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Extensions of Mendelian Genetics
Chapter 4 Lecture
Concepts of Genetics Tenth Edition
Factors that cause deviation from normal monohybrid and dihybrid ratios: X-linkage lethal alleles codominance and incomplete dominance multiallelism epistasis genetic linkage environmental conditions epigenetics segregation distortion
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4.1 Alleles Alter Phenotypes in Different Ways New alleles are produced by mutation. The solution to understanding the laws of heredity were found by finding and studying mutations. • wild-type vs. mutant • loss-of-function mutation (e.g. albinism; tyrosinase mutation) • gain-of-function mutation (e.g. lactose tolerance; lactase persistence) • neutral mutation (non-coding, silent mutation)
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4.2 Geneticists use a variety of symbols for Alleles The initial letter of the name of a recessive trait, lowercased and italicized, denotes the recessive allele. The same letter in uppercase refers to the dominant allele. d, dwarf; D, tall in the garden pea example. Wild type= + mutant= initials of the name of the mutant trait examples: +/+; e+/e; +/e; e/e. e stands for ebony body color.
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4.3 Incomplete (partial) dominance is a condition when neither allele is dominant over the other
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occurs when one allele does not mask the phenotype of the other allele, and the phenotype of the heterozygote is intermediate Example: flower color in snapdragons, Antirrhinum
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4.4 In codominance, the influence of both alleles in a heterozygote Is clearly evident
(There’s no “blending” of phenotypes)
Codominance Occurs when phenotypes of both alleles are expressed Simple ratios in monohybrid F2 because phenotype=genotype 1:2:1 Classic example is the ABO blood group system • gene encodes surface antigens on red blood cells • antigen = molecule recognized by immune system antibody • three alleles exist, IA and IB are codominant and IO is recessive to both
Codominance and multiple alleles
ABO blood groups
Genotype Antigen Phenotype Antibodies in serum IAIA, IAIO A A Ab against B IBIB, IBIO B B Ab against A IAIB A, B AB No Ab IOIO Neither O Ab against A&B
Donor blood type (red cells) Recipient
A B AB O
A B AB O
+ - - + - + - + + + + + - - - +
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4.5 Multiple alleles of a gene may exist in a population
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Bombay Phenotype
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More than 100 alleles
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4.6 Lethal Alleles Represent Essential Genes
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Recessive Lethal Mutations
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Dominant Lethal Mutations Huntington’s disease typical age of onset is about 40 causes gradual nervous and motor degeneration
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4.7 Combinations of Two Gene Pairs With Two Modes of Inheritance Modify the 9:3:3:1 Ratio
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4.8 Phenotypes Are Often Affected by More Than One Gene
One gene masks or modifies the effect of another gene in epistasis. Epistasis causes deviation from Mendelian phenotypic ratios. Let’s mate a AB blood type male Bombay carrier with an AB blood type female Bombay carrier. Expectation of blood types if did not consider Bombay locus (H)=1:2:1
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Epistasis causes deviation from Mendelian dihybrid phenotypic ratios
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Examples of modified dihybrid ratios
Two locus interactions may show: dominant epistasis: a dominant allele at one locus masks the expression of the alleles at a second locus Pea flower color: 9:7 recessive epistasis: a recessive genotype at one locus masks the expression of the alleles at a second locus Mouse coat color: 9:3:4 novel phenotypes: can emerge when two loci act simultaneously to produce a trait mouse coat color 9:3:4 Squash shape: 9:6:1
Note: these ratios are just examples, and can vary depending on the traits in question
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4.9 Complementation Analysis Can Determine if Two Mutations Causing a Similar Phenotype are Alleles
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4.10 Expression of a Single Gene May Have Multiple Effects
Pleiotropy
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4.11 X-Linkage Describes Genes on the X Chromosome
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X-Linkage in Drosophila
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4.12 In Sex-Limited and Sex-Influenced Inheritance, an Individual’s Sex Influences the Phenotype
Sex-limited traits are found in one sex only.
Sex-influenced traits show up in both sexes, but expression of such traits may differ between the two sexes.
Genotype Phenotype Female Male HH Hen-feathered Hen-feathered Hh Hen-feathered Hen-feathered hh Hen-feathered Cock-feathered
Genotype Phenotype Female Male BB Bald Bald Bb Not bald Bald bb Not bald Not bald
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Penetrance is the percentage of individuals that show at least some degree of expression of a mutant genotype. Retinoblastoma, the most malignant form of eye cancer, arises from a dominant mutation of one gene, but only 75% of people who carry the mutant allele develop the disease.
Expressivity reflects the range of expression of a mutant genotype. For instance, one or both eyes may be affected in retinoblastoma.
The same genotype does not always produce the same phenotype
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Variable expressivity as illustrated among eyeless homozygous mutants in Drosophila.
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Genetic suppression: mutant alleles of Gene 2 suppresses the expression of the mutant phenotype of Gene 1. Positional effect: the physical location of a gene in relation to other genetic material may influence its expression.
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4.13 Genetic Background and the Environment May Alter Phenotypic Expression
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Temperature Effects: An Introduction to Conditional Mutations Temperature-sensitive mutation in Siamese cats
3 wks old 11 wks old 5 months old
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a Himalayan rabbit and a Siamese cat show dark fur color on the muzzle, ears, and paws
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Genomic (or parental) imprinting: a condition where the expression of a trait depends on whether the trait has been inherited from a male or a female parent. Imprinting is a heritable phenotype but is not controlled by the genotype of the DNA (epigenetic)
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