174 Locating Genes on Chromosomes 



two genes that are very close together. AVhen dealing with 

 genes that are far apart, we find that sometimes two or even 

 more breaks may occur between them. When one chiasma is 

 formed between two genes, the break and fusion of the broken 

 chromatids need not always occur at the same place, for as the 

 result of chance it should occur at any place between the two 

 genes with equal frequency. No matter where the break be- 

 tween two genes occurs, the results are exactly the same, pro- 

 vided that only one such break occurs. If, however, two breaks 

 and exchanges occur at the same time, and two chiasmata are 

 formed between the two genes, the results will be different. 



There are several ways in which two chiasmata can be formed 

 betw^een two given loci of a chromosome, as are shown in Chap- 

 ter 13, depending upon which chromatids are involved in form- 

 ing each chiasma. If the same two chromatids are involved in 

 each break so that two reciprocal chiasmata are formed between 

 genes A and C, the four gametes resulting will be exactly the 

 same as if no chiasma had occurred between these genes (Fig. 

 55a). It may fairly be asked how it is known when two chi- 

 asmata were formed between A and C and when none was formed 

 since the genetic results are the same in each situation. The 

 answer is that there is no way of knowing unless three heterozy- 

 gous genes, such as A, B, and C, are present and one crossover 

 occurs between A and B and the second between B and C. Then, 

 as illustrated in Fig. 55b, the two noncrossover chromatids would 

 he A B C and ab c and the two crossover chromatids would be 

 a B c and Ab C. It is only when three such genes are present 

 that the organisms produced by a gamete in which the two 

 crossovers occurred can be distinguished from those in which 

 none occurred. 



When two crossovers are formed between two genes, the phe- 

 nomenon is known as double crossing over, and the a B c and 

 Ab C gametes are double crossover gametes. Naturally, double 

 crossing over does not occur between two given genes as fre- 

 quently as single crossing over, but it does have the effect of 

 appearing to reduce the amount of crossing over that occurs 

 between them. For that reason, data involving three genes are 

 more accurate than data which include the two extreme genes 

 only; and this is more noticeable the farther apart the two 



