The Chromosomal Basis Of Inheritance
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The Chromosomal Basis Of Inheritance

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    The Chromosomal Basis Of Inheritance The Chromosomal Basis Of Inheritance Presentation Transcript

    • The Chromosomal Basis of Inheritance
    • Chromosome Theory of Inheritance
      • Developed by Walter Sutton & Theodor Boveri in 1902
        • “ Mendelian genes have specific loci on chromosomes, and it is the chromosomes that undergo segregation and independent assortment.”
    • Drosophila Melanogaster
      • Thomas Hunt Morgan, Columbia University
      • His experiments with fruit flies ( D. melanogaster) confirmed that Mendel’s “heritable factors” ( ) are located on chromosomes
      • Why fruit flies?
        • Quick-breeding (2 weeks)
        • Produce many offspring (hundreds!)
        • Have only 4 pairs of chromosomes
          • 3 pairs of autosomes, 1 pair of sex chromosomes
    • Morgan’s Experiments
      • Wild Type vs. Mutant Phenotypes
        • Wild type:
          • “ normal” phenotype for a character/trait
          • Examples: red eyes
          • Symbolized by w + (example)
        • Mutant phenotype:
          • traits that are alternative to the wild type
          • Example: white eyes
          • Symbolized by w (example)
        • Note:
          • Gene symbols are taken from the first mutant discovered
    • Sex-Linked Genes
      • Genes located on a sex chromosome (X or Y in humans) are called sex-linked genes
      • Morgan mated a white-eyed male with a red-eyed female
      • All F 1 offspring had red eyes 
        • Concluded that wild type (red eyes) is dominant
      • F 2 generation was 75% red:25% white (classic ratio)… but white-eye showed up in males ONLY
      • Morgan concluded that the gene for eye color is located only on the X chromosome
        • Since red is dominant ( w + ) over white (w), females would need 2 w + alleles to have white eyes, while males would need only one
    • Linked Genes
      • Definition:
        • Genes located on the same chromosome that tend to be inherited together in genetic crosses because they are part of a single chromosome that is passed along as a unit.
      • Thomas Hunt Morgan’s example:
        • Morgan observed that there were disproportionate numbers of wild-type (gray-normal wings), and double mutant (black-vestigial wings) flies among the offspring
          • These were the phenotypes of the original parents
          • This is because the genes for body color and wing size are located on the same chromosome in fruit flies and are therefore usually inherited together
      Wing Size Body Color vg = vestigial wings vg + = normal wings b = black b + = gray Mutant Wild Type
    • Genetic Recombination
      • Recombination of Unlinked Genes:
        • 50% frequency of recombination is observed for any 2 genes that are located on different chromosomes
        • Basis for Recombination: Random alignment of homologous chromosomes during metaphase I of meiosis
        • Parents: YyRr (yellow, round) and yyrr (green, wrinkled)
          • Some offspring are (yellow, wrinkled) and (green, round)…
          • This is recombination! 
    • Genetic Recombination
      • Recombination of Linked Genes:
        • Linked genes do NOT assort independently because they are located on the same chromosome and tend to move together through meiosis and fertilization
        • But how does genetic recombination occur at all then??
          • Crossing over between homologous chromosomes during prophase I of meiosis!
    • Genetic Maps
      • By studying recombination data, it is possible to create genetic maps
      • If two genes are far apart on a chromosome there is a higher probability that a crossover event will separate them than if the two genes are close together
      • Linkage maps show the sequence of genes along a chromosome
    • Chromosomal Basis of Sex in Humans
      • There are 2 varieties of sex chromosomes in mammals: X and Y
      • A person who inherits two X chromosomes (one from each parent)  female
      • A person who inherits one X chromosome and one Y chromosome  male
      • 50/50 chance either way!
    • Sex-Linked Disorders in Humans
      • In addition to determining sex, sex chromosomes contain many genes that determine traits that are unrelated to sex
      • Far more males have sex-linked genetic disorders… XY vs. XX
        • A female must inherit the recessive allele (if it is a recessive disorder) from BOTH parents in order to exhibit it
        • A male must only inherit the recessive allele from one of his parents in order to exhibit it
    • Sex-Linked Disorders in Humans
    • Sex-Linked Disorders in Humans
      • Color blindness
      • Duchenne muscular dystrophy
        • 1/3500 males in US
        • Progressive weakening of the muscles and loss of coordination
      • Hemophilia:
        • Sex-linked recessive trait defined by the absence of a certain protein required for blood clotting
        • Prolific in royal families of Europe (intermarriage)
    • Alterations of Chromosome Number
      • During meiosis, nondisjunction occasionally occurs
        • The members of a pair of a homologous chromosomes do not move apart properly during meiosis I OR
        • Sister chromatids fail to separate during meiosis II
      • The other chromosomes are usually distributed normally
    • Alterations of Chromosome Number
    • Alterations of Chromosome Number
      • Aneuploidy:
        • Abnormal chromosome number
        • Occurs when a gamete with one more or one less chromosome than normal unites with another, normal gamete
      • Trisomy:
        • Chromosome is present in triplicate (instead of duplicate)
        • 2n + 1
      • Monosomy:
        • Only one chromosome is present in each homologous pair (instead of 2)
        • 2n - 1
    • Alterations of Chromosomal Structure
      • Breakage of a chromosome can lead to 4 types of changes in chromosome structure:
        • Deletion:
          • chromosomal fragment lacking a centromere is lost during cell division
        • Duplication:
          • chromosomal fragment joins its homologous chromosome
    • Alterations of Chromosomal Structure
        • Inversion:
          • chromosomal fragment attaches to the original chromosome in the reverse orientation
        • Translocation:
          • chromosome fragment joins a nonhomologous chromosome
    • Human Disorders due to Chromosomal Alterations
      • Down Syndrome
        • Trisomy 21
          • 3 chromosomes for chromosome 21
        • Results in mental retardation, characteristic facial features, heart defects, etc.
        • 1/700 children in US
    • Human Disorders Due to Chromosomal Alterations
      • Klinefelter Syndrome
        • XXY
        • 1/2000 births
        • Male sex organs, but testes are small
        • Feminine body characteristics
      • Turner Syndrome
        • X-
        • Phenotypically female, but sex organs do not mature at adolescence