68 HEREDITY AND VARIATION 



(h) Understand from reading and discussion how the concept of 

 genes which can occur in alternate states and which exist in pairs in 

 all zygotes but singly in mature germ cells can be derived from data 

 of the kind you have collected. What is the principle of segregation? 

 Why is it that your data do not conform precisely to the theoretical 

 3 : 1 and 1 : 1 ratios expected, respectively, in second filial and back- 

 cross generations? Consider the effect that differences in viability 

 would have on survival under natural conditions. Write an explana- 

 tion of the course of inheritance of whatever recessive character ex- 

 pression you have studied. 



Exercise 3. — Further Breeding Experiments. 



(i) If time permits, flies which differ with respect to two char- 

 acters conditioned by pairs of genes located on different autosomes 

 can be crossed. When the Fo generation has been obtained the char- 

 acter expressions are found to be associated in all possible ways, that 

 is, four classes of progeny occur. Understand how these results can 

 be explained in terms of the principle of independent assortment and 

 secondary hypotheses. Note again that perfect ratios are not ob- 

 tained, and consider the various factors that contribute to the dis- 

 crepancy. 



(j) The progeny of the Fi and backcross generations of reciprocal 

 crosses between individuals differing with respect to two characters 

 conditioned by two pairs of genes carried by the X-chromosomes will 

 demonstrate sex-linkage, linkage, and crossing over. 



