THE PHYSICAL BASIS OP INHERITANCE 291 



precisely correspond to the original Mendelian types. Soon after chromo- 

 somes became well known, it had been found that the cells of each species 

 or kind of organism have a definite and unchanging number of chromo- 

 somes. In man, for example, every cell (except matured germ cells) 

 has 48 chromosomes (24 pairs); in the fruit fly Drosophila, every 

 cell has 8 (4 pairs) ; and in the garden pea, every cell has 14 chromosomes 

 (7 pairs). 



Bug Type. But now certain exceptions were discovered. In some 

 species of bugs (Hemiptera), the cells of the females have an even number 

 of chromosomes and those of the male have an odd number, 1 less than 

 in the female. Further study showed that all the eggs of such a species 

 contain the same number of chromosomes (half the female number) but 

 that the spermatozoa are of two kinds as regards chromosome number: 

 half of the spermatozoa have the same number found in the egg; the 

 other half of the spermatozoa have 1 chromosome less. For example, in 

 one species of bug, the females have 12 chromosomes in each body cell, 

 and all the mature unfertilized eggs have 6 chromosomes; but the body 

 cells of the male contain only 11 chromosomes, and half of the mature 

 sperm have 6 chromosomes and half only 5. It seems fairly evident from 

 this that an egg (always with 6 chromosomes) fertilized by a sperm with 

 6 chromosomes will produce a 12-chromosome zygote and develop into a 

 female, whereas an egg that is fertilized by a 5-chromosome sperm will 

 produce an 11-chromosome zygote and develop'into a male. 



Drosophila Type. Later work brought to light other types of "sex 

 determination." In Drosophila and in man, the number of chromosomes 

 is the same in the two sexes, but in both these species one pair of chromo- 

 somes, the so-called "sex chromosomes," are alike (XX) in the female 

 and unlike (XY) in the male. As a consequence, when the human egg is 

 formed, it will always contain 23 ordinary chromosomes (autosomes) and 

 1 X chromosome, whereas the sperm are of two sorts — female-producing 

 spermatozoa with 23 autosomes and an X chromosome, and male-produc- 

 ing spermatozoa with 23 autosomes and a Y chromosome. 



Poidtry Type. In poultry and in many of the Lepidoptera (moths and 

 butterflies) a type of sex determination exists that appears to be just the 

 reverse of that in man and Drosophila. Here the male has two X chromo- 

 somes (XX), and the female an X and a Y. Accordingly, all sperm are 

 alike, containing one set of autosomes and an X chromosome, and the 

 female produces two kinds of eggs — one with a set of autosomes and an X 

 chromosome, the other with a set of autosomes and a Y chromosome. 



The various types of correlation between sex and chromosomes that 

 are well known in animals may be tabulated as follows (here AA denotes 

 a diploid set of autosomes; A, a haploid set): 



