By chance, half of their sons should be normal and half should be doubly affected, if there is no recombination. III-4, the color blind child, is a recombinant. In this pedigree the best estimate of linkage distance is 12.5 centimorgans. Of course, several other families will have to be added to this sample for meaningfully accurate estimates to be made. In autosomal linkage studies, this does not hold.
Recombination can, and does, occur in both oogenesis and spermatogenesis.
Useful pedigrees for linkage studies using classical methods are quite limited. Other methods must be employed. These are discussed quite well in Gelehrter, Collins, and Ginsburg, 2nd ed., Chapters 9 and 10, and need not be replicated here. The student is referred to these chapters and is responsible for all of the material they contain. Sample questions from this material are included.