Sex-linked Traits

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Sex-linked Traits
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Karyotype

• Karyotype a picture of chromosomes in a dividing
  cell that are arranged in pairs by size.
• First 22 pairs are autosomes
• Last pair are the sex chromosomes

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Autosomal Traits

• The majority of traits that we inherit from our
  parents are not dependent on our sex ?
• autosomal traits.
• Genetic traits such as tongue rolling and having
  a widows peak will appear equally in both sexes.

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Sex-linked traits

• However, certain traits are dependent upon ones
  sex. They are determined by genes that are found
  on the sex chromosomes sex-linked traits.
• X-linked traits located on the X chromosome
• Y-linked traits located on the Y chromosome
• We will focus on one X-linked recessive disorder,
  hemophilia.

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Hemophilia

• A disorder in which there is an improper clotting
  of blood. In turn, a little scratch or bruise can
  lead to serve bleeding or even death.
• Since this is a recessive trait or disorder
• h allele for hemophilia, H no hemophilia
• In addition, to show that the trait is X-linked,
  we will use the following notation
• Ex Female with hemophilia Xh Xh
• The alleles will be shown as superscripts of the
  X chromosome

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Hemophilia (Contd)

• What would be the genotype(s) of a female who
  does not have hemophilia?
• XH XH or XH Xh
• This female, XH Xh, is known as a carrier because
  she carries one recessive allele on one of her X
  chromosomes, but does not show the trait
• Because males only have one X chromosome
• Male with hemophilia- Xh Y (this allele is not
  present on the Y chromosome, so there is no
  superscript).
• What is the genotype of a male without
  hemophilia?
• XH Y
• Why do you believe that more males than females
  have hemophilia?

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Why are males more likely to have hemophilia?

• A male only has to inherit the recessive allele
  from his mother.
• Xh Y
• A female on the other hand has to inherit a
  recessive allele from both parents, which has a
  lower probability.
• Xh Xh

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Punnett Squares for Sex-linked traits

• A punnett square can still be used to find the
  probability that offspring will possess a certain
  sex linked trait.
• Suppose A woman has hemophilia. She marries a
  man, who also has hemophilia. What is the
  probability that they will have a child with
  hemophilia?

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Xh
Xh
Xh Xh XhXh
XhY XhY
Xh
Y
100 chance that child will have hemophilia.
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Another Scenario

• Suppose A woman is a carrier of hemophilia. She
  marries a man, who has hemophilia. What is the
  probability that they will have a child with
  hemophilia?

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XH
Xh
XHXh XhXh
XHY XhY
Xh
Y
There is a 50 chance that they will have a child
who has hemophilia. Genotypic ratio- 1 XHXh 1
XhXh 1 XHY 1 XhY Phenotypic ratio 1 normal
female (carrier) 1 female with hemophilia 1
normal male 1 male with hemophilia