Chromosomal Basis of Inheritance

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Chapter 15

• Chromosomal Basis of Inheritance

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Relating Mendelian Ideas to Chromosomes

• Mendel had no idea what was causing the traits
  seen in his pea plants
• Biologists discovered in 1900 that chromosomes
  behave in ways that explain Mendels Laws

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Figure 15.1
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Thomas Morgan and Fruit Flies

• Morgan began working out mendelian genetics ideas
  on fruit flies
• He breed flies for a year and discovered one male
  that had white eyes (red eyes are wild type)
• He then bred this white eyed male with a red eyed
  female
• Result all red eyed offspring (F1 generation)
• F1 x F1 31 ratio of red to white eyes
• But only males showed white eyed traits
• Evidence of sex linked genes

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Fig. 15.3
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Linked Genes

• Humans have 46 chromosomes and thousands of genes
• Therefore some genes are found on the same
  chromosome and these genes are usually inherited
  together as a unit
• Morgan then found a fly which had a black body
  and vestigial wings (short wings) Wild type is
  gray body with normal wings
• b gray bblack and vgvestigial wing vgnormal
  wing

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Fig 15.4
bbvgvg
b b vg vg
bb vgvg
b b vg vg
Expected Ratio 1111 Observed
Ratio 965944206185
bb vgvg bbvgvg bbvgvg bbvgvg
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Independent Assortment and Crossing Over Produce
Genetic Recombinants

• Sometimes offspring show phenotypes not seen in
  either parent
• YyRr x yyrr
• Some offspring show green round seeds not seen in
  either parent
• This type of recombination is seen in unlinked
  genes (on different chromosomes)
• Due to Independent assortment

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Recombination of Linked Genes

• How do you produce genetic recombinants for genes
  that are found on the same chromosome
• Crossing over
• Recall Morgans second experiment
• If no crossing over and no linked genes then we
  expect a 1111 ratio
• But if they are linked we should expect a 1100
  ratio
• We saw neither
• Morgan explained this by concluding that
  homologous chromosomes exchange portions during
  meiosis

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Fig 15.5
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Chromosomal Basis of Sex
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Sex-linked genes and patterns of inheritance

• Sex-linked genes genes found on the X
  chromosome
• Fathers pass all sex linked alleles to their
  daughters but no their sons
• Why is this true?
• Mothers pass sex linked allele to daughters and
  sons
• Because males only have on locus on the X
  chromosome we see much more recessive sex-linked
  disorders in males

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X-inactivation in Female Mammals

• Females have two X chromosomes
• But in each cell one of the X chromosomes becomes
  inactive leaving one X to produce proteins
• This leads to a female becoming somewhat of a
  mosaic
• Example recessive allele on the X prevents
  production of sweat glands
• A female that is heterozygous for this allele
  (one normal X and one X with the sweat gland
  preventer) will have patches of skin where there
  are no sweat glands

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Fig 15.10
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Errors and Exceptions in Chromosomal Inheritance

• Alteration of number or structure of chromosomes
  cause disorders
• Name one kind of disease that is caused by an
  inaccurate number of chromosomes

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Alteration of Chromosome Number

• Mishap in meiosis that causes Nondisjunction

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Alteration of Chromosome Structure

• Breakage of a chromosome can lead to four types
  of changes in the structure
• Deletion part of a chromosome us broken off
• Duplication piece that is broken off becomes
  attached to a sister chromatid
• Inversion broken piece reattaches to the
  chromosome but upside down
• Translocation broken piece joins a
  non-homologous chromosome

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Fig 15.13
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Human Disorders due to Chromosomal Alterations

• Frequency of abnormal number of chromosomes is
  quite high in humans
• Why dont we see lots of disorders?
• Most embryos with abnormal number of chromosomes
  are spontaneously aborted
• Some survive birth and beyond
• Trisomy 21

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Fig 15.14
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Human Disorders cont.

• XXY males Klinefelter syndrome
• Feminized males
• XYY males no well-defined syndrome, although
  they are taller than normal
• Why doesnt this extra Y cause more trouble
• XXX females no effect
• X females Turner syndrome
• Sterile, and lack developed sex organs

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Human Disorders cont.

• Chromosome 5 deletion cri du chat
• Cry of the cat