Chapter 7: Extending Mendelian Genetics

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Chapter 7 Extending Mendelian Genetics
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Section 1 Chromosomes Phenotype
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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

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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

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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 theyre
  recessive

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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

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Section 2 Complex Patterns of Heredity
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Complex Patterns

• Incomplete dominance - heterozygous individual
  shows a blending of the two homozygous phenotypes
• Ex. Four oclock plants
• RR red flowers
• rr white flowers
• Rr pink flowers

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• 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

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• 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

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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

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Section 4 Human Genetics Pedigrees
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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 Mendels peas
• Ex. widows peak, Huntingtons disease

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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

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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

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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

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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

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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
• Turners syndrome female only has 1 X
• Klinefelters syndrome male has XXY
• Caused by non-disjunction, which is when
  chromosomes fail to separate during meiosis