Sex-Linked Traits

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Sex-Linked Traits
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Genetic Counseling

• Sometimes its a good idea to know the odds . . .
  . Especially when dealing with sex-linked traits.

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Its all about the SEX!

• Sex-linked genes are located on the 23rd
  chromosome (sex chromosome).
• Sex chromosomes are X and Y.
• XY Boy
• XX Girl
• Sex-linked genes are shown as SUPERSCRIPTS on the
  X chromosome.

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Sex-Linked Traits

• Sex-linked traits can be Dominant or Recessive.
• A dominant a recessive
• What would be the genotypes of a male and female
  that have a Sex-linked Dominant trait and do not
  express (have) the trait?
• Expresses Trait Male - XA Y Female - XA
  XA or XA Xa
• No Expression Male - Xa Y Female - Xa Xa
• What would be the genotypes of a male and female
  that have a Sex-linked Recessive trait and do not
  express the trait?
• Expresses Trait Male - Xa Y Female - Xa
  Xa
• No Expression Male - XA Y Female - XA
  XA or XA Xa
• 
  
  (Carrier)
• Most Sex-linked traits are Recessive!

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Sex-Linked Recessive Traits

• More males are affected than females.
• An affected son can have parents who have the
  normal phenotype. (XAY, dad) x (XAXa, mom)
• For a daughter to have the trait, her father must
  also have it. Her mother must have it or be a
  carrier. (XaY, dad) (XaXa or XAXa, mom)
• The trait often skips a generation from the
  grandfather to the grandson.
• If a woman has the trait (XaXa), ALL of her sons
  will be affected.
• Pedigrees show only female carriers--no male
  carriers.

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Examples of Sex-Linked Recessive Disorders

• Red/Green Colorblindness Difficulty perceiving
  differences between colors (red or green, blue or
  yellow).
• Hemophilia Absence of one or more proteins
  necessary for normal blood clotting.
• Deafness
• Cataracts opacity in the lens that can lead to
  blindness
• Night blindness (Nyctalopia) rods do not work
  so that can not see in the dark
• Glaucoma pressure in the eye that can lead to
  optic nerve damage and blindness
• Duchenne Muscular Dystrophy progressive
  weakness and degeneration of skeletal muscles
  that control movement due to absence of
  dystrophin (protein that maintains muscle
  integrity). Mainly in boys, onset 3-5 yrs, by 12
  years cant walk, and later needs respirator.

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To See or Not To See

• Color-Blind
• -- cannot see the numbers or designs in the
  circles.
• -- What do you see?

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Color Blind Recessive SL Trait
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Color-Blind Pedigrees

• Draw a pedigree showing a cross between a man
  with normal vision and a woman who carriers the
  color-blind trait.
• Color-blind is a recessive trait.
• Genotypes of Parents
• Male XR Y Female XR Xr

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Does yours look like this?

• Pedigree of color-blind trait.
• Genotypes of Parents
• Male XR Y Female XR Xr

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Can a female be color-blind?

• Think about it.
• If so, how? Why?

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YESshe can!

• If dad (XrY) is color-blind and mom (XRXr) is a
  carrier
• 50 chance XrXr
• Shes color-blind.

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Hemophilia

• It is a rare blood disorder.
• The blood does not clot.
• A hemophiliac will bleed freely, while a
  normal person will eventually stop bleeding
  because a scab forms.
• Hemophiliacs require medical intervention to stop
  the flow of blood.
• Usually they are given clotting factors, which
  help them form scabs.
• Hemophilia is inherited.
• About 1 out of 5,000 males are born with
  hemophilia each year.

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Pedigree Royale Hemophilia