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Chapter 4: Modification of Mendelian Ratios

Chapter 4: Modification of Mendelian Ratios. Allele. *Wild-type allele. *Mutant allele. Conventional symbols for alleles: recessive allele - initial letter of the name of the recessive trait, lowercased and italicized dominant allele - same letter in uppercase . P Generation.

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Chapter 4: Modification of Mendelian Ratios

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  1. Chapter 4: Modification of Mendelian Ratios Allele *Wild-type allele *Mutant allele Conventional symbols for alleles: recessive allele - initial letter of the name of the recessive trait, lowercased and italicized dominant allele - same letter in uppercase

  2. P Generation White Red RR rr Gametes r R F1 Generation Pink Rr Gametes r R Sperm F2 Generation r R R RR Rr Eggs 1 1 1 1 1 1 r rr Rr 2 2 2 2 2 2 In incomplete dominance, F1 hybrids have an appearance in between the phenotypes of the two parents.

  3. Hypercholesterolemia • Dangerously high levels of cholesterol in the blood. • Is a human trait that is incompletely dominant. • Heterozygotes have blood cholesterol levels about 2X normal. • Homozygotes have blood cholesterol levels about 5X normal. Hh HH hh Homozygous for ability to make LDL receptors Heterozygous Homozygous for inability to make LDL receptors GENOTYPE LDL LDL receptor PHENOTYPE Cell Normal Mild disease Severe disease

  4. Codominance: Condition in which the phenotypic effects of a genes alleles are fully and simultaneously expressed in the heterozygote. Example: MN Blood group - red blood cells contain a transmembraneglycoprotein (glycophorin); two different forms of this protein exist, M and N

  5. Multiple Alleles Blood Group (Phenotype) Examples: *Table 4.1: over 100 alleles at a given locus in Drosophila *ABO Blood group in humans Red Blood Cells Genotypes Carbohydrate A IAIA or IAi A Carbohydrate B IBIB or IBi *Characterized by the presence of glycoprotein antigens on the surface of red blood cells *Distinct from the M and N antigens *Also exhibits codomiance B AB IAIB ii O

  6. Lethal Alleles: Example: Coat color in mice *A = agouti = wild-type allele *AY = yellow = mutant allele

  7. Epistasis Example: In Drosophila, gene:eyeless *Black (B) is dominant to brown (b) *Second gene responsible for allowing pigment to be deposited in hair, C = presence c = absence (colorless)

  8. *Gene product interactions

  9. Pleiotropy is the impact of a single gene on more than one character. Pleiotropy Multiple traits (e.g., sickle-cell disease) Single gene

  10. Polygenic inheritance Single trait (e.g., skin color) Multiple genes • Polygenic inheritance is the additive effects of two or more genes on a single phenotype.

  11. P Generation aabbcc (very light) AABBCC (very dark) F1 Generation AaBbCc AaBbCc Sperm F2 Generation 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 Eggs 1 1 20 15 15 6 6 64 64 64 64 64 64 64

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