NATURE OF PLANTS 143 



their action in some way by the opposite allelomorphic factor 

 and reappear in the offspring when hybrid tall blue sweet peas 

 are crossed among themselves. In cases of this nature where 

 certain factors are suppressed and others are active, the inactive 

 factors are termed recessive and the active ones are termed 

 dominant. Tallness and blue are dominant factors, dwarfness 

 and red are recessive factors. 



In the above examples we see that the hereditary factors are 

 segregated and distributed to the gametes in a very definite 

 way. There is one important exception to this rule. A large 

 number of instances is known where one factor is usually or 

 always associated with another factor. That is, these two 

 factors are not separated from one another in the formation of 

 the gamete and consequently both are transmitted to the 

 gamete. Factors that behave in this way are said to be linked 

 and linkage is to be contrasted with the segregation mentioned 

 above. One of the most striking illustrations of linkage is seen 

 in color blindness. The factor for this characteristic is always 

 linked with the factor for sex. Therefore this factor is distributed 

 differently to the two sexes. The offspring of a color-blind male 

 and a normal female would not show this characteristic at all 

 because the sons are absolutely normal and the daughters, though 

 having the factor for color blindness, do not show it, since this 

 factor is recessive to the normal sight factor. The daughters, 

 therefore, can alone transmit this factor, and one half of their sons 

 by a normal male will be color blind. 



In the above examples very simple cases have been taken in 

 order to bring out more clearly the principles involved. We 

 have spoken of only one factor being lodged in a chromosome. It 

 is now known that many factors may be associated with each 

 chromosome and the evidence at present makes very probable 

 the belief that these factors are arranged in a line in the chromo- 

 some and are separated from each other by definite distances. 

 The factors have also been spoken of as though they were the 

 sole cause of the characters. The factor for redness is merely 

 the deciding cause of this color. A great many factors are 

 really necessary to cause the production of a given character, 



