ment. It has been helpful, accordingly, to assume that each inheritable trait 

 depends upon something in one of the chromosomes. 



Specific determiners, or genes, are apparently arranged in each chromosome 

 in a series, like beads on a string. The genes, or determiners, in each chromo- 

 some tend to remain associated, or linked, although they may be transmitted 

 independently. 



Although the inheritance of characteristics and the behavior of the chromo- 

 somes are remarkably parallel, it is probable that each "character" depends 

 upon the interaction of several genes, and that each determiner produces 

 several effects in addition to the one we happen to observe. 



The chief problem in dealing with plants and animals, from the breeders' 

 point of view, is to get organisms that can transmit combinations of desirable 

 qualities. Breeders and experimenters have succeeded in producing strains 

 that maintain such combinations, quite distinct from any "natural" species. 



EXPLORATIONS AND PROJECTS 



1 To study the inheritance of certain traits in rats, cross a hooded rat with an 

 albino; then mate the hybrid generations among themselves. Tabulate the results 

 for the two successive generations. Compare the distinctive traits in parental, first- 

 hybrid, and second-hybrid generations. Interpret results. 



2 To study the inheritance of traits in guinea-pigs, cross male and female pigs 

 having contrasting characters, such as rough coats versus smooth coats, long hair 

 versus short hair, solid color versus spotted appearance, agouti versus nonagouti, or 

 black versus albinism.^ Tabulate results and note conclusions as to which traits 

 are dominant and which recessive. 



3 To demonstrate the 1 :2 :1 ratio by chance combination, work in pairs and 

 flip two coins 100 times. Record the heads and tails for each double throw. The 

 theoretical ratios are both heads, 25 times; 1 head, 1 tail, 50 times; both tails, 25 

 times. Compare results with the theoretical expectations. Combine the results of 

 several sets of trials; compare the total with the theoretical expectations. Note un- 

 usual deviations from "expected" results. 



4 To work out interpretations and probabilities in hereditary phenomena, make 

 diagrams and calculations in various concrete or imaginary situations, such as the 

 following: 



a. Pure smooth peas are crossed with pure wrinkled peas. Note (1) the appear- 

 ance of the hybrid generation; (2) the genetic make-up of hybrids. (Use capital S 

 to represent dominant smoothness and small s to represent recessive wrinkledness.) 

 Show (1) the probable appearances in the following generation if the hybrid in- 

 dividuals are mated; (2) the genetic make-up of the various types; and (3) the 

 ratios of the different phenotypes and the ratios of the different genotypes. 



^The gestation period in guinea-pigs is 65 days. The pigs can be fed on the complete diet as 

 given on page 112, or upon commercial rabbit chows supplemented by green foods and milk. If 

 green grass or clover is available, it may well constitute the bulk of the diet. 



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