Lecture Notes: Mendelian Genetics Part 2 - Inheritance Modes

Overview

• Focus on inheritance patterns, particularly sex-linked pedigrees.
• Pedigree charts: Diagrams showing the occurrence of phenotypes across generations.
• Key terms: affected, unaffected, carriers.

Key Concepts

Pedigree Chart Basics

• Symbols:
  • Diamond: Unknown sex
  • Dark symbol: Phenotype in question
  • Shaded symbol: Heterozygotes
• Lines:
  • Horizontal line: Parents
  • Vertical line: Offspring
  • Triangle: Twins
  • Crossed line: Deceased
• Analyses: Determines dominant/recessive patterns.

Sex-Linked Traits

• X-linked disorders: Most common due to larger X chromosome.
  • Males are hemizygous (only one X chromosome).
  • X-linked genes not passed from father to son.
• X-linked recessive:
  • More common in males, can skip generations.
  • Carriers: Unaffected females pass it to sons who express the trait.
  • Example: Hemophilia and Duchenne muscular dystrophy.
• X-linked dominant:
  • Occurs every generation, not passed from affected males to sons.
  • Examples: Fragile X syndrome, hypophosphatemic rickets, amelogenesis imperfecta.

Y-Linked Traits

• Only found in males and always expressed.
• Example: Swire syndrome.

Sex-Influenced Traits

• Expression differs by sex. Example: Early balding.

Imprinting

• Gene expression depends on parent origin.
• Examples: Prader-Willi syndrome, Engelmann syndrome.

Autosomal Inheritance Patterns

Autosomal Dominant Disorders

• Normal allele is recessive; abnormal allele is dominant.
• Observed in every generation.
• Examples: Achondroplasia, Huntington's disease, neurofibromatosis.

Autosomal Recessive Traits

• Must inherit two recessive alleles for expression.
• Carrier Parents: 25% chance of affected child.
• Examples: Cystic fibrosis, Tay-Sachs disease, albinism, phenylketonuria (PKU).

Important Concepts

• Balancing Selection: Maintains alleles that could be lethal but advantageous in heterozygote form.

Checkpoint Questions

• Question 1: Mom is a carrier (heterozygous). Answer: Heterozygous.
• Question 2: X-linked dominant disorder carrier mom and unaffected dad. Answer: All of the above (sons and daughters could be unaffected, son could be affected, daughter could be a carrier).
• Question 3: Autosomal recessive disorder carrier parents. Answer: 25% chance of affected child.

Conclusion

• This lecture emphasized understanding pedigree charts and genetic patterns for counseling and prediction, especially in high-risk populations.

Next Lecture

• Complex traits and epigenetics.