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AP Bio Ch. 15, part 2
AP Bio Ch. 15, part 2
AP Bio Ch. 15, part 2
AP Bio Ch. 15, part 2
AP Bio Ch. 15, part 2
AP Bio Ch. 15, part 2
AP Bio Ch. 15, part 2
AP Bio Ch. 15, part 2
AP Bio Ch. 15, part 2
AP Bio Ch. 15, part 2
AP Bio Ch. 15, part 2
AP Bio Ch. 15, part 2
AP Bio Ch. 15, part 2
AP Bio Ch. 15, part 2
AP Bio Ch. 15, part 2
AP Bio Ch. 15, part 2
AP Bio Ch. 15, part 2
AP Bio Ch. 15, part 2
AP Bio Ch. 15, part 2
AP Bio Ch. 15, part 2
AP Bio Ch. 15, part 2
AP Bio Ch. 15, part 2
AP Bio Ch. 15, part 2
AP Bio Ch. 15, part 2
AP Bio Ch. 15, part 2
AP Bio Ch. 15, part 2
AP Bio Ch. 15, part 2
AP Bio Ch. 15, part 2
AP Bio Ch. 15, part 2
AP Bio Ch. 15, part 2
AP Bio Ch. 15, part 2
AP Bio Ch. 15, part 2
AP Bio Ch. 15, part 2
AP Bio Ch. 15, part 2
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AP Bio Ch. 15, part 2

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  • 1. Ch. 15 The Chromosomal Basis of Inheritance 15.3-15.5
  • 2. Main Ideas 1. 2. 3. 4. 5. Sex-linked inheritance and issues Aneuploidy Chromosomal mutations Genomic imprinting Non-nuclear DNA
  • 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. 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. 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.  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. 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. 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.  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. 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. 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. 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. 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.  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. Quick Think Can a male cat be a calico or tortoiseshell? Explain your reasoning.
  • 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.  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. 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. Trisomy 18 – Edward’s syndrome low birth weight, mental retardation, extra fingers and toes
  • 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. 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.  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. 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. Alterations of Chromosome Structure  Chromosomal include mutations  Deletion  Inversion  Duplication  Translocation  Often occur during meiosis  Chromosomes break & rejoin incorrectly
  • 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. Inversion Mutations A double break where the piece reattaches, but backwards  Hemophilia is a inversion on the X chromosome
  • 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. Translocation mutation A piece breaks off one chromosome and reattaches to a different chromosome  Burkitt’s lymphoma is caused by this
  • 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. 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.  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. 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

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