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Incomplete PenetranceIncomplete Penetrance The phenotype is not always observed among The phenotype is not always observed among

individuals carrying the genotype – express or notindividuals carrying the genotype – express or not DD or Dd - only 80% show polydactylyDD or Dd - only 80% show polydactyly

Variable Expressivity• A phenotype that varies in intensity

Polydactyly two extra digits on each hand and foot vs. one extra digit on one foot Individuals with the same genotype for familial hypercholesterolemia have varying levels of symptoms

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PleiotropyPleiotropyOne gene has many symptoms One gene has many symptoms or controls several functionsor controls several functions

Photo © North Wind Picture Archives

Figure 5.5a

Figure 5.5b

Sickle cell anemia: Sickle cell anemia: autsomal recessiveautsomal recessive1:400 births of people 1:400 births of people from African descentfrom African descent

HH: normalHH: normalHh: no sickle cellHh: no sickle cellhh: sickle cellhh: sickle cell

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Genetic HeterogeneityGenetic Heterogeneity

• Different genes can produce identical phenotypes

• Genes may encode for different enzymes in a biochemical pathway

Clotting disorders

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PhenocopyPhenocopy Appears inherited but is caused by the environmentAppears inherited but is caused by the environment May have symptoms that resemble an inherited trait or May have symptoms that resemble an inherited trait or

occur within familiesoccur within familiesExamples:Examples: Exposure to teratogensExposure to teratogens

Thalidomide causes limb defects similar to inherited Thalidomide causes limb defects similar to inherited phocomeliaphocomelia

InfectionInfection AIDS virus can be passed from mother to child, looking AIDS virus can be passed from mother to child, looking

like it is inheritedlike it is inherited

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MitochondrionMitochondrion Organelle providing cellular energyOrganelle providing cellular energy Contains small circular DNA Contains small circular DNA No crossing over or DNA repairNo crossing over or DNA repair High exposure to free radicalsHigh exposure to free radicals Mutation rate is greater than nuclear DNA Mutation rate is greater than nuclear DNA 37 genes without noncoding sequences37 genes without noncoding sequences Mitochondrial genes are transmitted from mother to Mitochondrial genes are transmitted from mother to

all of her offspringall of her offspring

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Mitochondrial Mitochondrial InheritanceInheritance

Figure 5.8

Figure 5.7

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Mitochondrial DisordersMitochondrial Disorders

Mitochondrial myopathies – weak musclesMitochondrial myopathies – weak muscles

Ooplasmic transfer technique can enable Ooplasmic transfer technique can enable woman to avoid transmitting a woman to avoid transmitting a mitochondrial disordermitochondrial disorder

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HeteroplasmyHeteroplasmy

Many copies of the mitochondrial genome per Many copies of the mitochondrial genome per cell cell

May have more than one allele for the same May have more than one allele for the same gene in the same cellgene in the same cell

HeteroplasmyHeteroplasmy is the condition where is the condition where mitochondrial DNA sequence is not the same mitochondrial DNA sequence is not the same in all copiesin all copies