340 BIOLOGY OF THE LABORATORY MOUSE 



mice as a possible clue to the nature of the factors causing mammary 

 cancer. 



Hybrids 



The only type of hybrid that will be discussed here is the one that is of 

 particular value in research, namely the first generation hybrid (Fi) obtained 

 by crossing two inbred strains. 



Genetic Characteristics 



Apart from the segregation of the X and Y chromosomes, all the 

 germ cells of an isogenic inbred line are genetically identical. It follows 

 that all the offspring obtained by crossing females of one isogenic strain 

 with males of another will have the same genotype. Thus, if one strain is 

 AABBccdd . . . and the other aabbccDD . . . , all the Fi hybrids will be 

 AaBhccDd . . . Offspring of the reciprocal cross will have the same 

 genotype again in the homogametic sex. Individuals of the heterogametic 

 sex (male, in mammals) will have the same genotype for their autosomes, 

 but a different set of sex-linked genes, their X and Y chromosomes being 

 derived from the opposite parents. 



There is one important respect, however, in which hybrids differ from 

 their parental strains: they will not breed true. Though genetically uni- 

 form, they are heterozygous for all genes which differ in the two parents. 

 The F2 will have, therefore, as a result of segregation in the germ cells 

 of the Fi, the maximum genetic variation possible with the genes provided 

 by the parental strains. Backcrosses of the Fi to either parental strain, 

 and outcrosses of the Fi to any other strain, will also give considerable 

 genetic variation in the offspring. 



Phenotypic Characteristics 



Average. — The average of a character in the Fi may fall between the 

 parental averages, it may correspond to either parent, or it may lie beyond 

 the parental values. 



Intermediate averages were obtained by Wright (20) in some of his 

 crosses of normal and polydactylous strains of guinea pigs. E. L. Green 

 and the author got similar results in two hybrids from strains of mice differ- 

 ing in number of presacral vertebrae. 



The hybrid will, of course, correspond to one of its parents when the 

 character difference is determined by dominant genes all carried by one 

 parent. Thus, the Fi of a cross between agouti and black strains will be 



