2>& 



INHERITANCE 311 



chromosome ; and another race in which one of the X chromosomes 

 has been broken into two nearly equal parts — one part being 

 united to one of the other chromosomes. Moreover, recent work 

 emphasizes the significance of the order of arrangement of the 

 genes in a chromosome ; alterations of the usual order resulting in 

 hereditary changes referred to as position effects. 



All such chromosomal aberrations, rare and radical as most of 

 them are, may be regarded, in a way, as a broad extension of the 

 principal of recombination. They exert their influ- 

 ence by new relations and proportions of the genetic 

 material, and offer new hereditary possibilities in the 

 event that they are not lethal. y* ^x 



Gene mutations apparently involve intrinsic al- x 



terations in the individual genes themselves or even l ®' 7. f 



the origin of new genes, and probably are the most the X chromo- 



significant changes in the hereditary complex. They some in Dro- 



presumably are a result of an alteration in the so P^ lla as a 



i i-i • pi ii i result ot non- 



physico-cnemical constitution ol the gene; although disjunction. 



some would assign them mostly to position effects. See Fig. 191. 

 It appears that usually only one of a pair of homol- f t ron J,r e \ 

 ogous genes mutates at a given time so the change 

 is extremely localized, and most frequently takes place just be- 

 fore or during maturation. Although gene mutations occur rel- 

 atively rarely, several hundred examples have been identified 

 in Drosophila, chiefly by the elaborate studies of Morgan and his 

 collaborators — studies involving upward of 25 million fruit flies. 

 These experiments have made this tiny insect the greatest contribu- 

 tor to our knowledge of genetics since Mendel's experiments with 

 peas, and justified the award of a Nobel Prize to Professor Morgan. 

 Perhaps the best-known example of gene mutation in carefully 

 pedigreed animals is the sudden appearance, at long intervals, of 

 a single white-eyed Drosophila in a true-breeding red-eyed stock. 

 The white-eyed mutant breeds true from its origin, and the genetic 

 data indicate that a specific point on one chromosome — it has 

 been mapped — suddenly changed so that the developmental 

 processes that formerly gave rise to the usual red eyes thereafter 

 produced white eyes. And similar evidence of so-called 'point 

 changes ' has been obtained in a number of other kinds of animals 

 and in plants that have been bred under controlled experimental 

 conditions. 



