i^ROBLKM 3. How New Hereditary Characters Appear 483 



plasm which was inherited. Now since can readily see that a change in one gene 

 we know more about genes and chromo- might produce most unexpected results, 

 somes, we reserve the name mutation For example, the gene for white eye in 

 only for those cases in which there is a Drosophila (a mutation) not only pro- 

 change in a single gene. duces white eyes instead of the normal 

 Great changes are possible. You have red but also makes slight changes in the 

 learned that a character is produced by other characters of the fly. Changes in 

 the action of a pair of genes. You must parts of chromosomes or in single chro- 

 now broaden that understanding. Many mosomes, or in sets of chromosomes 

 characters are inherited in a more com- would, therefore, produce even more 

 plicated way. Frequently a character is marked changes. The offspring produced 

 the result of several pairs of genes by such chromosomal changes may 

 working together. And besides this, usu- therefore be very^ different from their 

 ally, one pair of genes has an effect on parents. This problem is easy to summa- 

 many characters. Since this is true you rize. See Exercise 4. 



Questions 



T. Give an example of a new character that appeared in a fruit fly. How 

 was it known that this character was not merely the result of a re- 

 shuffling of genes but a change in the chromosomes of the germ plasm? 

 About how frequently may one expect such a new character to appear 

 in the fruit fly? 



2. Define mutation and tell what is known about mutations, stating where 

 thev occur, how they may affect the organism and so on. 



3. How can the rate of mutation be increased? What scientist did this 

 first? 



4. Explain the connection between chromosome numbers and new char- 

 acters; mention a specific plant. 



5. What else may happen to chromosomes to produce new characters? 



6. How, by whom, and when was the word mutation first used? How- 

 does our present use of the word differ from the original meaning? 



7. What new ideas must you add to what you originally learned about a 

 gene producing a character? Explain how a tiny mutation might cause 

 a great change in an organism. 



Exercises 



I. Assume that whiteness appears as a mutation in the sperm cell in a 

 species of dark animals. This sperm cell fertilizes a normal egg. If the 

 gene that causes the whiteness is a recessive one, what is the appearance 

 of the animal developing from the fertilized egg? Suppose the same muta- 

 tion appeared in an egg and in the sperm that fertilized it. What must be 

 the genetic make-up of all the cells of the animal developing from such a 



