Chromosomal Inheritance


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Chromosomal Inheritance

  1. 1. Chromosomal Inheritance
  2. 2. Chromosomal Theory of Inheritance <ul><li>Chromosomes contain the units of heredity (genes) </li></ul><ul><li>Pair chromosomes segregate during meiosis, each sex cell has half of the number of chromosomes found in a somatic cell. (Mendel’s law of segregation) </li></ul><ul><li>Chromosomes assort independently during meiosis (Mendel’s law of independent assortment) </li></ul><ul><li>Each chromosome contain many different genes </li></ul>
  3. 3. Sex Chromosomes <ul><li>Sex chromosomes (X and Y) vs. autosomes (chromosomes 1-22), Sex cells and somatic cells. </li></ul><ul><li>Homogametic sex -- that sex containing two like sex chromosomes. In most animals species these are females (XX). Each egg only contain one X chromosome. </li></ul><ul><li>Heterogametic sex --- that sex containing two different sex chromosomes . In most animal species these are XY males. Each sperm will contain either an X or Y. Therefore the father determines whether the offspring is a boy or a girl (50/50 chance) </li></ul>
  4. 4. Sex Linkage <ul><li>X A = Locus on X chromosome </li></ul><ul><li>XX females </li></ul><ul><ul><li>X A X A , X a X a - homozygotes </li></ul></ul><ul><ul><li>X A X a – heterozygote (carrier) </li></ul></ul><ul><li>XY male </li></ul><ul><ul><li>X A Y, X a Y </li></ul></ul><ul><ul><li>no carriers in males, therefore they are more susceptible to x-linked traits. </li></ul></ul>
  5. 5. Red/white eye color in Drosophila <ul><li>In females: </li></ul><ul><ul><li>X R X R , X R X r = red-eye female </li></ul></ul><ul><ul><li>X r X r = white-eyed females </li></ul></ul><ul><li>In males: </li></ul><ul><ul><li>X R Y = red-eye male </li></ul></ul><ul><ul><li>X r Y = white-eyed male </li></ul></ul>
  6. 6. Examples of Sex Linked Traits <ul><li>Hemophilia - Recessive </li></ul><ul><li>Red-Green Color Blindness - Recessive </li></ul><ul><li>Muscular Dystrophy - Recessive </li></ul><ul><li>Fragile X syndrome - Dominant </li></ul>
  7. 7. Nondisjunction <ul><li>abnormal number of autosomal chromosomes when chromosomes fail to separate during replication. </li></ul><ul><li>2n – 1 = monosomic </li></ul><ul><li>2n + 1 = trisomic </li></ul>
  8. 8. Nondisjunction
  9. 9. Nondisjunction - Examples <ul><li>Down's -- trisomy 21 mean life expectancy 17 years. Short in stature, round face and mental retardation </li></ul><ul><li>Patau's -- trisomy 13 mean life expectancy 130 days </li></ul><ul><li>Edward's --- trisomy 18 mean life expectancy a few weeks </li></ul>
  10. 10. Sexual Determination - Males <ul><li>Single Y = male, so XXY, XYY, XXXY all male </li></ul><ul><li>Klinefelter Syndrome – XXY or XXXY. Male due to Y chromosome, Testes and prostrate underdeveloped, some breast formation, no pubic or facial hair, subnormal intelligence. </li></ul><ul><li>Jacob’s Syndrome – XYY. Males are usually taller than average, and tend to have speech and reading problems </li></ul>
  11. 11. Sexual Determination - Females <ul><li>Turner’s Syndrome – X0. Female with bull neck, short stature, nonfunctional ovaries, no puberty </li></ul><ul><li>Metafemale – 3 or more X chromosomes. No apparent physical abnormality except menstrual irregularities. </li></ul>
  12. 12. Chromosomal Mutation <ul><li>Permanent change in chromosome structure. </li></ul><ul><li>Caused by exposure to radiation, organic chemicals, viruses, replication mistakes. </li></ul><ul><li>Only mutations in sex cells are passed onto the next generation. </li></ul>
  13. 13. Structural Changes in Chromosomes <ul><li>Inversion – occurs when a chromosome segment turns around 180 degrees. </li></ul><ul><li>Translocation – is movement of chromosomal segments to another non-homologous chromosome </li></ul><ul><li>Deletion – occurs when a portion of the chromosome breaks off. </li></ul><ul><li>Duplication – when a portion of a chromosome repeats itself. </li></ul>
  14. 14. Deletions and Duplications
  15. 15. Inversions and Translocations
  16. 16. Linkage <ul><li>When genes are on the same chromosome, they are called linked. They can show departures from independent assortment </li></ul><ul><li>If genes on the same chromosome are sufficiently far apart, they can segregate independently through crossing over. </li></ul>
  17. 17. Gene Mapping <ul><li>By studying cross-over (recombination) frequencies of linked genes, a chromosomal map can be constructed </li></ul><ul><ul><li>Distant genes are more likely to be separated by crossing-over than genes that are closer together. </li></ul></ul><ul><ul><li>Each 1% of recombination frequency is equivalent to 1 map unit </li></ul></ul>
  18. 18. Crossing Over Produces Recombinations
  19. 19. Constructing Gene Maps <ul><li>Crossing over frequencies can be used to construct gene maps. For example, </li></ul><ul><ul><li>Crossing over frequency of genes A and B is 3%, genes B and C is 9% and genes A and C is 12%. </li></ul></ul><ul><li>3 mu 9 mu </li></ul><ul><li>A B C </li></ul>
  20. 20. Human Genome Project <ul><li>Map of the all of the genes on the human chromosomes. </li></ul>
  21. 21. Pedigree Analysis
  22. 22. Modes of Inheritance <ul><li>Autosomal dominant allele [e.g., Huntington's Disease, brown eyes] </li></ul><ul><ul><li>A phenotype associated with an autosomal dominant allele will, ideally, be present in every individual carrying that allele. It will be present in close to 50% of the individuals. </li></ul></ul><ul><ul><li>Affected children usually have affected parents </li></ul></ul><ul><ul><li>Two affected parents can produce an unaffected child </li></ul></ul><ul><ul><li>Both males and females are affected equally. </li></ul></ul>
  23. 23. Modes of Inheritance <ul><li>Autosomal recessive alleles [silent carriers] </li></ul><ul><ul><li>albinism, cystic fibrosis, certain types of hemophilia, Tay-Sachs disease, PKU, blue eyes. </li></ul></ul><ul><ul><li>A pedigree following a trait associated with an autosomal recessive allele is often marked by a skipping of generations. That is, children may express a trait which their parents do not. </li></ul></ul><ul><ul><li>In such a situation, both parents are heterozygotes, also known as silent carriers . </li></ul></ul><ul><ul><li>Close relatives who reproduce are more likely to have affected children. </li></ul></ul><ul><ul><li>Both males and females will be affected with equal frequency </li></ul></ul><ul><ul><li>A low number of individuals normal affected </li></ul></ul>
  24. 24. Modes of Inheritance <ul><li>Sex-linked dominant alleles [sex linkage] </li></ul><ul><ul><li>A sex linked dominant allele has a variation on the pattern displayed by autosomal dominant alleles. That is: </li></ul></ul><ul><ul><ul><li>one-half of the offspring of an afflicted heterozygote female will be similarly afflicted (gender independent). </li></ul></ul></ul><ul><ul><ul><li>only the female progeny of males will be afflicted (because the male donates an X chromosome to his female progeny). </li></ul></ul></ul><ul><ul><li>As with any sex-linked allele , males can pass the allele only on to their daughters, not their sons. </li></ul></ul>
  25. 25. Mode of Inheritance <ul><li>Sex-linked recessive alleles </li></ul><ul><ul><li>red-green color blindness, certain types of hemophilia. </li></ul></ul><ul><ul><li>More males affected than females </li></ul></ul><ul><ul><li>An affected son can have parents who have the normal phenotype </li></ul></ul><ul><ul><li>For a female to have the characteristic, her father must also have it and the mother must be a carrier. </li></ul></ul><ul><ul><li>If a woman has the characteristic all her sons will have it </li></ul></ul><ul><ul><li>The characteristic often skips a generation from grandfather to grandson </li></ul></ul>