Sex and the Chromosome (or how gender traits are inherited)

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Sex and the Chromosome(or how gender traits are
inherited)
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Some insects

• X-O system
• XXfemale
• Xmale (no second chromosome)

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Birds, Fish and some insects

• Z-W system
• ZWfemale
• ZZmales
• Determination is which chromosome is in the ovum
  (not the sperm)

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Bees and ants

• Haplo-diploid system
• No sex chromosome
• Female are diploid
• Males are haploid (developing from an
  unfertilized ova)

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Humans and other mammals

• Two varieties of sex chromosomes
• X Y
• XXfemale
• XYmale

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XY system

• Only relatively short segments at the either end
  of the Y are homologous to the X
• X Y rarely crossover

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Making babies

• In both testes (XY) and ovaries (XX), the two sex
  chromosomes segregate independently during
  meiosis
• Each ovum (from female) receives an X
• Sperm ½ X ½ Y
• During conception
• 50-50 chance of either sperm fertilizing the ovum
• Therefore 50-50 chance of producing a male or
  female offspring

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Genetic control of gender

• In humans anatomical signs of gender first appear
  would the 2nd month after conception
• In 1990 a gene on the Y chromosome was identified
  which is required for development of the testes
  (SRY gene)
• In individuals possessing SRY gene the embryonic
  region develops into testes
• Activity of SRY gene triggers a cascade of
  biochemical, physiological, and anatomical events
  because it regulates may other genes
• Other genes on the Y regulate production of
  functional sperm
• In the absence of these genes, an XY individual
  is male but does not produce normal sperm
• In individual lacking SRY gene, the embryonic
  region develops into ovaries

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Sex linked genes

• Sex chromosomes (especially the X) have a number
  of other genes
• Sex-linked genegene on the X chromosome
• Fathers pass sex linked alleles
• to all their daughters
• None of their sons
• Mothers pass sex linked alleles to both sons and
  daughters

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Inheritance of Sex-Linked Traits

• Normal mother father with trait
• XAXA x XaY
• Carrier mother normal father
• XAXa x XAY
• Carrier mother father with trait
• XAXa x XaY

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Recessive sex linked traits

• Heterozygous femalescarriers
• Chance of female inheriting 2 recessive mutants
  on the X are unlikely
• Hemizygous males (only one X)has trait
• There fore males are more likely to exhibit
  sex-linked recessive disorders than females

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Think about it

• Colorblindness is a recessive trait. Two people
  with normal vision have two sons, one colorblind
  and one with normal vision. If the couple also
  has daughters, what proportion of them will have
  normal vision? Use a Punnett square to explain
  your answer.

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Think about it

• Redgreen color blindness is caused by a
  sexlinked recessive allele. A colorblind man
  marries a woman with normal vision whose father
  was colorblind. What is the probability that
  they will have a colorblind daughter? What is
  the probability that their first son will be
  colorblind? (Note The two questions are worded
  a bit differently.)

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Think about it

• A man with hemophilia (a recessive, sexlinked
  condition) has a daughter of normal phenotype.
  She marries a man who is normal for the trait.
  What is the probability that a daughter of this
  mating will be a hemophiliac? That a son will be
  a hemophiliac? If the couple has four sons, what
  is the probability that all four will be born
  with hemophilia?

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Color blindness

• A color blind daughter may be born to a color
  blind father if the mother is a carrier
• Odds are low

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Duchenne muscular dystrophy

• Affects 13500 males in US
• Individuals rarely live past early 20s
• Due to the absence of an X linked gene for
  dystrophin (key muscle protein)
• Progressive weakening of muscles and loss of
  coordination

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Hemophilia

• Absence of one or more proteins required for
  blood clotting
• Prolonged bleeding because blood clots form very
  slowly
• Bleeding in muscles and joints can be painful and
  lead to serious damage
• Treatment involves IV injections of missing
  protein

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Think about it

• Pseudohypertrophic muscular dystrophy is an
  inherited disorder that causes gradual
  deterioration of the muscles. It is seen almost
  exclusively in boys born to apparently normal
  parents and usually results in death in the early
  teens. Is this disorder caused by a dominant or a
  recessive allele? Is its inheritance sexlinked
  or autosomal? How do you know? Explain why this
  disorder is almost never seen in girls.

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Being Female

• Only one of the X chromosomes is actually active
• Males and Females essentially have same dose of
  female genes
• During female development one of the X chromsomes
  per cell condenses into a compact Barr Body
• Barr body lies along the inside of the nuclear
  envelope
• Barr Body may reactivate in ovarian cells which
  produce ova

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Barr Body

• Selection of which X chromosome will form the
  Barr Body occurs randomly in embryonic cells at
  time of X inactivation
• Females are a mosaic of two types of cells
• Some inactive maternal X, some inactive paternal
  X

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Mosaic

• After an X is inactivated in a cell, all the
  mitotic descendants of the cell have the same
  inactive X
• If a female is heterozygous for a sex-linked
  trait
• ½ of her cells will express the trait
• ½ of her cells will not

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Examples of mosaic pattern of inheritance

• In cats
• Orange and black tortiseshell coat color is due
  to patches of cells expressing orange allele and
  other patches expressing nonorange allele
• In humans
• Women possessing X-linked mutation preventing
  development of sweat glands
• If Heterozygous
• Patches having sweat glands
• Patches lacking sweat glands

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How does Barr body form

• X-inactivation involves modification of DNA by
  attachment of methyl (--CH3) groups to cytosine
  nucleotides on one of the X chromosomes
• XIST (x-inactive specific transcript gene)
• Active only on Barr body chromosome
• Produces multiple copies of an RNA that attaches
  to the X chromosome on which they were made
• This inactivates X
• The mechanism connecting XIST RNA and DNA
  methylation is unknown
• Determination of X chromosome to be inactivated
  is also unknown
• Go to the film

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Using pedigrees to determine inheritance

• Is the trait shown in this pedigree autosomal
  dominant, autosomal recessive, sex-linked
  dominant, or sex-linked recessive? If
  sex-linked, is the trait on the X or the Y
  chromosome?

Hemophilia Sex-linked on the X chromosome
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Using pedigrees to determine inheritance

• Is the trait shown in this pedigree autosomal
  dominant, autosomal recessive, sex-linked
  dominant, or sex-linked recessive? If
  sex-linked, is the trait on the X or the Y
  chromosome?

Marfans Syndrome Autosomal Dominant
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Using pedigrees to determine inheritance

• Is the trait shown in this pedigree autosomal
  dominant, autosomal recessive, sex-linked
  dominant, or sex-linked recessive? If
  sex-linked, is the trait on the X or the Y
  chromosome?

Hairy Ears Sex-linked on the Y chromosome
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Using pedigrees to determine inheritance

• Is the trait shown in this pedigree autosomal
  dominant, autosomal recessive, sex-linked
  dominant, or sex-linked recessive? If
  sex-linked, is the trait on the X or the Y
  chromosome?

Albinism Homozygous Recessive