178 SECTIONAL ADDRESSES; 



According ro Morgan the frequency of separation of linked 

 characters is a measui'e of tHe distance apart in the chromosome of the 

 loci for the factors concerned, and it becomes possible to map their 

 position in the chromosome relatively to one another. In this attempt 

 to find in cytological happenings a basis for the observed facts of 

 inheritance our conception of the material unit in the sorting-out 

 process has been pushed beyond the germ cell and even the entire 

 chromosome to the component sections and particles of the latter 

 structure. 



To substantiate the ' chromosome ' view the primary requisite was 

 to obtain proof that a. particular character is associated with a particular 

 chromosome. With this object in view it was sought to discover a 

 type in which individual chromosomes could be identified. Several 

 observers working on different animals found that a particular chromo- 

 some differing in form from the rest could be traced at the maturation 

 division, and that this chromosome was always associated with the sex 

 character in the following manner. The female possessed an even 

 number of chromosomes so that each egg received an identical number, 

 including this particular sex -chromosome. The male contained an 

 uneven number, having one fewer than the female, with the result 

 that half the sperms received the same number as the egg including 

 the sex-chromosome, and half were deficient in tliis particular chromo- 

 some. Eggs fertilised with spenns containing the full number of 

 chromosomes developed into females, while those fertilised with sperms 

 lacking this distinctive chromosome produced males. Morgan made 

 the further discovery in the fruit fly Drosophila ampelophila that certain 

 factors controlling various somatic characters were located in the sex- 

 chromosome. The inheritance of these characters and of sex evidently 

 went together. A male exhibiting the dominant condition of such a 

 sex-linked character bred to a recessive female gave daughters all 

 dominant and sons all recessive (fig. 2), but in the reciprocal cross both 

 sons and daughters proved to be all dominants (fig. 1). Since the 

 mother with the dominant factor contributed it to all her children 

 (fig. 1), whereas, where the father bore it, it descended only to his 

 daughters (fig. 2), it was apparent that the female was homozygous 

 and the male heterozygous for the somatic character. Further, 

 although no distinction is observable in this species between the sperms, 

 the occvuTence of this sex-linked form of inheritance indicated that here, 

 as in the other cases mentioned, it is the female which behaves as a 

 homozygote for the sex character and the male as a heterozygote, the 

 sex-chromosomes of some sperms differing presumably in character, 

 though not in appearance, from those of others. The sperms of 

 Drosophila are therefore conceived as of two kinds, one containing the 

 same sex-chromosome as the eggs, the so-called X chromosome, and 

 the other a mate of a different nature, the Y chromosome, which 

 appears to be inert and unable to carry the dominant allelomorphs. 

 If. now, we suppose the factor for the sex-linked somatic character 

 to be located in the X chromosomes we understand whv the dominant 

 female, which is XX, and therefore furnishes an X chromosome 

 to every egg, should contribute the dominant character to all her 



