238 Mechanism of Mendelian Heredity 



X and Y. In Abraxas and in the fowl the female 

 is heterozygous for sex and the male homozygous. 



3. If the chromosomes are the vehicle for Mendelian 

 heredity it should be possible to show that the various 

 hereditary characters which follow Mendel's law must 

 be distributed over the various chromosomes; and it 

 should be possible to find out which characters are 

 contained in the same chromosome. It has already 

 been stated that sex-linked heredity is intelligible on 

 the assumption that the X chromosome carries the sex- 

 linked characters. T. H. Morgan and his pupils have 

 shown with the greatest degree of probability that 

 corresponding linkages occur in the other chromosomes 

 and that there are in Drosophila exactly as many groups 

 of linkage as there are different chromosomes, namely 

 four.^ 



Mendel had found that when he crossed two species 

 of peas differing in regard to two pairs of characters, 

 he obtained in the F2 generation results which he 

 calculated on the assumption that the segregation of the 

 two pairs of characters in the sex cells of the hybrids 

 took place independently of each other. To illustrate 

 by an example: When crossing a yellow round pea 

 with a green wrinkled variety in which the characters 

 round and yellow are dominant, green and wrinkled 

 recessive, all the hybrids of the F, generation had the 



» Morgan, T. H., Sturtevant, A. H., Muller, H. J., and Bridges, C. B., 

 The Mechanism of Mendelian Heredity. New York, 191 5. 



