Lecture Notes: Understanding Hemophilia and Sex-Linked Traits

Introduction to Platelets

• Platelets are fragments of cytoplasm important in blood clotting.
• Disorders like hemophilia affect platelets, causing excessive bleeding even from minor cuts.
• Modern treatments significantly improve outcomes.

Hemophilia

• Definition: A sex-linked, recessive genetic disorder affecting platelets.
• Genetics: Different from basic Mendelian traits; alleles are on sex chromosomes.

Sex Chromosomes Overview

• Definition: Humans have 46 chromosomes; 2 are sex chromosomes (X and Y).
• Sex chromosomes determine gender:
  • XX = Female
  • XY = Male
• Misconception correction: X and Y names are not based on their shapes.
• Sex-linked Traits: Typically on the X chromosome due to its larger size and more genes.

Genetics of Hemophilia

• Alleles:
  • 'H' = allele for not having hemophilia (dominant)
  • 'h' = allele for having hemophilia (recessive)
• Genotype Representations:
  • Females: XHXH or XHXh (no hemophilia), XhXh (hemophilia)
  • Males: XHY (no hemophilia), XhY (hemophilia)
  • Note: Males cannot be carriers due to having only one X chromosome.

Applying Punnett Squares

• Purpose: Predict the probability of offspring inheriting hemophilia.
• Steps:
  1. Identify parent's genotypes:
     • Mother (carrier): XHXh
     • Father (non-hemophilia): XHY
  2. Arrange genotypes on Punnett square:
     • Mother across the top, father on the side.
  3. Cross genotypes:
     • Write X before Y, dominant before recessive.
• Results:
  • Genotype ratio: Determined from cross outcomes.
  • Phenotype ratio: 75% chance no hemophilia, 25% chance hemophilia in sons.

Key Takeaways

• Boys are more likely to inherit sex-linked recessive disorders due to having one X chromosome.
• Other sex-linked disorders can include color blindness.

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