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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

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)

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.

4.3 Incomplete (partial) dominance is a condition when neither allele is dominant over the other

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

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

+ - - + - + - + + + + + - - - +

4.5 Multiple alleles of a gene may exist in a population

Bombay Phenotype

More than 100 alleles

4.6 Lethal Alleles Represent Essential Genes

Recessive Lethal Mutations

Dominant Lethal Mutations Huntington’s disease typical age of onset is about 40 causes gradual nervous and motor degeneration

4.7 Combinations of Two Gene Pairs With Two Modes of Inheritance Modify the 9:3:3:1 Ratio

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

Epistasis causes deviation from Mendelian dihybrid phenotypic ratios

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

4.9 Complementation Analysis Can Determine if Two Mutations Causing a Similar Phenotype are Alleles

4.10 Expression of a Single Gene May Have Multiple Effects

Pleiotropy

4.11 X-Linkage Describes Genes on the X Chromosome

X-Linkage in Drosophila

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

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

Variable expressivity as illustrated among eyeless homozygous mutants in Drosophila.

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.

4.13 Genetic Background and the Environment May Alter Phenotypic Expression

Temperature Effects: An Introduction to Conditional Mutations Temperature-sensitive mutation in Siamese cats

3 wks old 11 wks old 5 months old

a Himalayan rabbit and a Siamese cat show dark fur color on the muzzle, ears, and paws

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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