HEREDITY 235 



Conjugation of the gametes. It is a matter of chance which kind 

 of ovum conjugates with which kind of spermatozoon. Thus : 



ABCD + ABCD = ABCDABCD (all paternal) 



b c d a b c d (all maternal) 

 B C D A b c d (mixed characters 



from both par- 

 ents.) 

 b C D AB cd do. 



and so on through 256 arrangements. 



Therefore hybridity " rings the changes," so to speak, on these 

 " loose " character-components. 



81C. '' Crossing-over " of the Chromosomes. We have 

 considered only a small number of characters that can so be 

 rearranged when mutually and indefinitely fertile races cross. We 

 take only 4 (haploid) chromosomes and, for simplicity, we assume 

 that each of these " carries " a factor that is responsible for the 

 appearance of a character. But there may be about 200 characters 

 in a species (Drosophila) and there are only 4 chromosomes, so 

 it appears that each of the latter must " carry " many characters. 

 The hypothesis (founded on evidence) is that a chromosome 

 carries a group of " linked " characters. Where there is sexual 

 mating it is such grouped characters that " go into the cross." 

 Thus instead of assortments and reassortments of single characters 

 (as in the above schemes) there may be assortments and reassort- 

 ments of " linked " or grouped characters, for instance, when 

 night-blindness is '' linked " with maleness (in men). Now we 

 must consider the conception of 



Genes. There are, we suppose, only 4 chromosomes but there 

 may be (say) 1 50 characters that can be observed to behave in the 

 Mendelian way. It can often be seen that a chromosome is made 

 up of a single, or double row of granules. This has suggested 

 that there are hypothetical counterparts, in the chromatin of the 

 ova and spermatozoa, of the adult bodily characters. We cannot 

 actually see these things that are the counterparts, but they are 

 supposed to be present in the chromosomes as entities called 

 genes. In the case of the fly, Drosophila, chromosome maps have 

 been made to show the distribution of the genes in the latter 

 structures and it is an essential part of Mendelian hypothesis that 

 the genes are arranged linearly. 



