AP Bio Ch. 15, part 2


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AP Bio Ch. 15, part 2

  1. 1. Ch. 15 The Chromosomal Basis of Inheritance 15.3-15.5
  2. 2. Main Ideas 1. 2. 3. 4. 5. Sex-linked inheritance and issues Aneuploidy Chromosomal mutations Genomic imprinting Non-nuclear DNA
  3. 3. Sex-linked genes exhibit unique patterns of inheritance  The XX, XY system in humans  The XX, X system in grasshoppers  The ZW, ZZ system in chickens  The diploid, haploid system in bees
  4. 4. Sex Chromosomes in Humans  Females - XX  Males XY  The Y chromosome is homologous with certain regions on the X chromosome  Parents pass on one sex chromosome to their children through their gametes   Woman always pass on an X chromosome Males pass on either an X (to make a daughter) or a Y (to make a son)
  5. 5. Sex Chromosomes in Humans  Early in development, the embryo has “generic” sex parts  Whether these parts develop into testes or ovaries is determined by the presence of hormones within the embryo  The “default” is a female Weeks of development
  6. 6.  On the Y chromosome is a gene called the SRY gene (sex-determining region of Y)  This gene codes for a protein that triggers other genes, leading to the development of a male
  7. 7. Quick Think Why do you think it is the presence of a Y chromosome that makes someone male, and not the presence of two X chromosomes that makes someone female? (i.e. - why is female the default)?
  8. 8. Inheritance of Sex-linked Genes  Sex-linked genes - genes on the sex chromosomes (may or may not be related to gender)  We see different inheritance patterns in males and females because females have 2 X chromosomes where males only have 1
  9. 9.  Women have 2 Xchromosomes, men have an X and a Y.  For women to express a recessive phenotype, they must inherit 2 Xchromosomes, both with the recessive allele.  For men to express the recessive phenotype, they need only 1 recessive X X-linked traits
  10. 10. Colorblindness  Colorblindness is recessive and is a mutation of a gene on the X chromosome  Men are more likely to be colorblind than women…WHY
  11. 11. Duchenne muscular dystrophy  1/3500 males in US  Progressive weakening of muscles  Loss of coordination  Death by early 20s  Mutation in X chromosome in the gene that codes for a muscle protein (dystrophin)
  12. 12. Hemophilia  Prolonged bleeding when injured  Clots slow to form  Caused by a mutation for a gene on the X chromosome that codes for a blood clotting protein  Can be treated but not cured Bleeding (bruising) after an injection
  13. 13. X inactivation in females  Even though females get 2 X chromosomes, 1 becomes inactive early in the development of the embryo  The inactive X condenses into a Barr body  This means that the cells of males and females each have 1 active X chromosome
  14. 14.  Which X becomes inactive in each embryonic cell is random  In some cells, the paternal X will do this  In other cells, the maternal X will do this  This leads to mosaicism in females  All cells that developed from the embryonic cells containing the maternal X will exhibit certain traits  The cells that develop from the embryonic cells containing the paternal X will exhibit other traits Example: tortoiseshell coat color in cats
  15. 15. Quick Think Can a male cat be a calico or tortoiseshell? Explain your reasoning.
  16. 16. Abnormal Chromosome Numbers  Occasionally, the chromosomes do not segregate properly during meiosis  This is called nondisjunction  This results in some gametes with too many chromosomes and some gametes with too few
  17. 17.  Having an abnormal number of chromosomes is a condition known as aneuploidy  Having 3 copies of a particular chromosome is called a trisomy  Having 1 copy of a particular chromosome is called a monosomy
  18. 18. Down Syndrome  Caused by a trisomy of chromosome 21  1/700 births in US  The result of nondisjunction during meiosis I  Risk increases with the age of the mother  Mental retardation, increased risk of other diseases, shortened life span, underdeveloped and likely sterile
  19. 19. Trisomy 18 – Edward’s syndrome low birth weight, mental retardation, extra fingers and toes
  20. 20. Abnormal Sex Chromosomes  Extra or missing sex chromosomes also cause abnormal phenotypes.  If an individual only has one sex chromosome, they have Turner syndrome (45, X karyotype).  These people are short, underdeveloped, have a web neck, and sterile females.
  21. 21. Abnormal Sex chromosomes  XXY syndrome (47, XXY karyotype) is called Klinefelter syndrome. These males are very tall and sexually underdeveloped, with diminished intelligence and some female phenotypes due to the extra X chromosome
  22. 22.  Polyploidy - having more than two complete chromosome sets   The normal state is having 2 of each chromosome (homologous pairs) When organisms have 3 or 4 of each chromosome, that is called polyploidy  Triploidy  Very  (3n), tetraloidy (4n), hexploid (6n), octapolid (8n) uncommon in animals Some fish and amphibians, mostly  Quite common in plants Recently discovered tetraploid mammal, rodent from Chile
  23. 23. Quick Think  Gene dosage, the number of active copies of a gene, is important to proper development.  Identify and describe two disorders that are the result of improper gene dosage
  24. 24. Alterations of Chromosome Structure  Chromosomal include mutations  Deletion  Inversion  Duplication  Translocation  Often occur during meiosis  Chromosomes break & rejoin incorrectly
  25. 25. Deletion mutations A piece of chromosome is lost during meiosis  Cri du chat is one condition that results distinctive cry, severe retardation, shortened life span
  26. 26. Inversion Mutations A double break where the piece reattaches, but backwards  Hemophilia is a inversion on the X chromosome
  27. 27. Duplication mutation A piece breaks off from one chromosome and reattaches to the sister chromatid  Fragile X syndrome is caused by this, 2nd most common form of mental retardation after Downs syndrome
  28. 28. Translocation mutation A piece breaks off one chromosome and reattaches to a different chromosome  Burkitt’s lymphoma is caused by this
  29. 29. Quick Think  At what point in gamete formation would these chromosomal mutations occur? During what particular process might the chromosomes be especially vulnerable to these mutations?
  30. 30. Exceptions to the chromosome theory of inheritance  Sometimes, the phenotype of the individual is different depending on which parent passed along the particular allele   If Example: in mice, there is a normal lgf2 gene and a mutant lgf2 gene the mother gives mutant & the father gives normal, the mouse grows to normal size  But, if the father gives a mutant allele, the mouse will be dwarf  This is called genomic imprinting
  31. 31.  In genomic imprinting – one copy of a gene is silenced during gamete formation by the addition (usually) of a methyl (-CH3) group to the cytosine nucleotides of that gene  Occurs in a small number of genes critical to embryonic development in animals  Therefore ONLY the maternal or paternal copy of that gene can be expressed in the offspring
  32. 32. DNA in organelles  DNA is also found in mitochondria and chloroplasts.  This DNA is not passed to gametes through meiosis like nuclear DNA is.  Mitochondrial DNA is only passed from Mother to child.  The genes in mitochondrial DNA code for proteins of the ETC & ATP synthase  Mutations in this DNA may contribute to nervous system disorders, diabetes, heart disease, & Alzheimer’s