Fig. 7. A classical example of Wright (1931) showing multifactorial inheritance with a threshold. The trait is presence of an extra toe in guinea pigs. Parental stocks show either three toes or four toes. A fraction of the F hybrids shows four toes ( dark shading ). Genetic analysis showed eight genes to be responsible for this trait. The number of animals showing this extra toe depends on the number of “plus” genes. (Data of Wright6: From Vogel F, Motulsky AG: Human Genetics Problems and Approaches, 3rd ed, p 211. Berlin, Springer-Verlag, 1997)