278 CHROMOSOMES IN HEREDITY: MECHANICAL 



this heterozygous type, AB- AB- will be produced, a homozygous 

 form with more extreme abnormality than the heterozygote. Such 

 a new form, however, usually results from pairing of B and C, and 

 this pairing involves crossing-over between these dissimilar chromo- 

 somes. When the changed C chromosome is lost, this crossing-over 

 makes no difference. But sometimes it is not, and we have a gamete 

 of the make-up ABC^ (C^ being a C chromosome which contains a 

 segment of B through crossing-over) and zygotes of the make-up 

 ABC, ABC^ and ABC^, ABC^ are produced. These are hetero- 

 zygous and homozygous for the C^ chromosome, and appear as 

 mutants from the original type. Such mutants must be taken to 

 be the products of secondary segregation in the polyploid. They 

 have been found in Primula kewensis and Nicotiana Tahacum (R. E. 

 Clausen, 1931), but it is in the hexaploid cereals that their effect on 

 inheritance has been studied in the greatest detail. Spelt oid wheats 

 and fatuoid oats both arise by these means (Winge, 1924 ; Huskins, 

 1927, 1928 ; Hakansson, 1931 ; Nishiyama, 1931, 1933 ; Philp, 

 1933)- Moreover, it has been possible to identify the parts of the 

 C chromosome whose loss is responsible for mutation in this way. 

 Heterozygous fatuoids, ABC, AB — , produce some offspring in which 

 the C chromosome has been replaced by a shorter chromosome 

 which has lost either the shorter arm which suppresses the fatuoid 

 character or the longer arm which conditions normal meiosis. 

 (Nishiyama, 1935 ; Uchikawa, 1934). Both these losses are pre- 

 sumably due to crossing-over between the middle of the C chromo- 

 some and an homologous segment near the end of B or some other 

 chromosome. 



The third circumstance of secondary structural change is by intra- 

 haploid pairing. This occurs in haploids, triploids and imbalanced 

 forms such as trisomic diploids and trisomic tetraploids. In all 

 these there are one or more unpaired chromosomes. If these contain 

 amongst themselves reduplicated segments they will pair, cross- 

 over and give secondary structural changes. There is now evidence 

 for every step in the process in different organisms. The reduplicated 

 segments have been found in the haploid complement of Drosophila 

 (Bridges, 1936 ; MuUer, 1935). The pairing between different 

 chromosomes in the haploid has been found in CEnothera (Catcheside, 



