DOCTRINE OF EVOLUTION AND RELATED IDEAS 503 



maternal and paternal chromosome of a pair contains genes for 

 just the same series of unit character (hereditary traits). For 

 this reason they are called "homologous" chromosomes; and 

 the homologous genes which they carry are at similar levels in 

 the chromosomes. Every unit character therefore is represented 

 by two genes in each such cell. 



4. In these homologous genes there is another complication. 

 The two genes, while relating to the same unit character, are not 

 necessarily identical. For example, the gene for "tallness" 

 and that for " dwarf ness" relate to the same quality, but differ 

 in their influence. The two homologous chromosomes might in 

 the case of peas carry two genes for tallness, or two for dwarfness, 

 or one for each, depending on the character of the parents. 



5. During the development of the body of an individual 

 plant or animal, these pairs of maternal and paternal chromo- 

 somes retain their independence within the nuclei. However, 

 in the maturation of the new generation of germ cells, whether 

 eggs or sperm, these homologous chromosomes actually unite in 

 pairs (see §50) and form the bivalent chromosomes. At a subse- 

 quent division these pairing chromosomes separate and pass to 

 two different cells (segregation) . In this way each daughter cell 

 (egg or sperm) receives only one of each pair of the homologous 

 chromosomes, and hence only one from each pair of genes. This 

 is the reduction division so frequently referred to, and seems to 

 explain how the "purity of the gametes" may be secured. If 

 the pairing chromosomes both carry the identical gene (as for 

 tallness), the germ cells would all be alike in respect to this 

 character. If however one of the pair of genes is for tallness 

 and the other for dwarfness, the resulting germ cells would be 

 wholly different in this character, and would carry a very differ- 

 ent inheritance. 



6. The fact that certain unit qualities tend to appear 

 together, as the linking of color blindness with sex in human 

 beings or color with sex in mating black Langshans and barred 

 Plymouth Rocks, is believed to be due to the tendency of certain 

 adjacent genes to hang together more tenaciously than the 

 average. This association is not, in all instances of linkage, an 

 absolutely fixed one. 



