INHERITANCE IN ORGANISMS 537 



as extra digits, which are not an evidence of disease, are also heritable. 

 Disease inheritance may be due in certain cases to the actual passing on 

 of disease-producing organisms from parent to offspring; but it is also 

 clear that diseases and abnormalities due to defective genes may be 

 inherited. Most of these defective genes, as has been stated above, are 

 recessive, but a few are dominant. Among the latter are those for dia- 

 betes insipidus, a disease characterized by an insatiable thirst and the 

 production of an excessive amount of urine; hereditary night blindness; 

 congenital cataract; and the presence of extra digits or of short digits on 

 either hands or feet. Among the recessive defects, aside from feeble- 

 mindedness, are those of dwarfness and certain types of nerve degenera- 

 tion causing paralysis. 



608. Sex Determination. — In all that has hitherto been said of 

 chromosomes reference has been made only to what are called ordinary 

 chromosomes, or autosomes, which exist in pairs. There are, however, 

 odd chromosomes, which, because of their connection with the determina- 

 tion of sex, are known as sex chromosomes. 



In the males of certain insects there are two sex chromosomes, recog- 

 nized as the .T-chromosome and the ^/-chromosome, the latter being the 

 smaller; but in the males of other animals there may be only one sex 

 chromosome and this an .r-chromosome. In both cases in the female 

 there are two .r-chromosomes. Experiments have shown that it is the 

 a;-chromosome which in some way determines sex. 



Since in chromosome reduction these odd chromosomes must go 

 to either one sex cell or the other (Fig. 327), it follows that in cases in 

 which the male has one ^--chromosome, only half the sperm cells will 

 contain such a chromosome, while, since the female has two, all of the egg 

 cells will contain one. If a sperm cell which contains an .r-chromosome 

 unites with the egg cell, then the zygote will contain two .r-chromosomes 

 and from it will develop a female; if, on the other hand, the sperm cell 

 without an .r-chromosome unites with an egg cell, the zygote will contain 

 only one r-chromosome and will be male. In case the male possesses 

 both an r-chromosome and a ^/-chromosome, then the r-chromosome 

 goes one way and the ?/-chromosome the other, and the result is that 

 there are again two types of sperm cells. If the sperm cell containing the 

 a;-chromosome unites with the egg cell also containing an r-chromosome, a 

 female is produced; but if the sperm cell containing the ?/-chromosome 

 unites with an egg cell, then a male is produced, with both an r-chromo- 

 some and a ^/-chromosome. 



In other cases instead of there being two kinds of sperm cells there are 

 two kinds of egg cells. In fowls and in moths females have either one 

 r-chromosome or both an r-chromosome and a ?/-chr.omosome, while the 

 males have the two r-chromosomes. In this case it is the type of egg cell 

 which is fertilized that determines sex. 



