Understanding Family Trees and Cystic Fibrosis

Introduction to Family Trees

• Family trees, also known as family pedigrees, are diagrams used to represent the genetic relationships within a family.
• Example discussed: Family tree for cystic fibrosis, a recessive inherited disorder.

Genetic Representation

• Recessive Disorder: Requires two recessive alleles to manifest the disease (e.g., cystic fibrosis).
  • Alleles are represented by lowercase 'f'.
• Dominant Allele: Represented by capital 'F', and does not cause cystic fibrosis.
• Genotypes:
  • Homozygous Recessive (ff): Individual has cystic fibrosis.
  • Homozygous Dominant (FF): Individual does not have cystic fibrosis.
  • Heterozygous (Ff): Individual is a carrier, does not have the disease but can pass on the recessive allele.

Symbols in Family Pedigrees

• Circles = Females
• Squares = Males
• Unshaded = Individual unaffected by condition
• Shaded = Individual has the condition
• Half-shaded = Carrier of the condition
• Note: A key is always provided in exams to explain symbols

Analyzing Pedigrees

• Each row represents a new generation.
• Horizontal lines connecting individuals indicate a couple.

Example Analysis

• Jenny: Unaffected female, genotype FF (homozygous dominant).
• Paul: Male carrier, genotype Ff (heterozygous).
• Children: Ron and Harry (unaffected, FF); Anna and Lucy (carriers, Ff).
• Sam's Wife: Affected female, genotype ff (homozygous recessive).

Using Pedigrees to Determine Disease Probability

• When asked about the probability of offspring having a condition, evaluate parents' genotypes.
• Example: Lucy (Ff) and Jack (Ff) having a child:
  • Use a Punnett Square to determine possible genotypes of the offspring.
  • Outcomes:
    • 1 in 4 chance (25%) of child being unaffected (FF).
    • 2 in 4 chance (50%) of child being a carrier (Ff).
    • 1 in 4 chance (25%) of child having cystic fibrosis (ff).

Conclusion

• Understanding family trees is crucial for predicting genetic conditions.
• Practice drawing and interpreting family pedigrees to accurately determine genotypes and probabilities in exams.