356 RADIATION BIOLOGY 



the orientation of one tetrad in this regard does not influence that of 

 another. 



If now the second pair of genes being considered should lie in the same 

 pair of chromosomes as the first pair, the frequency of recombination 

 found among the gametes is lower than 50 per cent, and these genes, lying 

 in the two different positions or loci in the same pair of chromosomes, are 

 said to be linked. That there is some recombination even of genes in the 

 same pair of chromosomes is due to the process of crossing over which 

 occurs during synapsis. This involves the breakage of one of the two 

 paternal and one of the two maternal chromatids of a tetrad at exactly 

 corresponding points, followed by cross-union between the broken ends, 

 in such wise that one of the chromatids formed is a combination derived 

 from the maternal member on one side, say the left, of the point of 

 breakage and reunion, and from the paternal member on the other side, 

 the right, while the other chromatid, complementarily, comes to have a 

 paternal left-hand portion and a maternal right-hand one. Thus if the 

 maternal chromosome had contained the genes A and B and the paternal 

 one the homologous but somewhat different alleles a and b, the two cross- 

 over chromatids, formed as a result of crossing over between the genes 

 at the two loci, would have the composition Ah and aB, respectively, 

 while the other two chromatids of the tetrad, being noncrossovers, would 

 still be of compositions AB and ah, respectively. 



In most species crossing over can take place at virtually any point along 

 a pair of conjugating chromatids. Hence, in the case of genes that lie 

 farther apart along the chromosome there will be a tendency to have a 

 higher frequency of crossover combinations formed than in the case of 

 genes that lie close together. Thus the frequency of crossing over can 

 be used, conversely, as an indication of the distance apart of genes, and 

 has lent itself to the plotting of "maps" showing the position of the genes 

 in the chromosome. In these linkage maps, that distance which for con- 

 venience is designated as "one unit" is a distance having 1 per cent of 

 crossing over within it. Crossing over can occur in more than one posi- 

 tion at a time in a given tetrad, but at distances closer than a given 

 length there is a tendency, called interference, which increases with 

 proximity, for crossing over at one point not to occur so readily as usual 

 when there happens to be crossing over at another point. The two 

 chromatids which participate in crossing over at one point can have one, 

 both, or neither of their members the same as those which participate at 

 another point in the tetrad. Both the frequency of crossing over and the 

 amount of interference are influenced by various physiological and 

 genetic conditions, as well as by external agents. They also vary some- 

 what from one chromosome region to another. In the neighborhood of 

 the centromere, crossing over is much less frequent than elsewhere for 

 a given physical length of the chromosome thread, and it is there 



