Sex Linked Genes

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Sex Linked Genes
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Sex Linked Genes

• Found on 23rd chromosome.
• If a gene is found only on the X chromosome and
  not the Y chromosome, it is said to be a
  sex-linked trait.
• Sex linkage is linked to the gender of the
  individual.

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X Chromosome
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Y Chromosome
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XR
XR
XR
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How we see sex linked genes

• Look at XX or XY chromosomes
• Place superscript letter above X chromosome.
• Female Homozygous (Dominant)
• XHXH
• Female Heterozygous (Carrier)
• XHXh
• Female Homozygous (Recessive)
• XhXh
• Male Dominant
• XHY
• Male Recessive
• XhY

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Give it a try
Parent
Father (Carrier)
Mother (Heterozygous Normal)
Gametes
Children
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Recessive vs. Dominant

• X-linked recessive disorders
• Common red-green color-blindness.
• Hemophilia.
• Duchenne muscular dystrophy.
• More males than females display the disease.
• X-linked dominant are very rare in humans (ie.
  hypophosphatemia) and affected males pass the
  condition only to their daughters who may pass
  this on to both sons and daughters.

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• Examples of Sex-linked Traits
• Red-green colorblindness
• Male Pattern Baldness
• Hemophilia
• Duchenne Muscular Dystrophy
• Hairy Ears
• Y Chromosome

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Hemophilia

• Hemophilia is an X-linked recessive disorder
• Inability to properly form blood clots.
• Until recently, hemophilia was untreatable, and
  only a few hemophiliacs survived to reproductive
  age because any small cut or internal
  hemorrhaging after even a minor bruise were
  fatal.
• Hemophilia is treated with blood transfusions and
  Factor 7.

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Hemophilia continued

• Hemophilia affects
• males much more frequently (1 in 10,000).
• females (1 in 100,000,000).
• Since males only carry one X chromosome, if that
  is defective, hemophilia will immediately show
  up.
• Females, carry two X chromosomes.
• If only one is defective, the other normal X
  chromosome can compensate. The woman will have
  normal blood clotting she will simply be a
  carrier of the recessive defective gene.

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• Sample Sex-linked Trait Problem
• In humans, red-green colorblindness is a
  recessive sex-linked trait.
• It is found on the X chromosome, not the Y.
• Because, males only have one X chromosome, they
  have a much greater chance of having red-green
  colorblindness.
• Females would have to be homozygous recessive in
  order to have red-green colorblindness.

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Color Blindness

• Parents Phenotypes
• Normal Vision Father x Normal Vision Mother
  (Carrier)

Phenotypes of Offspring Females Normal
Vision Males Normal Vision Males
w/ Colorblindness
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Color Blindness

• Parents Phenotypes
• Normal Vision Father x Colorblind Mother

Phenotypes of Offspring Females Normal
Vision Males Normal Vision Males
w/ Colorblindness
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First, lets take a look at Queen Victorias son
Leopolds family. His daughter, Alice of Athlone,
had one hemophilic son (Rupert) and two other
childrena boy and a girlwhose status is
unknown. a) What is the probability that her
other son was hemophilic? b) What is the
probability that her daughter was a carrier?
Hemophilic? c) What is the probability that
both children were normal?