Genetic relatedness. Family trees reflect r, the genetic relatedness between individuals. Mary and John have 2 children: Bill and Judy. Bill marries Karen, and they have 1 child: Trudy. Judy marries Michael, and they have 1 child: Phil.

• Q: What is r between siblings Bill and Judy?
  • A: Siblings have an r of 0.5, as do parent/offspring.

• Q: What is r between grandparents/grandchildren?
  • A: 0.25 (0.5 * 0.5); also aunt(uncle)/niece(nephew)

• Q: What is r between first cousins Trudy and Phil?
  • A: 0.125 (0.5 * 0.5 * 0.5).