Chromosomes:%20Vessels%20For%20The%20Genes

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ChromosomesVessels For The Genes

• Timothy G. Standish, Ph. D.

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Reasons Mendels Work Was Ignored

• There was no Physical element in which Mendels
  inherited particles could be identified.
• By the turn of the century, chromosomes had been
  discovered (physical particles) and biologists
  were better at math.

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ChromosomesThe Physical Basis of Inheritance

• 1866 Mendel published his work
• 1875 Mitosis was first described
• 1890s Meiosis was described
• 1900 Mendel's work was rediscovered
• 1902 Walter Sutton, Theodore Boveri and others
  noted parallels between behavior of chromosomes
  and alleles.

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Chromosomal Theory of Inheritance

• Genes have specific loci on chromosomes.
• Chromosomes undergo segregation (meiosis) and
  independent assortment,
• Thus alleles of genes are independently assorted.

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Chromosomal Theory of Inheritance
Telophase II
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Independent Assortment
As long as genes are on different chromosomes,
they will assort independently
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Two Genes On One Chromosome
Telophase II
As long as genes on the same chromosome are
located a long distance apart, they will assort
independently due to crossing over during
prophase I of meiosis
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Thomas Hunt Morgan

• First to associate a trait (gene) with a
  chromosome.
• Worked with fruit flies (Drosophila melanogaster)
• Why fruit flies?
• Short generation time ( 2 weeks)
• Survives and breeds well in the lab
• Very large chromosomes in some cells
• Many aspects of phenotype are genetically
  controlled.

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Drosophila Mutations
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More Drosophila Mutations
Wild Type
ebony body ee
white eyes ww
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X Chromosome Human and Drosphila Genes Are Easy
To Find

• In humans and Drosophila, males are XY
• Thus males are haploid for the X chromosome
• Because of this, recessive genes on the X
  chromosome show up far more commonly in male than
  female phenotypes

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Morgans Discovery Of An X-Linked Drosophila Gene
A white-eyed male was discovered
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The Key To Morgans Discovery

• The key to Morgans discovery was the observation
  that all the white eyed individuals in the F2
  generation were males
• Without this vital data on the association of
  white eyes with being male, the gene for white
  eyes could have been seen as a simple recessive
  trait on an autosome
• This illustrates the importance of recording all
  the data possible and being alert to the
  possibility of interesting things being present
  in the data
• Fate favors the prepared mind (Louis Pasture)

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Human X-linked Recessive Genes

• Brown enamel - Tooth enamel appears brown rather
  than white
• Hemophilia - Two types
• A - Classic hemophilia, deficiency of blood
  clotting factor VIII
• B - Christmas disease, deficincy of blood
  clotting factor IX

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X-linked Recessive GenesRelated to sight

• Coloboma iridis - A fissure in the eyes iris
• Color Blindness - Two types
• Deutan - Decreased sensitivity to green light
• Protan - Decreased sensitivity to red light
• Congenital night blindness - Not due to a
  deficency of vitimin A
• Mocrophthalmia - Eyes fail to develop
• Optic atrophy - Degeneration of the optic nerves

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Royal Pedigree
Edward Duke of Kent (1767-1820)
Victoria Princess of Saxe-Coburg (1786-1861)
Albert of Saxe-Coburg (18XX-18XX)
Victoria Queen of England (1819-1910)
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Variation In Chromosome Number - Polyploidy

• Polyploid individuals have more than two sets of
  chromosomes
• Many important commercial plants are polyploid
• Roses
• Navel oranges
• Seedless watermelons
• Polyploid individuals usually result from some
  sort of interruption during meiosis

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Variation In Chromosome Number - Aneuploidy

• Polyploid humans are unknown, but individuals
  with extra individual chromosomes are known.
• Having extra chromosomes or lacking some
  chromosomes is called aneuploidy
• Aneuploid individuals result from nondisjunction
  during meiosis

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Aneuploidy In Humans

• Most human aneuploids spontaneously abort
• The most viable variations in chromosome number
  are those that deal with the sex chromosomes
• XO - Turners Syndrome - Phenotypically females
• XXX- Super females
• XYY - Super Males - On average tend to be
  larger and less intelligent
• XXY - Klinefelters Syndrome - Phenotypically
  male
• Of the non-sex chromosome aneuploidys, Downs
  Syndrome, extra chromosome 21, tends to be the
  most viable
• Downs Syndrome is more common in children of
  mothers who gave birth after age 40

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Gene Dosage

• There seem to be elegant mechanisms for
  maintaining the correct dosage of genetic
  material in each cell
• When aneuploidy causes a change in the relative
  dose of one chromosome, problems result

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The Lyon Hypothesis

• Having extra chromosomes causes problems (ie
  Downs Syndrome)
• Men have only one X chromosome and they are
  normal (at least they think so)
• Women have two X chomosomes and they are normal
• Mary Lyon proposed that the extra dosage of X
  chromosome that women have is compensated for by
  turning off one of the X chromosomes.
• This turned off chromosome can be observed as a
  Bar Body in metaphase female nuclei

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Consequences of X Chromosome Dosage Compensation

• Early during development, X chromosomes are
  randomly turned off in female cells
• All daughter cells have the same X chromosome
  inactivated as their parental cell.
• Thus, females are a mosaic of patches of cells
  some patches expressing the genes on the paternal
  X chromosome, other patches expressing the
  maternal X chromosome

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Consequences of X Chromosome Dosage Compensation
At some pont (probably later than the 4 cell
stage) half the X chromosomes are turned off
Daughter cells inherit the mother cells
combination off and on X chromosomes
Because of dosage compensation, females are
thought to be a mosaic of patches of cells with
each patch expressing the same X chromosome, but
none expressing both chromosomes
Different patches of cells inherit different act
X chromosomes
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Why Calico Cats Are Usually Female

• Orange coat color is a sex-linked trait in cats
  (it is on the X chromosome)
• A female cat heterozygous for orange, has skin
  patches expressing the orange X with the other X
  chromosome turned off. In other patches the
  opposite occurs.

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