HEREDITY IN SOMATIC CELLS 367 



cis arrangement with it, the point of the exchange can be immediately 

 deduced from the phenotvpe. For instance, if segregants homozygous 



for y arc selected from a stock with the genotype 



pro pa ha y 

 crossovers between paba and y will be only yellow in phenotype, cross- 

 overs between pro and paha will be yellow and paha-lcss, while cross- 

 overs between pro and the centromere will be yellow, pabaAess, and 

 prolineless. If the proximal markers are in trans arrangement with the 

 selected marker, the position of the exchange must be ascertained by 

 further analysis. It should be evident that the farther the selected 

 marker is from the centromere, the more useful it will be for mapping 

 the arm of the chromosome that contains it. 



Table 12.1 shows a sample of data collected for mapping chromosome 

 I. The centromere can be located by noting that point of change in 

 the direction with which homozygosis for one marker is always associated 

 with homozygosis for a series of others. In the example, this is between 

 an and pro. The distances on the map in Table 12.1 are expressed as 

 fractions of the total amount of exchange in each arm, which is set at 

 100. This is done because the absolute incidence of mitotic crossing 

 over is small, and estimates of mitotic recombination fractions are un- 

 reliable. A comparison of mitotic and meiotic maps made on this basis 

 shows that, although the order of genes is the same, the rate of exchange 

 between them differs during the two processes. 



Diploids that have become homozygous at one or more loci but are 

 still heterozygous at others may give rise to homozygotes for the latter 

 by further crossing over in series, as shown in Figure 12.7. Or at any 

 time they may be reduced to the haploid state by somatic reduction, a 

 process which brings about recombination among nonhomologous 

 chromosomes without any relation to crossing over. This can be seen 

 from an analysis of the diploid described in Table 12.1. Its composition 

 with regard to four chromosomes was: 



Chromosome I 

 Chromosome II 

 Chromosome V 

 Chromosome VIII 



su ribo an + + + bi 



n ■ 



pro paba y + 



+ 



Selection yielded 126 yellow y w^ haploids. These were all acr^ and, 

 with one exception, ad'^; the ad exception is probably a coincidence 



