Chapter 11 

 LOCATING GENES ON CHROMOSOMES 



Since there are many different genes on a chromosome, they 

 must be arranged in some sort of order. In a number of plants 

 and animals the exact or approximate places at which many 

 of the genes are located have been determined. The work of 

 locating genes is tedious but not difficult, once the principles of 

 the technique are understood. The general method is genetical 

 and is based upon crossover data. It gives the order in which 

 the genes occur and their distances apart as well as can be 

 determined from crossover data, but it does not necessarily give 

 the exact points at which the genes are found on the chromo- 

 somes. This method can be followed by a cytological method 

 which will locate them much more precisely on the chromosome. 



The genetic method is based upon the assumptions that cross- 

 ing over is due to breaks in the chromatids, that these breaks 

 occur purely by chance, and that the possibility that a break 

 can occur is the same for all parts of a chromosome. If these 

 hypotheses are true, it follows that the farther apart two genes 

 are, the greater the chance that a break will occur between 

 them, and, when tested genetically, the higher the percentage of 

 crossovers between them. In practice, the percentages of cross- 

 ing over between various genes are obtained experimentally, and 

 from that information the genes are mapped in their order on 

 the chromosome. In mapping genes, a unit of distance must be 

 used, just as in mapping cities or anything else. The unit used 

 in genetics is one per cent of crossing over, called a map unit 

 or a unit of map distance. As shown in the last chapter, the 

 percentage of crossing over is influenced by both internal and 

 external conditions. When the internal conditions are known 

 they can be discounted, but when they are not known they may 

 lead to erroneous conclusions. The external conditions can be 

 controlled, and ''normal" or "standard" conditions are used 

 when obtaining crossover data for the purpose of mapping genes. 



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