chapter 5a lecture

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Chapter 5Chapter 5

Extensions and Extensions and Exceptions to Exceptions to

Mendel’s Laws Mendel’s Laws


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Exceptions to Mendel’s LawExceptions to Mendel’s Law

Mendel’s traits showed two distinct forms Mendel’s traits showed two distinct forms Most genes do not exhibit simple inheritanceMost genes do not exhibit simple inheritance Genotypic ratios persist but phenotypic ratios may Genotypic ratios persist but phenotypic ratios may

vary because of interactions betweenvary because of interactions between AllelesAlleles Other genesOther genes Non-nuclear genesNon-nuclear genes Segregation of genes on same chromosomeSegregation of genes on same chromosome EnvironmentEnvironment


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Lethal Alleles Lethal Alleles Some allele combinations are lethalSome allele combinations are lethal

Figure 5.1b


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Multiple AllelesMultiple Alleles An individual carries two alleles for each gene An individual carries two alleles for each gene A population can have many alleles among the individual membersA population can have many alleles among the individual members ExamplesExamples

Four basic phenotypes: Four basic phenotypes:

Classic PKU with severe mental retardationClassic PKU with severe mental retardationmoderate PKUmoderate PKUmild PKUmild PKUasymptomatic PKU, phenylalanine is excreted into the urineasymptomatic PKU, phenylalanine is excreted into the urine

PKU gene has over 300 alleles resulting in four basic phenotypesPKU gene has over 300 alleles resulting in four basic phenotypes

Genes can mutate in many ways in their DNA sequenceGenes can mutate in many ways in their DNA sequence


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Incomplete DominanceIncomplete DominanceThe heterozygous phenotype is distinct from either homozygous phenotype It may be an intermediate phenotype


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Incomplete DominanceIncomplete Dominance• Humans: hypercholesterolemia: gene determines the number of cell membrane receptors that bring cholesterol into the cell

Figure 5.2


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Codominant Codominant AllelesAlleles

Figure 5.3


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Codominant AllelesCodominant AllelesBoth alleles are expressed in the heterozygotesBoth alleles are expressed in the heterozygotes

Example:Example:

The ABO gene encodes a cell surface protein The ABO gene encodes a cell surface protein A allele produces A antigenA allele produces A antigen B alleles produce B antigenB alleles produce B antigen O allele does not produce antigensO allele does not produce antigens A and B alleles are dominant over OA and B alleles are dominant over O Alleles A and B are codominantAlleles A and B are codominant


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Offspring from Parents Offspring from Parents with Blood Type A and Blood Type with Blood Type A and Blood Type

B B

Figure 5.4


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EpistasisEpistasis

One gene affects the action of a second gene

Example: H gene is epistatic to the ABO gene.• H protein attaches a molecule to the cell surface to which the A or B antigens are attached• hh genotype = no H protein• Without H protein the A or B antigens can not be attached to the cell• All hh genotypes have the phenotype of type O, although the ABO blood group can be anything (A, B, AB, or O)


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EpistasisEpistasis

One gene affects the action of a second geneWithout H protein the A or B antigens can not be attached to the cell• All hh genotypes have the phenotype of type O, although the ABO blood group can be anything (A, B, AB, or O)

Genotype PhenotypehhAB OHhAB ABHhOO O

chapter 5a lecture

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