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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. X R. X R. X R.

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Sex Linked Genes

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  1. Sex Linked Genes

  2. 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.

  3. X Chromosome

  4. Y Chromosome

  5. XR XR XR

  6. 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

  7. Give it a try… Parent Father (Carrier) Mother (Heterozygous Normal) Gametes Children

  8. 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.

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

  10. 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.

  11. 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.

  12. 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.

  13. 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

  14. Color Blindness • Parent’s Phenotypes: • Normal Vision Father x Colorblind Mother Phenotypes of Offspring: Females Normal Vision Males Normal Vision Males w/ Colorblindness

  15. 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?

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