Genetics 169 



generation (which means the offspring from these parents) are repre- 

 sented by the formula, F x . The offspring of F x in turn are known as 

 F 2 , and so on, the F representing a filial generation. 



In many cases the various characteristics, which the genes deter- 

 mine, may be independent of each other; but, just as certain chemical 

 elements have an affinity for each other, so there are various types of 

 characters that often link themselves in the same way. This is known 

 as linkage. Color of hair and the direction in which the hair grows, 

 such as curliness, straightness, or whorls, are often linked. Then there 

 are also certain types of sex linkage by which we mean that there are 

 certain characters, such as plumage in fowls and eye-color in flies, which 

 are almost always concomitant with the sex of the individual. 



Much has been written on sex-determination in the past, though 

 it is only recently that any progress has actually been made in this field. 

 It has been found that sperm cells possess an extra or accessory 

 chromosome (called an X-chromosome by American writers, and a 

 heterotropic chromosome by Europeans). (Fig. 30, A.) When such a 

 sperm cell fertilizes an egg, a male is produced, while, when an egg 

 containing the regular even number of chromosomes is fertilized by a 

 sperm with an even number of chromosomes, a female is produced. 



Interpreting these findings of the cytologists, biologists now believe 

 that there is such an extra chromosome in both egg and sperm, but 

 that in the egg, this X-chromosome divides as do the others, although 

 this division is delayed until some time after the other chromosomes 

 have divided in the maturation divisions. This means that the 

 X-chromosome of the sperm is really a double chromosome which fails 

 to separate during spermatogenesis and consequently goes over to one 

 of the two sperm-cells entire. 



Then, in some organisms this X-chromosome has actually been seen 

 to be made up of a larger and a smaller portion, while in the female of 

 the same species both parts of the chromosome are of equal size. When 

 the accessory chromosome is thus divided into two parts of different 

 sizes, the smaller is called the Y-chromosome. 



It follows from this that, if unit characters are carried by the genes 

 of the X-chromosome, all organisms in which the sperm carry an 

 X-chromosome, must necessarily transmit the characters of the X-chro- 

 mosome to the female offspring only, while females can transmit them 

 equally to all offspring. Similarly in those organisms in which eggs 

 may lack one chromosome, the female can transmit characters only to 

 their sons, while males can transmit to their offspring of both sexes. 

 This is the explanation of sex-linked transmission as shown in men who 

 are color-blind. Such men transmit this defect to their daughters, and 

 the daughters can in turn transmit it to all their sons and daughters. 



There are exceptions to this. A usual sex-linked characteristic such 

 as color-blindness, is sometimes transmitted from father to son directly. 

 This is explained by Bridges as being due to what he terms a "non~ 



