The Chromosomal Basis Of Inheritance


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The Chromosomal Basis Of Inheritance

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