Lecture Notes: Platelets and Genetics of Hemophilia

Platelets

• Platelets are cell fragments critical for blood clotting.
• Disorder: Hemophilia
  • Affects the ability of blood to clot.
  • Even minor cuts can cause continuous bleeding.
  • Treatments exist that have improved outcomes.

Hemophilia Characteristics

• Genetics: Sex-linked, recessive trait (non-Mendelian)
• Sex Chromosomes:
  • Humans typically have 46 chromosomes (23 pairs).
  • 23 chromosomes from each parent.
  • Sex chromosomes determine sex (XX for females, XY for males).

Sex-linked Traits

• Traits found on sex chromosomes, mostly on the X chromosome.
• Types:
  • X-linked traits (most common)
  • Y-linked traits (exist but rarer)

Solving Punnett Squares for Hemophilia

• Genotypes for Hemophilia:
  • Female: XHXH or XHXh (carrier) = no hemophilia; XhXh = hemophilia
  • Male: XHY = no hemophilia; XhY = hemophilia

Example Problem

1. Determine Parent Genotypes:
   • Female (carrier, no hemophilia): XHXh
   • Male (no hemophilia): XHY
2. Set up Punnett Square:
   • Place female genotype on top, male on the side.
   • Fill in the square to determine offspring genotypes.
3. Results:
   • 75% chance offspring without hemophilia
   • 25% chance offspring with hemophilia

Key Points on Punnett Squares

1. Do not assume a trait is sex-linked
   • Many traits are on autosomes (non-sex chromosomes).
2. Sex-linked recessive traits
   • More common in XY genotypes (e.g., certain colorblindness).
3. Sex-linked dominant traits can exist
   • One dominant allele needed for expressing trait.
4. Complexity of genetic disorders
   • Not all follow single-gene inheritance (e.g., preeclampsia).
   • Many involve multiple genes and environmental factors.
5. Animals differ in sex chromosome configurations
   • Birds (ZW), Platypus (multiple X and Y chromosomes).

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End Note: Always stay curious!