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Chapter 7: Extending Mendelian Genetics Section 1: Chromosomes & Phenotype 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 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 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 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 Section 2: Complex Patterns of Heredity 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 • 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 • 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 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 Section 4: Human Genetics & Pedigrees 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 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 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 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 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 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