BMS100 Chapter 20 Outline - Genetics
  1. genes
    1. genes = discrete segments of DNA molecules
    2. genes dictate the structure and function of cells by controlling production of enzymes and other proteins
    3. each inherited trait is controlled by two genes, one from each parent
    4. dominant alleles (genes) - have effects that appear in the offspring
    5. recessive alleles (genes) - have effects that are masked by a dominant gene (effects are nonexistent or do not appear in the offspring)
    6. example: albinism
      1. dominant gene - A - causes normal melanin (pigment) production (prevents albinism)
      2. recessive gene - a - does not contain the correct code for melanin production
      3. genotype - the combination of genes (alleles)
      4. with two genes, there are three possible genotypes
        • AA (homozygous dominant)
        • Aa (heterozygous)
        • aa (homozygous recessive)
      5. In cases of complete dominance, only one (1) dominant gene is required to produce the dominant phenotype.
      6. phenotype - the appearance or function of the individual
      7. in cases of complete dominance, there are two possible phenotypes
        • dominant phenotype (normal skin color in this case)
        • recessive phenotype (albino in this case)
      8. "carrier" individuals: genotype is heterozygous; phenotype is "normal" (recessive gene is masked by dominant gene)
      9. If two carriers of albinism produce children, what is the probability that a child will be an albino? (Use Punnet square.)
    7. cystic fibrosis (CF) - pp. 542-543
      1. dominant gene codes for fully functional "pump" that transports chloride ions out of cells (necessary for normal consistency of mucus)
      2. recessive gene codes for defective "pump" - mucus is too "thick" - causes breathing and digestive difficulties, respiratory infections
      3. If one parent is a CF carrier, and the other is a normal, non-carrier...
        • What is the probability of having a child with CF?
        • What is the probability of having a child who does not have CF?
        • What is the probability of having a child who is a carrier?

  2. codominance - example: blood types (A, B, AB, O)
    1. dominant genes: IA, IB
    2. recessive gene: i
    3. genotypes and corresponding phenotypes
      • IAIA or IAi - type A
      • IBIB or IBi - type B
      • IAIB - type AB
      • ii - type O
    4. What blood types are possible among the children of a type O mother and a type A father who is heterozygous?
    5. What blood types are possible among the children of a type A mother and a type B father if both parents are heterozygous?

  3. karyotypes / gametes / sex determination
    1. each chromosome is a coiled mass of chromatin (DNA)
    2. each cell contains 46 chromosomes, arranged into numbered pairs of decreasing size
      1. autosomes - 22 pairs (within pair, appear nearly identical to each other)
      2. sex chromosomes - 1 pair (may or may not match)
    3. female karyotype: 22 pairs of autosomes and one pair of X chromosomes
    4. male karyotype: 22 pairs of autosomes and one X chromosome and one Y chromosome
      • Y chromosome normally contains "SRY," a dominant gene that causes an embryo to develop as a male
    5. meiosis - nuclear division that produces gametes (sperm and ova)
    6. during meiosis, each member of a chromosome pair is directed to a different daughter cell
    7. each gamete contains 22 autosomes plus one sex chromosome
      1. 100% of the ova produced contain 22 autosomes plus one X
      2. 50% of the sperm produced contain 22 autosomes plus one X chromosome ("X sperm")
      3. the other 50% of the sperm produced contain 22 autosomes plus one Y chromosome ("Y sperm")
    8. at fertilization: sperm (22 autos. + 1 sex chromo.) + ovum (22 autos. + 1 sex chromo.) >>> zygote (22 pair autos. + 1 pair sex chromosomes)
      1. "X sperm" + egg ---> 22 pairs of autosomes + 2X (female)
      2. "Y sperm" + egg ---> 22 pairs of autosomes + X + Y (male)

  4. sex-linked genes
    1. Y chromosome normally contains dominant "SRY" gene
    2. X chromosome contains many genes (for color vision, blood clotting, etc.)
    3. example: genotypes and phenotypes in color vision
      • XC = X chromo. with dominant gene (supports color vision)
      • Xc = X chromo. with recessive gene (does not support color vision)
      • genotypes and phenotypes for X-linked color blindness:
        • XCXC - female (normal vision, not a carrier)
        • XCXc - carrier female, normal vision
        • XcXc - color-blind female (why are color-blind females so rare?)
        • XCY - male with normal color vision
        • XcY - color-blind male
    4. If a mother is a carrier of an X-linked recessive gene, and the father has normal color vision, what genotypes and phenotypes are possible among their children?

  5. autosomal dominant conditions
    1. caused by a dominant gene
    2. example: Huntington's disease (loss of coordination, deterioration of mental functions)
    3. typically begins near age 40 & causes death an average of 16 years after onset
    4. If a person is heterozygous for HD, and their mate is free of the gene, what are the chances of having a child who will eventually have HD?
    5. Can HD "skip a generation," as can CF and albinism?
Chapter 20 Study Questions - 22, 23, 24, 25, 38, 39, 40
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Fall 2009