AP Bio Ch. 15, part 1


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AP Bio Ch. 15, part 1

  1. 1. Linked Genes, X-linked inheritance, and Mutations Ch. 15
  2. 2. Genes are on Chromosomes • This is obvious to us, but it has only been known for a few decades
  3. 3. Chromosome Theory of Inheritance • The work of scientists in the early 1900s • Says 2 things: 1. Genes occupy specific loci (positions on chromosomes) 2. Chromosomes undergo segregation and independent assortment during meiosis
  4. 4. Thomas Hunt Morgan and his fruit flies • The first solid evidence associating a specific gene with a a specific chromosome came from Thomas Hunt Morgan, an embryologist • Morgan’s experiments with fruit flies provided convincing evidence that chromosomes are the location of Mendel’s heritable factors
  5. 5. Fruit flies were a good choice… • They breed at a high rate • A generation can be bred every two weeks • They have only four pairs of chromosomes
  6. 6. What Morgan did… • He spent a year looking for variant individuals, those that differed from the normal, or “wild” phenotype • Traits alternative to the wild type are called mutant phenotypes
  7. 7. He found a white eyed male… • He mated it to a “wild” type red eyed female and got all red eyed offspring in the F1 generation • What does that tell us? • He let the F1s mate, and got the classic 3:1 ratio of red:white eyes, but only in males • What does that tell us? • Morgan figured out that the gene for eye color was on the X chromosome • His work supported the chromosome theory of inheritance
  8. 8. Morgan’s other work • Each chromosome has hundreds or thousands of genes • Genes located on the same chromosome that tend to be inherited together are called linked genes
  9. 9. Morgan experimented to see inheritance of two traits • Morgan crossed flies that differed in traits of body color and wing size
  10. 10. What he found… • He found that the body color and wing size traits were usually inherited together and the offspring looked like the parents, mostly…
  11. 11. What he found… • Some of the offspring had phenotypes different from the parents, but in lower ratios than expected • Offsrping with phenotypes like the parents called parental types • Offspring with new phenotype combos called recombinant types • 50% frequency of recombination = genes on 2 different chromosomes Gametes from yellow-round heterozygous parent (YyRr) Gametes from greenwrinkled homozygous recessive parent (yyrr) Parental-type offspring Recombinant offspring
  12. 12. What he found… • He discovered that some genes can be linked, but there is a process that can break their connection: crossing over • This led to the development of genetic maps that determine the location of each gene on a chromosome based on the frequency of recombination • Distance between genes expressed as map units • 1 map unit = 1% recombination frequency
  13. 13. Sex-linked genes • An organism’s sex is an inherited phenotypic character determined by the presence or absence of certain chromosomes • Mammals like humans have an XX or XY system of inheritance • Other organisms have other systems
  14. 14. Genes on the sex chromosomes are called sex-linked genes • Some diseases on the X chromosome: • Color blindness • Rare in females, mild disease • Duchenne muscular dystrophy • 1 in 3500 males in US gets it • Lack the gene for the muscle protein dystrophin • Muscles get weaker and lose coordination • Usually don’t live past 20s • Hemophilia • Lack the protein to cause clotting • Don’t clot normally
  15. 15. Barr bodies • In mammalian females, 1 of the 2 X chromosomes is inactivated during embryonic development • The inactive X condenses into what is called a Barr body (we can see it under the microscope) • If she is heterozygous for a sex-linked trait, she will be a mosaic for that trait
  16. 16. • Some cells have the maternal X inactivated • These cells have the orange color • Some cells have the paternal X inactivated • These cells have the black color • All cells in the ovaries have active X chromosomes
  17. 17. Chromosomal mutations • In nondisjunction, pairs of homologous chromosomes do not separate normally during meiosis • As a result, one gamete receives two of the same type of chromosome, and another gamete receives no copy
  18. 18. What results… • Aneuploidy - a zygote produced from a normal gamete and a gamete produced by nondisjunction • Offspring with this condition have an abnormal number of a particular chromosome
  19. 19. Very rare among animals Common in plants, some fish, some amphibians What results… • Trisomy - having 3 copies of a particular chromosome • Monosomy - having just one copy of a particular chromosome • Polyploidy is a condition in which an organism has more than two complete sets of chromosomes Recent research has shown that this Chilean rodent is a tetraploid
  20. 20. Chromosomal breakage • Breakage of a chromosome can lead to four types of changes in chromosome structure: • Deletion removes a chromosomal segment
  21. 21. Deletion mutation • Example: retinoblastoma (eye tumors)
  22. 22. Chromosomal breakage • Duplication repeats a segment
  23. 23. Duplication mutation – fragile X syndrome
  24. 24. Chromosomal breakage • Inversion reverses a segment within a chromosome
  25. 25. Hemophilia A – inversion mutation patient was given injection in buttocks
  26. 26. Chromosomal breakage • Translocation moves a segment from one chromosome to another
  27. 27. Translocation mutation causes Burkitt’s lymphoma Tumors on hand from cancer
  28. 28. Down syndrome • Trisomy 21 - 3 number 21 chromosomes • 1 in 700 children in US • Frequency increases with age of mother
  29. 29. Trisomy 18 – Edward’s syndrome low birth weight, mental retardation, extra fingers and toes
  30. 30. • Klinefelter syndrome is the result of an extra chromosome in a male, producing XXY individuals Trisomy of sex chromosomes • Monosomy X, called Turner syndrome, produces X0 females, who are sterile; it is the only known viable monosomy in humans