ABSTRACT
The fact on which the building up of a cross-over map is based has been mentioned in Chap, i; the additive theorem of linkage states that if A, B, and C are three closely linked factors the linkage value between A and C is the sum of the values between A and B, and B and C. A, B and C can therefore be taken to lie at definite points along the length of a chromosome, the distances between any two being proportional to the linkage value between these two. A chromosome may therefore be represented by a simple straight line with the position of the genes marked on it according to a scale in which unit distance represents one crossing over per 100 gametes (the "Morgan," a name not in general use).
