NORMAL INHERITANCE 



Indeed they (white mice) always bring forth white ones. 



— Pallas, 1766. 



For many years it has been known that when white mice 

 are crossed with pure-bred grays, the immediate offspring 

 will be gray and in the second generation albinos will reap- 

 pear (36). The true nature of this transmission, however, was 

 first clearly understood in 1900, when Mendel's Law was 

 rediscovered. 1 



Work since that time has shown that the majority of 

 house-mouse characters are inherited in the same simple 

 fashion as albinism. Most of the fancy characters are reces- 

 sive like albinism, which, when crossed with the wild gray 

 type, produces all grays in the first generation and one 

 recessive out of four in the second generation. 



The mechanics of such inheritance is clearly known. The 

 physical determiners underlying these hereditary variations 

 are located in the minute rod-shaped bodies (chromosomes) 

 within the nucleus of each cell of the body- In all mouse 

 body-cells there are twenty kinds of these chromosomes 

 (see Fig. 9) and two of each kind, one of each pair having 

 been received from the father and one from the mother. 

 These chromosomes are reduced from the diploid (double) 

 number to the haploid (single) number at the formation of 

 the gametes (eggs and sperm), so that each gamete contains 

 only one of each pair. When two gametes unite (an egg with 



1 litis, 1924 (.SO), p. 68, says: "We have already heard from Fr. Hornish and 

 Inspector Nowotny that Mendel raised mice in one of his two rooms, and not only 

 white ones but also grays, and crossed them with each other. It is very possible that 

 through these more dramatic researches the revelation of dominance and segrega- 

 tion appeared to him for the first time. Indeed, he mentions nothing about it. 

 This is not to be wondered at because industry in natural science at once made an 

 ecclesiast suspicious in the eyes of many clerical zealots, to whom the undertaking 

 of animal breeding appeared highly immoral." 



