1 86 Heredity and Environment 



one X chromosome in the male; all homozygous females that 

 have the factor in both X chromosomes die, while only those sur- 

 vive that are heterozygous for this factor. (Such a heterozy- 

 gous female produces in equal numbers eggs with and without 

 the lethal factor and if she is bred to a normal male all of the 

 daughters are viable though half of them carry the lethal factor 

 in one of the X chromosomes, but all of the males that receive 

 the lethal factor are non-viable since the male has only one X 

 chromosome, while all the males that survive lack this factor 

 altogether. Thus the sex ratio in this case is 2 females to I male. 

 Other lethal factors have occurred in other chromosomes of 

 Drosophila but they were first studied and are most easily demon- 

 strated in the X chromosome. 



b. Other Cases of Linkage. In addition to characters which 

 are sex-linked other characters may be linked together in hered- 

 ity without being associated with sex. Up to 1916 Morgan had 

 found and studied about one hundred mutations of the fruit fly, 

 Drosophila, which are inherited in four groups, all the charac- 

 ters of each group usually going together. At that date there had 

 been found in the first group 47 different characters, in the sec- 

 ond 27, in the third 22 and in the fourth 2. Corresponding with 

 the number and size of these groups there are four pairs of chro- 

 mosomes in Drosophila, three of which are large and one is very 

 small (Fig. 65). The sex chromosomes (XX in the male, XY 

 in the female) constitute one of the large pairs and the genes of 



FIG. 65. CHROMOSOMES (DIPLOID) OF D. melanogaster. The sex chromo- 

 somes are the lower ones in each case, XX in the female and XY in the 

 male, Y being J-shaped. There are three other pairs of chromosomes. 

 (From Morgan.) 



