A LABORATORY MANUAL 



FOR GENETICS' 



COURSES in genetics have been 

 on the rapid increase in the 

 last few years, but the sub- 

 ject is so new that they have 

 scarcely yet crystallized into any 

 very definite form or content as given 

 in different institutions. With the ful- 

 ler growth and rounding-out of the 

 subject, however, and with the advent 

 of more suitable texts, there is evidently 

 a tendency toward a more definite 

 order in the presentation of at least the 

 more elementary facts. The value of 

 laboratory work to supplement that of 

 the classroom has doubtless been appre- 

 ciated by all who have taught genetics, 

 but here the selection of the most suit- 

 able exercises has been even more dififi- 

 cult than a choice of material for lecture 

 presentation. Obviously, for an under- 

 standing of Mendelian inheritance, 

 nothing else is so valuable to the stu- 

 dent as the actual performing of ex- 

 periments involving crosses, and the 

 study of dominance, segregation, re- 

 combination, and other such phe- 

 nomena, in at least the first and second 

 filial generations. Fortunately, the very 

 characteristics that have made Droso- 

 phila the form par excellence for gene- 

 tic research, make it also well adapted 

 for laboratory class purposes. This 

 fact has been recognized by numerous 

 teachers of genetics in this country, 

 who have been making use of Droso- 

 phila in connection with their elemen- 

 tary teaching for some years. It is 

 accordingly not surprising to find that 

 breeding experiments with the "vine- 

 gar fly" form the backbone of the 

 course in this the pioneer of laboratory 

 manuals in genetics. 



It seems somewhat presumptuous to 

 attempt to give an estimate of a man- 



ual of directions without first having 

 tried them out in practice, even by one 

 who has been giving much of the same 

 work to his own classes. The old 

 proverb asserts that the real test of a 

 pudding is not to be gained by inspec- 

 tion, but by chewing the string; this 

 is even more true of laboratory man- 

 uals than of puddings, and there has 

 been no opportunity to apply the string 

 test in the present case. Nevertheless, 

 one may, perhaps, be allowed to hazard 

 an opinion on the basis of parallel 

 experience. 



The principal experiments outlined 

 on Drosophila are along obvious lines, 

 and are designed to illustrate normal 

 'Mendelian inheritance, sex-linkage, and 

 mono and dihybrid ratios. Experiment 

 VI involves the question of factor 

 groups (corresponding to the chromo- 

 some pairs), but no experiment on the 

 degree of linkage of two factors in 

 the same group is given. The advis- 

 ability of including this in an elemen- 

 tary course may, without doubt, be 

 questioned, but where as much time 

 can be devoted to the course as in this 

 one, it might, perhaps, well be ofifered 

 at least as a substitute. The character 

 difi^erences selected for the breeding ex- 

 periments are well chosen, and a feature 

 that will doubtless appeal to those who 

 give the work to successive classes of 

 elementary students is the provision of 

 alternate characters for each exercise, 

 which can be substituted from year to 

 year. Two alternatives are given with 

 each exercise. 



The suggestive work on variation in 

 plants is based on readily accessible ma- 

 terial, and the suggestion of having the 

 student prepare an herbarium of varia- 

 tion is a good one where such material 



1 Genetics Laboratory Manual, by E. B. Babcock and J. L. Collins. First edition, xi -|- 56 pp. 

 McGraw-Hill Book Company, Incorporated ; New York, 1918. 



39 



