222 HEREDITARY TRANSMISSION [CH. 



for each mitosis. Since, however, there are two chromo- 

 somes of each sort in the diploid nucleus, it was supposed 

 that the small units of which they were composed were 

 interchangeable, and that therefore the chromosomes of one 

 mitosis were not necessarily identical with those of the pre- 

 ceding, but that the chromosomes of each homologous pair 

 were made up from a double set of microsomes. The micro- 

 somes would be in two series, a, b, c,d... and a', b f , c ', d' ..., 

 arranged in this order, but when the mitotic chromosomes 

 are formed from the resting nucleus a is interchangeable with 

 a ', b with b', and so on. It has been seen in the preceding 

 chapter that there is evidence for the compound nature of 

 chromosomes, at least in some species, but the evidence 

 for their general individuality as complete wholes does not 

 favour this hypothesis of the interchange of parts in the 

 resting nucleus. 



The matter has entered on a new phase since the discovery 

 and thorough investigation of the phenomenon known in 

 England as gametic coupling, and in America as linkage, first 

 described by BATESON and PUNNETT in the Sweet Pea, but 

 now known in many plants and animals, and most fully 

 investigated by MORGAN and his associates in the fly Droso- 

 phila. The essence of gametic coupling is that if varieties 

 are crossed which differ in two or more characters, the 

 characters that are associated in the parents tend to be 

 associated in the gametes of the heterozygous offspring. If, 

 for example, an individual bearing the characters A and B 

 is mated with one bearing their allelomorphs a and b, the 

 heterozygote Aa Bb, instead of producing in equal numbers 

 four kinds of gametes bearing AB, Ab, aB, ab, produces 

 great excess of AB and ab and relatively few Ab and aB. 

 This "coupling" in transmission by the heterozygote of 

 characters that were associated in the parents strongly 



