THE LINEAR DIFFERENTIATION OF THE CHROMOSOMES IO3 



haploid mycelia are developed. By a micromanipulation technique the 

 ascospores can be isolated and grown individually and their genetic 

 constitution determined. We shall consider the data on crossing-over 

 relating to the factor P (pale ascus as against normal p) and the alter- 

 native sex factors known as + and — . Four-type "tetrads'* occur; 

 owing to the extra mitotic division, the tetrads here consist of groups 

 of four pairs of ascospores instead of four single spores. The cross-over 

 percentage was determined by the number of recombinations in the 

 usual way and was 22 • 5 per cent. 



Other evidence that crossing-over takes place between chromatids 

 and not between chromosomes can be produced if we make the assump- 

 tion, which will be shown to be true in higher forms, that the centro- 

 meres of the two chromosomes always disjoin from one another in the 

 first division. If this is the case, at the centromere the two sister chroma- 

 tids from one chromosome are always separated from the two sisters 

 from the other chromosome (a reductional separation). The only way 

 in which two non-sister chromatids can get to the same pole after the 

 first division is by the occurrence of a cross-over between chromatids, 

 when a separation of this Itind (equational separation) is possible for 

 the regions distal to the crossing-over. Now it can easily be shown 

 that equational separation does occur. If we have two factors which 

 are not linked and if they are both separated reductionally at the first 

 division, they must give tetrads consisting of two pairs of similar spores 

 (2-type tetrads), while if either or both are separated equationally at 

 the first division and reductionally at the second, 4-type tetrads will 

 result (see Fig. 49). 



Very many investigations on non-linked factors have revealed the 

 presence of such 4-type tetrads and therefore the occurrence of a cross- 

 over, in the chromatid stage, between the segregating factor and the 

 attachment constriction. Segregation of this kind is known as chromatid 

 segregation, since the mechanism segregates the two allelomorphs from 

 one another when they are in a body consisting of four chromatids 

 instead of segregating them from a body consisting of two chromosomes. 



The considerations we have just discussed enables one to obtain an 

 estimate of the distance, in cross-over units, between a certain factor 

 and the centromere. We have seen that if in a heterozygote a cross-over 

 occurs (in the chromatid stage) between the factor and the centromere, 

 one of the two chromatids going to each pole of the ist division will 

 contain one allelomorph and the other the other, and the segregation 

 of the faaor will not occur until the second division. The fact that segre- 



