Transcript Sex Linked Genes

Sex Linked Genes
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.
X Chromosome
Y Chromosome
XR
XR
XR
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
Give it a try…
Parent
Father (Carrier)
Gametes
Children
Mother (Heterozygous Normal)
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
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.
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.
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.
Color Blindness
Parent’s Phenotypes:
• Normal Vision Father x Normal Vision Mother (Carrier)
Phenotypes of Offspring:
Females Normal Vision
Males Normal Vision
Males w/ Colorblindness
Color Blindness
Parent’s Phenotypes:
• Normal Vision Father x Colorblind Mother
Phenotypes of Offspring:
Females Normal Vision
Males Normal Vision
Males w/ Colorblindness
First, let’s take a look at Queen Victoria’s son Leopold’s family. His
daughter, Alice of Athlone, had one hemophilic son (Rupert) and two
other children—a boy and a girl—whose 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?