102 



AN INTRODUCTION TO MODERN GENETICS 



stration, which we must now examine, that crossing-over takes place 

 during the stage when the chromosomes are divided into chromatids. 

 The consequences of crossing-over (in the sense of the breakage 

 hypothesis) would be different if it occurred between two whole 

 chromosomes or between two out of four chromatids. In the first case 

 the four gametes formed from a germ-mother cell would consist of two 

 similar pairs, in the second case all four would be dissimilar. In an 

 ordinary diploid organism, the gametes resulting from one mother cell 

 cannot be separately identified, and the condition of affairs cannot be 

 directly determined. It is otherwise, however, in organisms with a well- 

 developed haploid phase : the four haploid spores are often aggregated 



2 STRAND 

 STAGE 



GAMETES 



Fig. 48. Two- and Four-Strand Crossing-Over. — With two-strand crossing- 

 over, shown above, the process of crossing-over is complete before the chromo- 

 somes have split into daughter chromatids, which will therefore form two pairs 

 of similars; and from them there must form two pairs of similar gametes. If crossing- 

 over takes place after the chromatids are formed, i.e. in the four-strand stage, 

 all four chromatids and therefore all four gametes, will be different. 



together in a cluster and can be isolated and their gametic constitution 

 separately determined. Until recently not very many cases of the 

 segregation of Unked factors have been analysed in diplo-haplonts, 

 since not many certain cases of linkage are known. Wettstein^ found 

 only 2-type tetrads in Funaria (i.e. crossing-over apparently between 

 chromosomes), but Allen^ (in Sphaerocarpus) has shown that 4-type 

 tetrads occur. In the last few years several examples of the inheritance 

 of linked factors in diplo-haplonts have been studied. The most com- 

 plete study is that of Lindegren on Neurospora, an Ascomycete.^ The 

 essential feature of the chromosome cycle is the formation, from the 

 fertilized zygotes, of an ascus in which the two reduction divisions 

 occur, giving four haploid cells which are arranged in a row, and 

 which then undergo a further, mitotic, division to give eight haploid 

 ascospores, arranged in a row as four pairs; from the ascospores the 

 ^ Wettstein 1924, 19286, ^ Allen 1926, 1935. ^ Lindegren 1933, 1936a, b. 



