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1. Alternative versions of genes (alleles) account for variation in inherited characters

Genetics: Review. 1. Alternative versions of genes (alleles) account for variation in inherited characters. 2. For each character, an organism inherits two alleles, one from each parent. 3. If two alleles differ, one is dominant, the other recessive.

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1. Alternative versions of genes (alleles) account for variation in inherited characters

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  1. Genetics: Review 1. Alternative versions of genes (alleles) account for variation in inherited characters 2. For each character, an organism inherits two alleles, one from each parent 3. If two alleles differ, one is dominant, the other recessive 4. The two alleles for each character segregate (separate) during gamete production.

  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 Variations in Mendel’s Laws In incomplete dominance, F1 hybrids have an appearance in between the phenotypes of the two parents.

  3. Variations in Mendel’s Laws 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. Variations in Mendel’s Laws Multiple Alleles Blood Group (Phenotype) Red Blood Cells Genotypes Carbohydrate A IAIA or IAi A Carbohydrate B IBIB or IBi B AB IAIB ii O

  5. Variations in Mendel’s Laws • Pleiotropy is the impact of a single gene on more than one character. Pleiotropy Multiple traits (e.g., sickle-cell disease) Single gene

  6. Variations in Mendel’s Laws

  7. Variations in Mendel’s Laws *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. Polygenic inheritance Single trait (e.g., skin color) Multiple genes Variations in Mendel’s Laws • Polygenic inheritance is the additive effects of two or more genes on a single phenotype.

  9. 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 Variations in Mendel’s Laws

  10. Variations in Mendel’s Laws *Gene interactions

  11. Sex Linkage II III XY IV or XX *1909 Thomas Hunt Morgan *Sex chromosomes *Autosomes Example: In Drosophila and all mammals sex chromosomes designated as X and Y XX=female XY=male

  12. Sex Linkage Any gene located on a sex chromosome is called a sex-linked gene. • Most sex-linked genes are found on the X chromosome.

  13. Variations in Mendel’s Laws *Phenotype depends on environment and genes

  14. How do we account for genetic variation? Cross over: *Independent assortment *Crossing over *Random fertilization Independent Assortment:

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