Chapter 7: Extending Mendelian Genetics

1. Chapter 7: Extending Mendelian Genetics

2. Section 1: Chromosomes & Phenotype

3. Autosomal Genes & Phenotype • Phenotype is determined by 2 copies of each autosomal gene • Mendel studied autosomal characteristics • Most traits in humans are determined by autosomal genes

4. Autosomal Disorders • Many genetic disorders are caused by recessive alleles on autosomes • NN: healthy • Nn: healthy; carrier for the disorder • nn: has disorder • Less genetic disorders are caused by dominant alleles on autosomes • NN and Nn: has disorder • nn: healthy

5. Sex-Linked Traits • Genes located on the sex chromosomes (X or Y) are called sex-linked genes • Genes on the Y chromosome are responsible for male characteristics only • X chromosome has many more genes that affect many traits • In males, all sex-linked traits are expressed • they only have one copy of each chromosome, so all alleles are expressed even if they’re recessive

6. X Chromosome Inactivation • Since females have 2 X chromosomes, one of them is randomly turned off in each cell • Because it is random, a female will have some cells with one X turned off, and others with the other X turned off

7. Section 2: Complex Patterns of Heredity

8. Complex Patterns • Incomplete dominance - heterozygous individual shows a blending of the two homozygous phenotypes • Ex. Four o’clock plants • RR = red flowers • rr = white flowers • Rr = pink flowers

9. Codominance - both alleles are dominant, so both are fully and separately expressed • Ex. Human ABO blood type - has multiple alleles (IA, IB, & i) • IAIA or IAi = type A blood • IBIB or IBi = type B blood • IAIB = type AB blood • ii = type O blood

10. Polygenic traits - traits produced by two or more genes, leading to a range of phenotypes • Ex. human skin tone, human eye color • In a polygenic trait, one gene may be epistatic, meaning it can interfere with the expression of the other genes • Ex. albinism in mammals

11. Genes & the Environment • Phenotype is usually a mixture of genes & the environment • Ex. genes determine your underlying skin tone & hair color, but sunlight can cause your skin to become darker and hair to become lighter

12. Section 4: Human Genetics & Pedigrees

13. Human Genetics • Humans follow the same basic patterns of heredity • The inheritance of many human traits can be very complex • Polygenic traits, the environment, etc… • Some human traits are single-gene traits that follow simple dominant and recessive patterns like Mendel’s peas • Ex. widow’s peak, Huntington’s disease

14. X and Y Chromosomes • X chromosome is 3x larger than the Y & has many more genes • Since females have 2 X chromosomes, they can be carriers for sex-linked traits • Sex-linked disorders are more common in males since they only have one X chromosome

15. Pedigree • Circle = female • Square = male • Shaded = person with trait • Unshaded = person without trait • Half shaded = carrier for a genetic disorder • Horizontal line = marriage • Vertical line & bracket = offspring • For some individuals in a pedigree, you can determine genotypes based on their phenotypes

16. Pedigrees: Tracing Autosomal Traits • If trait is recessive, shaded individuals would be nn, unshaded would be NN or Nn, carriers would be Nn • If trait is dominant, unshaded would be nn, shaded would be either NN or Nn

17. Pedigrees for sex-linked traits • Shaded females must be homozygous recessive XnXn • Unshaded females could be XNXN or XNXn - carrier • Shaded males must have the recessive allele XnY • Unshaded males must have the dominant allele XNY

18. Karyotype • Picture of all the chromosomes in a cell (see pg. 217) • Can show gender as well as any chromosomal abnormalities • Down syndrome: extra chromosome #21 • Turner’s syndrome: female only has 1 X • Klinefelter’s syndrome: male has XXY • Caused by non-disjunction, which is when chromosomes fail to separate during meiosis