LINKAGE OF POLYGENES 



the seed weight distribution in Fo, but no major discontinuities 

 appeared. The heritable variation is thus polygenic. But when Sax 

 determined the average weights of the seeds in his three classes, 

 distinguished by the seed colour gene, viz. homozygous coloured 

 (PP), heterozygous coloured {Pp) and white (pp), he found that 

 they differed. The PP plants had the largest seeds and the pp plants 

 the smallest, just as the coloured parental strain had larger seeds 

 than the white one. 



Such an association between size and colour would be expected 

 if some, at least, of the polygenes controlling seed size were linked 

 with the major gene controlling colour. And since the major gene 

 must be carried in the nucleus, so must the polygenes. On one point, 

 however, the experiment is not decisive. That part of the difference 

 in seed size which was associated with the colour difference could 

 have been a secondary effect of the major gene which itself deter- 

 mined the colour difference. In other experiments, however, this 

 possibility can be ruled out. We may take, as an example, an experi- 

 ment done by Mather and Harrison on the polygenic system con- 

 trolling variation in the number of hairs on the undersides of the 

 4th and 5th abdominal segments of Drosophila melanogaster. With 

 hair-number the class groupings to be used in the frequency 

 distribution are of course decided for us, just as though we could 

 not measure height except in whole inches. 



Drosophila melanogaster, it will be recalled, has three large pairs of 

 chromosomes and one small pair. This last is, in fact, so small that 

 it can be neglected for most purposes. Most of the genes belonging 

 to any polygenic system will be found on the three larger ones. Now 

 those three large chromosomes can be marked by certain gene 

 differences which are capable of being followed by the mendelian 

 method, and which are particularly convenient because hetero- 

 zygotes for each of them can be recognized phenotypically. The genes 

 used in the experiment were Bar (jB) affecting the shape of the eye. 

 Plum (P/n) affecting the colour of the eye and Stubble [Sh) affecting 

 the shape of certain bristles (though not, it should be remarked, 

 altering the number of abdominal hairs so far as is known). B marks 

 the X chromosomes, Pm the second chromosome (II) and Sh the 

 third (III). These genes were combined with inverted pieces of 

 chromosome, which as we shall see in Chapter 6 reduce, or in 



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