VIII. HEREDITY: MENDEL'S LAW 



In the experiment on the life-cycle of the fruit fly (Drosophila) you were 

 given a pair of flies which differed from each other in a single character, as long 

 wings and short wings. (If blowflies were provided for that experiment, a pair 

 of fruit flies will now be given to you and should be examined according to the 

 directions under II, F, i.) The offspring resulting from such a pairing between 

 unlike individuals are therefore hybrids, and it becomes a matter of great interest 

 to find out what will be the appearance of the offspring, as we may then discover 

 how the characters of animals behave in heredity. 



A. FIRST HYBRID GENERATION 



When the offspring appear note carefully the character of the wings (or other 

 feature which was selected for the experiment). Are they all alike, or of two 

 kinds, like the parents, or do they resemble one parent more than the other, or 

 are they intermediate? What is the meaning of the terms "dominant" and 

 "recessive" as applied to a pair of characters such as we are dealing with here (A)? 



A fresh bottle with banana will be provided for each student into which he is 

 to put one or two pairs of flies from his first generation and raise a second genera- 

 tion. The transference of the flies may be accomplished by dropping a small bit 

 of cotton soaked in ether into the bottle and taking out the flies after they have 

 become unconscious, or by holding a bottle over the end of the old culture bottle 

 until a few flies have flown into it, then holding the bottle over the new culture 

 bottle until they have flown out again. In doing this make use of the tendency 

 of Drosophila to fly toward the light. 



B. SECOND .HYBRID GENERATION 



In the second hybrid generation note again the character of the wings (or 

 other feature selected) and determine the number of individuals with each kind 

 of wing length. This behavior of a hereditary character is called Menders 

 law or alternative inheritance. If we suppose that every female fly of the first 

 hybrid generation gave rise to two kinds of eggs in equal numbers, one bearing 

 the character "long wings" and the other the character "short wings"; and that 

 similarly every male of the first hybrid generation produced two kinds of sperms 

 in equal numbers; and that in fertilization all possible chance combinations 

 occurred (i.e., each kind of sperm fertilizes both kinds of eggs), what kind and 

 proportions of offspring would we expect mathematically? Thus if L is taken 

 to represent the character "long wings" and s the character "short wings" we 



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