192 bulletin: museum of compaeative zoology. 



(J)) The Principle of Ser/regation. The appearance of white mice, as 

 just described, in the second hybrid generation, follows from the prin- 

 ciple of segregation. Tlie primitive germ-cells of the primary hybrid 

 contain both parental characters. D (dominant) and R (recessive), but 

 in the maturation of the germ-cells the two are separated, so that the 

 ripe gerni-cell (or gamete) contains either D or R, but not both. This 

 is demonstrably true in both sexes. Accordingly there are ova, D and R, 

 and spermatozoa, I) and R. If dominants and recessives are produced 

 by each parent in equal abundance, and they unite at random, the sorts 

 of zygotes resulting and their relative frequencies of occurrence will be 

 expressed by the product, — 



D-\- R (ova) 



D -\- R (spermatozoa) 



DJ) -{-2 D (R)* -\-RR (zygotes). 



One individual in four will be a pure dominant, DB (gray in the 

 case of mice) ; likewise one in four will be a pure recessive, RR (white 

 in mice) ; while two in four will be hybrids, D (R), like their parents, 

 the primary hybrids, though indistinguishable in appearance from the 

 pure dominant, I)D. 



2. Mosaic Inheritance. 



An important exception to the two principles just stated needs to be 

 noted. In cases otherwise conforming to Mendel's law, tliere sometimes 

 occur exceptional hybrid individuals in which the normal dominance of 

 one character is not realized, but the two alternative characters coexist 

 in a patchwork or mosaic arrangement. Such a condition is illustrated 

 in the case of piebald, or spotted, mice. 



Segregation of characters does not commonly occur in the formation of 

 the gametes pi'oduced by mosaic individuals. The gametes, as well as the 

 parents, are mosaic, DR. For when two mosaic individuals are mated, 

 they commonly produce only mosaic offspring ; and when a mosaic is 

 mated with a pui'e recessive, RR., no recessive offspring are as a rule 

 produced. These facts show clearly that the ordinary mosaic individual 

 forms no ])ure recessive gametes; in other words, that segregation does 



* Tlie parenthesis is used to indicate tliat the recessive character, though 

 present, is not visible. Wlienever the recessive cliaracter alone is present in an 

 individual [as iu {ltR)'\, it will of course be visible; but whenever the recessive 

 character is present together with the dominant [as in the two individuals Z> (/?)], 

 the recessive character will not be visible. 



