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Variations to Mendel’s Laws
Extensions and Exceptions
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Alterations to Mendel’s Ratios
In these cases, genotypic ratio is as Mendel predicted but phenotypic ratio is altered Lethal allele combinations Multiple alleles Different dominance relationships Epistasis Penetrance and expressivity Pleiotropy Genetic heterogeneity Phenocopies
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Lethal Allele Combinations
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Multiple Alleles Give a range of phenotypes
Each of us has 2 alleles for any given gene 1 from mom and 1 from dad There can be many different alleles for a gene Different alleles are formed by mutations
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Example of Multiple Alleles -PKU
Phenylketonuria (PKU) Enzyme that breaks down phenylalanine is deficient Phenylalanine accumulates There are hundreds of possible alleles Allelic combinations give rise to different phenotypes Severe mental retardation Moderate PKU Mild PKU Asymptomatic PKU
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Different Dominance Relationship –Incomplete dominance
The heterozygous phenotype is intermediate between the homozygous dominant and the homozygous recessive Blended phenotype
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Different Dominance Relationship –codominance
Phenotypes of both alleles are expressed
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Codominance –Blood Type
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Codominance –Blood Type
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Epistasis One gene affects the expression of another
Example: If a dog has the hairless gene, the genes that affect hair color will not be expressed
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Penetrance and Expressivity
Describe degrees of inheritance Due to multifactorial inheritance Expression of a gene is influenced by other genes and by environment Penetrance =all-or-none expression of a gene 100% of the people who inherit mutant amyloid precursor protein (mAPP) develop Alzheimer disease so mAPP is 100% penetrant Expressivity =severity of gene expression Polydactyly has variable expressivity Some have an extra toe and others have an extra toe and an extra finger.
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Pleiotropy Gene affects several functions Example: Porphyria variegata
Phenotype is varied Example: Porphyria variegata
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Genetic Heterogeneity
Different genes produce the same phenotype Example: Hearing loss may be due to one of 132 different genes that follow autosomal recessive inheritance B, b =gene for hearing loss type 1 R, r =gene for hearing loss type 2 BBrr bbRR BbRr deaf deaf NOT deaf
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Phenocopy An environmentally caused trait that appears to be inherited
Example: Phocomelia is a rare genetic disorder whose effects are mimicked by the teratogen, thalidomide Example: AIDs transmission from mother to offspring
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Mitochondrial Genes
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Mitochondrial Inheritance Pattern
Mitochondrial genes are passed from mothers to offfspring. Only females pass on the genes
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The 37 Mitochondrial Genes
24 encode proteins important for protein synthesis Mutations can have devastating effects 13 encode proteins needed for energy production Mutations often affect skeletal muscle and cause fatigue
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Heteroplasmy A mutation can occur in one mitochondrial DNA ring and not another. When the mitochondria divide, different batches of daughter mitochondria are produced (some with the mutation, some without) It is therefore possible to have mutant mitochondrial DNA in some tissues but not others Causes variation is expressivity of a mitochondrial disease depending on which tissues/organs have cells with mutated mitochondrial DNA
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Linkage Two genes on the same chromosome may “co-segregate”
Example: Dihybrid cross of pea plants with purple flowers (Pp) and long pollen grains (Ll)
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Figure 5.10 Parents P p P L p l L l Genotype PpLl Genes not linked
Genes linked Self-cross Self-cross Figure 5.10
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Parents F1 P p P L p l L l Genotype PpLl Genes not linked
Genes linked Self-cross Self-cross F1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Parents F1 P p P L p l L l Genotype PpLl Genes not linked
Genes linked Self-cross Self-cross F1 Female gametes PL Pl pL pl Female gametes PL pl PL PL Male gametes Pl pl Male gametes pL pl
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Parents F1 P p P L p l L l Genotype PpLl Genes not linked
Genes linked Self-cross Self-cross F1 Female gametes PL Pl pL pl Female gametes PL pl PPLL PPLl PpLL PpLl PPll Ppll ppLL ppLl ppll Male gametes PL Pl pL pl Male gametes PL pl PPLL PpLl ppll
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Parents F1 Phenotypic ratio 3: Phenotypic ratio 9:3 P p P L p l L l
Genotype PpLl Genes not linked Genotype PpLl Genes linked Self-cross Self-cross F1 Female gametes PL Pl pL pl Female gametes PL pl PPLL PPLl PpLL PpLl Male gametes PL Pl pL pl Male gametes PL pl PPLL PpLl ppll PPLl PPll PpLl Ppll PpLL PpLl ppLL ppLl Phenotypic ratio 3: PpLl Ppll ppLl ppll Phenotypic ratio 9:3
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Parents F1 P p P L p l L l Genotype PpLl Genes not linked
Genes linked Self-cross Self-cross F1 Female gametes PL Pl pL pl Female gametes PL pl PPLL PPLl PpLL PpLl Male gametes PL Pl pL pl Male gametes PL pl PPLL PpLl ppll PPLl PPll PpLl Ppll PpLL PpLl ppLL ppLl Phenotypic ratio 3: PpLl Ppll ppLl ppll Phenotypic ratio 9:3:3
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Parents F1 Phenotypic ratio 9:3:3:1 P p P L p l L l Genotype PpLl
Genes not linked Genotype PpLl Genes linked Self-cross Self-cross F1 Female gametes PL Pl pL pl Female gametes PL pl PPLL PPLl PpLL PpLl Male gametes PL Pl pL pl Male gametes PL pl PPLL PpLl PPLl PPll PpLl Ppll PpLl ppll PpLL PpLl ppLL ppLl Phenotypic ratio 3:1 PpLl Ppll ppLl ppll Phenotypic ratio 9:3:3:1
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Crossing Over May Disrupt Linkage
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Linkage Maps The farther apart 2 genes are, the more likely their linkage will be disrupted during crossing over % recombination tells us the relative location of the genes
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