Lin\age and Cross-over 265 



when the character is present in both the father and the 

 maternal grandfather. 



Lmkage and Cross-over 



It is a fact that certain traits appear to be determined 

 by something associated with a particular chromosome. It 

 is a fact that pairs of alternative characters are not all inde- 

 pendent of one another, as Mendel had supposed. These 



Eye characters 



XX m xo m ?, 



Fig. j6. Color-blindness in a Female 



When color-blindness occurs in a female it seems to result from a meeting of 

 two atfected X chromosomes as follows: in the daughter of a color-blind male 

 (female in F^), carrying two X chromosomes of which only one is afifected (that 

 derived from the father), mates with a color-blind male, whose single X chromo- 

 some is also affected. In the offspring, F^, all the females get one X chromosome 

 from the color-blind father, and one half of them get from the mother the 

 affected X chromosome. The result is that^one half the females in F^ are color 

 blind. One half the males are also color blind, but in their case the color- 

 blindness is determined by the X chromosome of the normal mother (in Fj), 

 not by the color-blindness of the father. 



two facts have led to the theory that each chromosome con- 

 tains a series of determiners, called genes by Morgan, and 

 that these genes occur in pairs — one in each of a pair of 

 chromosomes. On this assumption we can explain why ap- 

 parently unrelated characters generally occur together. Not 

 only color-blindness and maleness, but, in the fruit fly, vari- 



