2 3 o HEREDITARY TRANSMISSION [CH. 



will be remembered from the account of the cytological 

 basis of sex-determination that in a number of insects, 

 including the Diptera, as well as in other forms, the male 

 has either an unpaired ^-chromosome or an unequal X-T 

 pair, while the female has two similar Jf-chromosomes, with 

 the result that two kinds of spermatozoa are produced, one 

 containing X and one without it, while all eggs are ^-bearing. 

 From this it follows that individuals produced from eggs 

 fertilised by an X-bearing spermatozoon contain two X- 

 chromosomes in their nuclei, and become females, while 

 those derived from eggs fertilised by a spermatozoon 

 lacking X contain only one ^-chromosome, and are males. 

 On the other hand, in two species of Lepidoptera it has been 

 found that all the spermatozoa are alike, while there are 

 two kinds of eggs, one containing and the other lacking a 

 chromosome present in the spermatozoa, and in this case 

 it seems that the presence of this chromosome in the egg 

 before fertilisation leads to the production of a male. If, 

 therefore, the hypothesis is correct that inherited, or at least 

 Mendelian, characters are determined by factors borne by 

 the chromosomes, it is to be expected that in those animals, 

 such as Diptera and Mammals, in which some spermatozoa 

 contain and others lack an X-chromosome, the factors 

 for any characters borne by the ^-chromosome will be 

 transmitted by males only to their female offspring, while 

 females will transmit them equally to their offspring of 

 both sexes. And conversely in the Lepidoptera, in which 

 the female-determining eggs are without one chromosome, 

 the female should transmit any characters borne by. this 

 chromosome only to their sons, while males should transmit 

 them to both sexes. Inheritance of the kind described does 

 exist, known as sex-limited or sex-linked transmission, and in 

 Drosophila among the Diptera, and in Man and the Cat 



