PROCEEDINGS OF THE OHIO ACADEMY OF SCIENCE. 365 



homozygous and the male heterozygous as regards sex. In a 

 few cases the opposite appears to be true, — the eggs are of 

 two classes and the spermatozoa alike ; thus the male is ho- 

 mozygous and the female heterozygous. Whether all animal and 

 plant species conform to these two types of sex inheritance it 

 is probably too early to predict ; but, at least, the discovery of 

 the sex chromosomes gives a promising clue to the long dis- 

 cussed but hitherto hopelessly tangled snarl of theory in regard 

 to sex determination. 



When the chromosomes are mentioned as the bearers of 

 hereditary units, it is not, of course, implied that there is a 

 chromosome for each character, — the very large number of 

 characters and the relatively very small number of chromosomes 

 effectually negative such an idea. Thus in the fruit fly, 

 Drosophila, which has been brought into such prominence through 

 the brilliant intensive researches of Morgan and his students, 

 there are but four chromosome pairs in the somatic cell, four 

 chromosomes in the gamete. If, then, a group of hereditary char- 

 acters is found in each chromosome, and if, as is generally be- 

 lieved, the chromosomes are somewhat permanent structures, it 

 follows that the characters borne by a single chromosome should 

 be closely associated in heredity, — in technical expression, should 

 be linked. In actual observation this linkage is a recognized 

 fact. It was first observed by Bateson and Punnett in 1906, and 

 its possible relation to the chromosome theory of inheritance was 

 suggested by Lock in the same year. Drosophila, with its rapid 

 breeding and extreme frequency of mutation, affords exception- 

 ally favorable material for the study of linkage; and since 1910 

 Morgan has recorded about a hundred linked characters. Of 

 these 47 are shown to belong to one great linkage group, 27 to 

 another, 22 to a third, and 2 to a fourth group. The corre- 

 spondence of four linkage groups and four chromosomes is 

 significant. 



But, while there is this very evident tendency to linked in- 

 heritance, the linkage is not complete, nor is it equal for all the 

 characters of a group. Thus, while two characters coming from 

 one parent may usually hold together in inheritance, one may 



