106 



AN INTRODUCTION TO MODERN GENETICS 



chromatids which need not be completely different, and the diploid 

 gametes can contain two chromatids which are partly alike and partly 

 different. The parts for which the two chromatids are alike have shown 

 equational reduction; that is the reduction has failed to separate the 

 two alike sister chromatids. The flies which develop from these eggs 

 are therefore known as equational exceptions and may show, in a 

 homozygous condition, a factor contained in a single dose in their 

 mother. 



^f 



A ?• 



\ 



Fig. 50. Crossing-Over in Triploids. — ^The upper row shows crossing-over in 

 the "2-strand" stage, i.e. before the chromosomes have split. The six chromatids 

 form three pairs of similars. Below is crossing-over in the "4-strand" stage, 

 i.e. after the split; the six chromatids are all different, and if two of them are 

 chosen, they may be alike in some parts but not in others. The first and third 

 chromatids, counting from the left, are recurrent double cross-overs, while the 

 second is a progressive. 



We have explained the occurrence of equational exceptions or 

 chromatid segregations, which is another name for the same pheno- 

 menon, as a consequence of crossing-over among the chromatids 

 between the factor in question and the centromere. If this is true the 

 frequency of equationals should be high for factors lying far away from 

 the centromere, for which there will be a high chance of a cross-over 

 between the factor and the centromere, and low for factors lying 

 nearer to it. The highest frequency which can be expected is that due 

 to pure chance; if one diploid gamete contains a certain factor, the 

 probability that it will also contain the same allelomorph from the 

 sister chromatid cannot be greater than the chance of choosing the 

 sister from among the five that are left, namely 20 per cent. The actual 

 frequencies of equationals accordingly vary from near the centromere 

 towards 20 per cent at the distal end of the X (actually the highest 



