244 BIOLOGY OF THE LABORATORY MOUSE 



showing linkage, so that most or perhaps all of these mark additional 

 chromosomes, making perhaps sixteen chromosomes in all with known 

 marker genes. 



No known mutant gene in the mouse, or in fact in any of the rodents, 

 sufficiently clear cut in its effects to serve as a "marker" gene, is sex- 

 linked. In view of the large size of the X chromosome and the ease with 

 which sex-linked mutations, if they occur, can be detected, this is a note- 

 worthy fact. 



In all cases of linkage sufficiently well tested to give critical evidence, 

 the crossover percentage has been higher in the female than in the male. 

 In accord with this, the number of chiasmata observed during gameto- 

 genesis is higher in the female than in the male.* 



Negative Linkage Data 



Most of the gene mutations that have been found in mice have been 

 tested for linkage against other known gene mutations. Where these tests 

 have led to the discovery of a linkage, this is indicated in the section on Gene 

 Mutations (p. 234). In the majority of cases no linkage has been found. 

 These negative linkage data are summarized in the accompanying table. In 

 this table all the genes are listed in the first vertical and also in the first 

 horizontal line. Where several genes lie on the same chromosome they are 

 listed as a unit. The crossover data for any two genes are found in the 

 rectangle where the horizontal line from one gene and the vertical line from 

 the other gene intersect. In a number of cases several tests have been made 

 for a single pair of genes. In such cases the data from one test only, that 

 involving the most animals or for other reasons the most satisfactory, have 

 been use. The data given at each intersection consist of the following: 



1. A number referring to a reference in the bibliography. 



2. An abbreviation indicating the type of cross used. The abbreviations 

 are: 



BC, a cross of the type AaBb X aabb 

 Fo, a cross of the type AaBb X AaBb 

 MC, a cross of the type AaBb X Aabb 



3. The observed crossover per cent plus or minus its standard error, or 

 where this cannot be given, the data themselves. In the case of backcross 

 (BC) data, the standard error has been calculated from the tables given by 



* Bryden, 1933, J. Genetics 27: 421-433. 



