The Cellular Basis 185 



These relations are shown schematically in the accompanying 

 diagrams (Fig. 60 a). 



2. LINKAGE OF CHARACTERS AND CHROMOSOMAL LOCALIZATION. 

 Finally the study of characters which are linked together in 

 heredity, joined with the study of the chromosomes and their dis- 

 tribution in the maturation and fertilization of the germ cells, has 

 not only confirmed the chromosomal theory of heredity but has 

 also shown that certain chromosomes carry the genes for certain 

 characters and has even indicated the relative positions of dif- 

 ferent genes in the chromosomes. 



Thanks to the work of Bateson, Morgan, and many others, it 

 is now known that many characters are linked together in inheri- 

 tance. Darwin had long ago noted that male albino cats with blue 

 eyes are usually deaf and many other cases of the association of 

 peculiar characters had been reported by earlier observers. In 

 1906, Bateson and Punnett found that sweet peas with purple 

 flowers usually have elongated pollen grains, whereas those with 

 red flowers have round pollen. Since 1910 Morgan and his pu- 

 pils have discovered about four hundred new characters, or 

 mutations in the pomace fly, Drosophila melanogaster, which are 

 usually linked together in four groups. 



a. Sex-linked Inheritance. The first cases of such linkage 

 studied by Morgan were in characters which are usually asso- 

 ciated with one or the other sex, but which may have nothing to 

 do with reproduction and may affect any part of the body. Such 

 characters are not necessarily limited to one sex or the other, as 

 are many primary and secondary sexual characters, but they may 

 appear in either sex though they are usually transmitted from 

 fathers to daughters, or from mothers to sons ("criss-cross" in- 

 heritance) in exactly the way in which the sex chromosomes (X) 

 are transmitted. Morgan has therefore concluded that the factors 

 for these characters are carried by the sex chromosomes and has 

 named them sex-linked characters. In the pomace fly, Drosophila, 

 he has discovered a large number of such characters which are 



