CHAPTER XX 



SOME COMPLICATIONS OF MENDELIAN 

 INHERITANCE 



Although experimental studies on heredity since 1900 have abundantly 

 verified Mendel's findings, it would be very misleading to imply that the 

 present-day concept of inheritance is as simple and direct as might be 

 expected from Mendel's original laws. We have already seen that the law 

 of "independent assortment" is limited to special conditions and must be 

 supplemented by the principles of linkage and crossing over when the 

 genes of a di- or polyhybrid are located in the same chromosome. In 

 the present chapter, we shall look at some of the simpler of other com- 

 plicating conditions that have been discovered by post-Mendelian 

 investigations. 



Absence of dominance. In all the crosses made by Mendel the 

 dominance of one allelomorphic gene over the other was a conspicuous 

 feature. Soon after 1900, however, crosses were found in which domi- 

 nance and recessiveness were not well marked. One of these was in the 

 old-fashioned garden flower, the four-o'clock, in which a red variety 

 crossed with a white variety gives an F\ that is pale red or pink and an 

 F 2 that consists of 34 red, ^ pink, and 34 white. This F% ratio of 1:2:1 

 is typical of a monohybrid cross when dominance is absent. 



A similar case had long been known to poultry breeders, although 

 it was not understood until after 1900. In the Andalusian breed of poultry 

 there occur black, white, and blue varieties. Of these, the blue was most 

 desired by fanciers, but although the black and white varieties breed 

 true, mating two blues results in 34 blacks, }4, blues, and 34 whites. It 

 is thus evident that blue is due to a heterozygous combination of the 

 gene for black and its allele, the gene for white, and the only way to 

 obtain a brood consisting entirely of blue chickens is to mate a black 

 with a white. Many other crosses that show lack of dominance are known, 

 as well as a large number in which the heterozygote, although much more 

 like one parent than the other, can be easily distinguished from it. 



Lethal genes. Among the multitude of genes found to be inherited 

 in various breeding stocks, a number are known that result in the death 

 of the organism if they are present in a homozygous condition. The first 



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