172 Locating Genes on Chromosomes 



If two genes give 5 per cent of recombination types when the 

 experiment is conducted under standard conditions, they are 

 said to be five map units apart on the chromosome. 



Location of Three Genes 



A rule widely followed in plotting genes is that if genes A and 

 B are known to be linked, and if gene C is found by experiment 

 to be linked with A, it must also be linked with B. This principle 

 follows from the fact that two linked genes are on the same 

 chromosome. If A and B are on the same chromosome, and if 

 A and C are found to be on the same chromosome, naturally B 

 and C must be on the same chromosome also. This statement 

 may seem too self-evident to be worth mentioning, but consider 

 the following situation. If genes A and B are linked with 20 per 

 cent crossing over, and if the heterozygote is backcrossed to the 

 double recessive, 80 per cent of the offspring would be parental 

 types and 20 per cent recombination types. Let us assume now 

 that A is tested with C and that 30 per cent of the offspring of 

 this testcross are found to be recombinations. Genes A and B 

 would then be on the same chromosome 20 units apart and genes 

 A and C would be on the same chromosome 30 units apart. If 

 they were arranged in the order C-A-B, genes B and C should 

 therefore be 50 units apart, as shown in this diagram: 



30 20 



50 



U a C B / cb organism is now crossed with one that is cb / cb, 

 there should be 50 per cent of parental types and 50 per cent 

 of recombinations among the offspring. It would, however, mean 

 a ratio of lCB:lCb: lcB:lcb, exactly the ratio that 

 would be obtained if the genes showed independent assortment. 

 If only genes B and C were investigated, there would be no evi- 

 dence that they were linked, but B must be linked with C since 

 both are on the same chromosome as A. 



In the diagram, the three genes were mapped although the 

 crossover percentage between B and C was not known. How 

 much information must be available to locate three genes in 



