696 HEREDITY AND CELL DIVISION 



showing that the factors which cause them do not go into the 

 gametes entirely independently. This, which is called linkage, 

 was discovered in the sweet pea by Bateson and Punnett in 1902, 

 but the great development of our knowledge of it followed the 

 use as a breeding animal of the small grey fruit fly, Drosophila, 

 and is due chiefly to Morgan and his school in New York from 

 1 9 10 onwards. 



In one of their experiments a wild-type fly, BBVV, was crossed 

 with a form having two recessive characters, a black body and 

 vestigial wings, bbvv, and gave a grey normal-winged hetero- 

 zygous Fi, BbVv. Females of this were then back-crossed to 

 bbvv males, giving this result 



$ BbVv X bbvvc^ 



BbVv bbVv Bbvv bbvv 



586 106 III 465 



On Mendelian principles equal numbers of the four types would 

 have been expected, but instead, of the 1268 gametes formed 

 by the heterozygous F^ female, 105 1 resemble those from which 

 it was derived, and only 217 are the new combinations bV and 

 Bv. Instead of 50 per cent, of the gametes being recombinations 

 only 217 out of 1268, or 17. i per cent. are. Later experiments 

 showed that, for a given pair of characters, this recombination 

 value is roughly constant, and may have any value from o to 50. 

 The former is a theoretical limit, which, if it is ever achieved, 

 must be indistinguishable from the pleiotropic effects of a single 

 factor. 



CHARACTERS AND CHROMOSOMES 



The position which we have now reached is that characters 

 are passed from parents to offspring by factors contained in the 

 gametes in such a way that only one factor concerned with a pair 

 of contrasting characters is present in a single gamete ; that 

 factors for different pairs of characters are inherited either 

 independently or in such a way that there is a definite percentage 

 association between two members of the pairs ; and that, with 

 this exception, the sorting-out and recombination of the factors 



