Genesis 1:27

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• Genesis 127
• 27 So God created man in his own image, in the
  image of God created he him male and female
  created he them.

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Sex Determinationand Sex Chromosomes

• Timothy G. Standish, Ph. D.

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What Is Male and Female?

• What characteristics define being male or female?
• Male
• Produce sperm . . .
• What are sperm?
• Female
• Produce eggs . . .
• What are eggs?
• Organisms like Chlamydomonas are isogamous,
  producing morphologically indistinguishable
  gametes

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Many Plants and Animals are Hermaphrodites

• Plants like corn and peas have both male and
  female parts which produced sperm and eggs
  respectively
• Some animals, for example Caenorhapditis elegans,
  are also hermaphroditic
• In both cases there are not genetic differences
  producing dimorphic genders, instead the various
  parts develop as a result of developmental
  differences within the organism
• C. elegans produces males via non-disjunction

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Sex Determination

• Two ways in which sex can be determined
• Environment
• Turtles - Temperature of development
• Some fish - Social structure
• Chromosomes - Three methods
• XO - Haploid/diploid ie bees, haploid males
  diploid females
• ZW - Heterogametic (ZW) females, homogametic (ZZ)
  males, ie birds
• XY - Heterogametic (XY) males, homogametic (XX)
  females, ie humans and Drosophila

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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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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, deficiency 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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Human Y Chromosome
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Human Y Chromosome
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Human Y Chromosome
Pseudoautosomal region (PAR)
Nonrecombining region of the Y (NRY)
Pseudoautosomal region (PAR)
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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
• Another way in which dosage of genetic material
  can be changed is via macromutations

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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 chromosomes 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
  Barr Body in metaphase female nuclei

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Consequences of X Chromosome Dosage Compensation

• During early 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 chromome

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Consequences of X Chromosome Dosage Compensation
At some point (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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Why Have More Than One Chromosome?

• If only one chromosome, no crossing over, thus
  all bad or good genes go to offspring ...