i86 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 1 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 191 6 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 



reiuu ""« 



IV I r 



Fig. 65. Chromosomes (Diploid) of D. ampelophikt. 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.) 



