstruction of the antigen, so all

ABO genotypes appear as Ophenotypes. This is sometimescalled the Bombay phenotype.


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Lethal alleles


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Continuous traits

Traits produced by the actions of multiple genesare polygenic. Variation is continuous, not discrete.

Effects of genes can either be additive or synergistic.

Different genes can have major or minor impacts withvarying weights.

Few continuous traits are solely polygenic. Mostare dependent on both genetic andenvironmental factors and are, therefore, calledmultifactorial traits.


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Eye color: A polygenic trait?

Number of dominant alleles at two

genes produces five phenotypes


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Pleiotropy

One gene controls manytraits (e.g. porphyriavariegata).

It is important toremember when lookingat multiple phenotypesdue to a single locus that

the mode of inheritancemay not be consistent.

mendel exceptions

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