LINKAGE, CROSSING-OVER, AND THE GERM PLASM 427 



pie allelomorphs in other animals. Nabours, for example, has de- 

 scribed a very interesting series of these for the grasshopper, Ues- 

 perotettix; the series of color characters in mice dealt with on page 407 

 are each allelomorphic to the others; Bellamy has worked out an 

 explanation of a very intricate piece of hereditary beha\ior in fish 

 hybrids that involves the use of the multiple-allelomorph scheme. 



Lethal factors again. — A study of the chromosome map of Droso- 

 phila melanogaster will show that many of the located factors are 

 lethal. This perhaps needs further explanation. Many of the factors 

 that we have previously dealt with in this insect are more or less unim- 

 portant to the life of the animal. Such things as shght changes in eye 

 color, body color, bristle arrangement, etc., are not very important, 

 nor do they affect the viability of their possessors. Some factors, on 

 the contrary, have been found to be so essential to the life of the indi- 

 vidual that their absence causes death. The loss of or detrimental 

 change in a vital character is known as a lethal factor. Of course all 

 such factors are recessive; otherwise they never could be inherited. 

 As it is, an individual may have a lethal factor in the heterozygous 

 condition, balanced by the normal dominant character. When, how- 

 ever, two such heterozygous individuals breed together, one-fourth of 

 their offspring, according to the simple Mendelian rule, will get the 

 lethal factor from both parents. These homozygous recessive lethal 

 zygotes cannot live; so the ratio that actually appears is one homo- 

 zygous normal to two heterozygous individuals (phenotypically nor- 

 mal), and that is all. One of the best-known cases of this kind of 

 hereditary behavior occurs La mice. Yellow mice when mated together 

 give one gray to two yellows. The production of grays from yellows 

 shows that the yellows were heterozygous, or hybrid yellow and gray. 

 It is noted that the litters of these mice average one-fourth smaller 

 than other mice. What becomes of the lost one-fourth? An exam- 

 ination of the uteri of yellow mice reveals a number of dead embryos 

 equal to the expected ratio of pure dominant yellows. These con- 

 stitute the missing class and prove the presence in yellow mice of the 

 recessive lethal factor associated with the yellow factor. 



LINKAGE IN OTHER ORGANISMS 



Lest the reader leave this chapter with the impression that linkage 

 is a phenomenon confined to flies and mice, we shall close this chapter 

 with the table on the next page taken from Castle, which shows how 

 widespread is the occurrence of linkage and crossing-over in both the 



