314 EVOLUTION AND GENETICS 



one half red-ej^ed heterozygous females and one half white-eyed 

 recessives. Those eggs which receive the y chromosome from the 

 male develop into males, which are one half white-eyed and one 

 half red-eyed. In this case the condition of the male is determined 

 entirely by genes received from its mother. 



The reciprocal cross of a homozygous red-eyed female with a 

 white-eyed male produces only red-eyed individuals. The females 

 are heterozygous, and so produce two types of eggs as in the 

 preceding example, while the males produce germ cells bearing 

 the X chromosome with the gene for red eyes and the y chromosome 

 with no genes, respectively. The F2 generation therefore contains 

 only red-eyed females, one half homozygous and one half hetero- 

 zygous, while the males arc one half red-eyed and one half white- 

 eyed (Fig. 181). 



Sex-linkage is known in other organisms, including man, but 

 not all characters associated with sex are necessarily due directly 

 to sex-linked genes. Secondary sexual characters may result 

 from hormones produced by the gonads, and may appear in the 

 opposite sex under abnormal conditions, although there is no 

 reason to suppose that the chromosomes are modified by the 

 unusual stimulus. 



Multiple Allelomorphs. Most known allelomorphic characters 

 appear in pairs which are independent of all other characters. 

 Exceptions have been discovered in Drosophila of which the 

 following data quoted from The Mechanism of Mendelian Heredity 

 by Morgan, Sturtevant, Muller and Bridges are an illustration: 



"1. If a white-eyed male of Drosophila is mated to a red-eyed 

 female, the F2 ratio of three reds to one white is explained by 

 Mendel's law, on the basis that the factor for red is the allelomorph 

 of the factor for white. 



"2. If an eosin-eyed male is mated to a red-eyed female, the 

 F2 ratio of three reds to one eosin is also explained if eosin and red 

 are allelomorphs. 



"3. If the same white-eyed male is bred to an eosin-eyed female, 

 the F2 ratio of three eosins to one white is again explained by 

 making eosin and white allelomorphs." 



The assumption that these three characters are allelomorphic 

 to each other carries with it as a necessary corollary the assumption 

 that their genes occupy the same position in the chromosome. 

 Data on crossing over between these and other characters bear 



