250 CHROMOSOMES IN HEREDITY : MECHANICAL 



combine them and use the conclusions in a joint attack on this prob- 

 lem. It is a fundamental problem, for on its solution depends our 

 understanding equally of the mechanics of structural changes in the 

 chromosomes and of the genetical consequences of these changes. 

 These affect all sexually reproducing organisms, both in the pairing 

 of their chromosomes and in the recombinations of their hereditary 

 characters. 



Let us consider first the evidence of experimental breeding. 



(ii) The Genetic Theory. In all organisms in which the inheritance 

 of a large number of mendelian factor-differences has been studied, 

 certain of these have been found to be " linked," j.^., when individuals 

 differing in two such factors are crossed, the proportion of new 

 combinations in the second generation is lower, and of old 

 combinations higher, than would be expected from free assortment 

 (according to Mendel's second law). The general conditions of this 

 linkage are similar in most structurally homozygous diploids that 

 have been studied, e.g., in Drosophila species, Zea Mays, Pharbitis, 

 Primula sinensis, Pisum sativum, etc., and observations of excep- 

 tionally high linkage in Le^is^^s, Apotettix, Cepea, Funaria, etc., are 

 intelligible as the result of an irregularity in the spacing 

 of genes in the chromosomes which is merely an exaggeration of 

 that now known in Drosophila. 



In 1909 Janssens suggested that the paired chromosomes broke 

 and rejoined at meiosis and that the chiasmata, whose structure 

 was then not clearly understood, were the result of this recombina- 

 tion. In the light of this chiasmatype theory Morgan in 1911 was able 

 to put forward the explanation of linkage that is now accepted and 

 has been the basis of genetical analysis since that time. Morgan 

 assumed (i) that linkage of factors is due to the specific particles or 

 genes that determine the characters concerned lying in the same 

 chromosome in a linear order, and (ii) that the recombinations of 

 factors are due to crossing-over or an exchange of homologous 

 segments containing those genes between partner chromosomes. 



Thus, if the factors are at opposite ends of chromosomes : ABODE 

 and abcde, crossing-over might give ABCde and ahcDE. Crossing- 

 over should be more frequent between A and C than between A 

 and B, owing to its occurring at any point along the chromosome. 



