Chapter 15 Chromosomal Basis of Inheritance

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Chapter 15 Chromosomal Basis of Inheritance
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Mendel Chromosomes

• Mendel was ahead of his time. 19th C cytology
  suggested a mechanism for his earlier findings.
  What did they find?
• Chromosomes and genes are both present in pairs
  in diploid cells.
• Homologous chromosomes separate and alleles
  segregate during meiosis.
• Fertilization restores the paired condition for
  both chromosomes and genes.

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Chromosome Theory of Inheritance

• Mendelian genes have specific loci on chromosomes
• Chromosomes are what physically undergo
  segregation and independent assortment.

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Morgans Fruit Flies

• Morgan first associated a specific gene with a
  specific chromosome.
• Why fruit flies?
• Breed quickly (two week generations)
• 4 pairs of chromosomes (3 pair of autosomes, 1
  pair of sex chromosomes)
• Females XX
• Males XY

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Morgans Fruit Flies

• Wild Type flies are the most common natural
  phenotype. (Red Eyes)
• After a series of crosses, Morgan produced
  mutants with white eyes.
• After a few generations, Morgan noted that only
  males displayed the white eyes.
• He concluded that certain genes are located on
  the sex chromosome and thus linked to sex.
• Sex-linked genes (ie hemophilia)

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Sex-linked Traits
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Sex-linked Traits

• Morgan concluded the gene with the white-eyed
  mutation is on the X chromosome. Y chromosome
  no info
• Males (XY) only need one copy of recessive allele
  to show trait.

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Linked Genes

• All genes located on the same chromosome tend to
  be inherited together.
• Chromosome passed on as a unit.
• Testcross results varied from those predicted the
  law of independent assortment.
• This showed that certain genes will assort
  together. (on same chromosome)

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Linked Genes
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Linked Genes

• Body color and wing shape are usually inherited
  together (same chromosome)

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Recombinants

• Where did the other phenotypes come from?
  (grey-vestigial and black normal)
• Genetic recombination offspring with new
  combinations of traits inherited from two parents
• How??
• independent assortment of genes (non-homologous)
• crossing over of genes (homologous)

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Recombinants

• Mendels dihybrid crosses produced recombinant
  genotypes.
• 50 parental 50 recombinant genotypes typical
  for nonhomologues
• Metaphase I
• YR, Yr, yR, and yr
• Seed shape and color tetrads are independent from
  one another

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Recombinants

• Linked genes tend to move together during
  meiosis/fertilization
• If Independent assortment of genes
• Expect a 1111 phenotype ratio
• If Complete linkage of genes
• 1100 ratio (all parental)
• Observed 17 recombinant flies
• Suggested Incomplete linkage of genes

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Crossing Over

• Prophase I homologous chromosomes can swap
  alleles
• More variable gametes than simple mendelian rules
  would predict

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Therefore, Crossing Over Explains
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Linkage Maps

• Ordered list of genetic loci along chromosome
• Based on recombination frequencies b/t two genes
• Higher of recombination further apart
• More places in between genes for crossing over to
  occur and separate the genes

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Linkage Maps

• The recombination frequency between cn and b is
  9.
• The recombination frequency between cn and vg is
  9.5.
• The recombination frequency between b and vg is
  17.

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Linkage Maps

• Map units are the distances between genes on a
  chromosome.
• 1 map unit 1 recombination
• 50 recombination so far apart that crossing
  over is all but certain
• Remember, 50 recomb. ind. assortment
  (non-homologous)
• Linkage maps show relative order/distance
• More recent studies show exact distances and
  order

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Sex Chromosomes
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X-Y Sex Determination

• X and Y behave as homologues
• Each egg receives an X from XX mother
• One sperm receives X and one Y
• Results in 50/50 chance of male or female
• SRY Gene
• Present (on Y) gonads develop into testes
  (male)
• Not present (no Y) gonads become ovaries
  (female)
• SRY also regulates other genes

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Sex-Linked Genes

• Sex chromosomes also contain other genes. (ie
  drosophila eye color)
• Females must be homozygous recessive to display
  trait (XX second X can mask recessive)
• Females can be carriers
• Males only need to inherit a single copy to show
  trait
• Can a male be a carrier?

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Sex-Linked Disorders

• Duchenne Muscular Dystrophy
• 1/3500 males
• Progressive muscular weakening
• Die by mid-20s
• Missing X-linked gene
• No production of dystrophin (muscle protein)

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Sex-Linked Disorders

• Hemophilia
• Absence of one or more clotting factors
• affected individuals cannot stop bleeding
  normally
• treated with protein injections

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Barr Bodies

• Only one of the females X chromosomes is active
• The other becomes a Barr body
• When assorted into an ova, the Barr body becomes
  activated again
• Which X becomes Barr body is random in each cell
• Approx. 50 express each allele (if hetero)

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X-Inactivation in Females
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Nondisjunction

• Errors with meiotic spindle
• Meiosis I Homologous tetrad doesnt separate OR
• Meiosis II Sister chromatids dont separate
• Some gametes receive two of the same type of
  chromosome and another gamete receives no copy

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Aneuploidy

• Results from fertilization involving nondisjoined
  gamete(s)
• Trisomy three copies of a particular chromosome
  (2n 1)
• Monosomy only one copy of a particular
  chromosome (2n 1)

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Down Syndrome

• Three copies of chromosome 21
• 1/700 children born each year
• Definite link b/t maternal age

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Aneuploidy in Sex Chromosomes

• XXY Male (Klinefelters Syndrome)
• Male sex organs, sterile w/ femininity
• XYY Males
• Tend to be taller than normal

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Aneuploidy in Sex Chromosomes

• XXX Females
• Will develop as normal females
• XO Females (monosomy)
• Immature females

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Changes in Chromosomes