300 CHROMOSOMES IN HEREDITY: MECHANICAL 



chromosome taking part in them. It now becomes possible, there- 

 fore, to investigate crossing-over cytologically. This has already- 

 been begun, as we have seen. It is important in bringing a different 

 technique to bear on the problem and also in extending the observa- 

 tions of crossing-over to organisms that are not suitable for analysis 

 by experimental breeding (Table 44). This not merely increases 

 the material available for observation, it enables us for the first 

 time to study crossing-over in homozygous organisms which have 

 no variation in their progeny to show crossing-over, and in sterile 

 hybrids which have no progeny to show variations. 



The first consequence of applying this method is that it reveals 

 for the first time the universal properties of crossing-over as an 

 essential part of the mechanism of sexual reproduction. 



We already know that chiasmata are a condition of metaphase 

 pairing and segregation in all homozygous organisms, in fact in all 

 organisms except a few sex-heterozygotes which will be considered 

 later. It follows, therefore, that crossing-over itself is a condition 

 of chromosome pairing and is approximately co-extensive with 

 sexual reproduction, occurring even sometimes when reduction is 

 suppressed. This leads to consequences of some importance. Thus 

 the detailed properties of crossing-over determined in the female 

 Drosophila and in Zea have an almost universal appHcation, for 

 they agree with the almost universally observed properties of 

 chiasma formation. 



This means that with sexual reproduction the unit of inheritance 

 is not the whole chromosome but the smallest part into which a 

 chromosome can be broken by crossing-over, that is, the gene. If 

 this were not so the number of possible types of organism in a 

 species would be an exponential function of the number of types of 

 whole chromosomes in it, instead of being a slightly lower function 

 of the number of types of genes. Every chromosome would have 

 the properties of a clone. With mutation rates and breeding 

 systems that are commonly known in the higher organisms there 

 would be fewer types than individuals. Groups of individuals 

 would be identical like the members of a clone and would be 

 separated from other groups by sharp cleavages. So far as we know 

 such an extreme system of reproduction occurs nowhere, but, as we 



