Chapter 19, Problem 30P

Congenital dislocation of the hip is a threshold condition in which the head of the femur (the femoral head) is out of its normal position relative to the bones that will form the socket of the hip (the acetabulum). This misplacement can lead to potentially serious orthopaedic problems later in life if the condition is not treated in infancy. Numerous studies have shown that (a) brothers and sisters of infants born with congenital hip dislocation are more likely to develop the condition than are the siblings of those without the condition. These studies also find that (b) more female infants than male infants have the trait, and (c) if the affected child is a girl, the risk to her siblings is lower than if the affected infant is a boy. Explain the meaning of the three observations (a, b, and c) in the context of proposing a threshold model that explains these observations.