168 GENERAL BIOLOGY 



The parents are often represented by the capital letter P. The first 

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

 sented by the formula, F . The offspring of F 1 in turn are known as 

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



In many cases the various characteristics that the genes determine 

 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 curli- 

 ness, straightness, or whorls, are often linked. Then there are also cer- 

 tain types of sex linkage by which we mean that there are certain char- 

 acters 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 chro- 

 mosome (called an X-chromosome by American writers and a hetero- 

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

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

 a 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 be- 

 lieve 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, al- 

 though this division is delayed until some time after the other chromo- 

 somes 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 of their sons and daughters. 



