3 



Recombination in Sexual Organisms 



The genius of Gregor Mendel s insight into heredity hes in the fact 

 that he conceived of it as discontinuous. While working with the garden 

 pea, he decided to focus his attention on one difference at a time, such 

 as that between pink and white blossoms. Putting aside for the moment 

 all other differences, he counted the numbers of each type of offspring 

 from the crosses he made and avoided the trap that caught his forebears. 

 They had tried to consider all characters at once and could not see the 

 simple laws that governed the inheritance of something as complex as an 

 individual. Mendel, on the contrary, postulated the existence of unit 

 factors and gave an abstract solution to his problem. The rules which 

 he established for the behavior of unit factors accounted in a formal way 

 for what he saw and provided a basis for predicting the results of future 

 experiments. 



In this chapter we will consider the transmission of hereditary factors 

 in sexual reproduction, starting not with the pea plant, as Mendel did, 

 but with some microorganisms which illustrate basic principles more 

 clearly. Subsequently, we will see how the same laws govern the genetic 

 behavior of higher organisms. 



SEGREGATION AND ASSORTMENT 



First, let us consider inheritance in the green alga, Chlamydomonas. 

 There are two "sexes, ' or more properly, two mating types, referred to 

 as mf"*" and mt~ because we cannot see any morphological difference be- 

 tween them. But they know the difference; mt^ individuals will choose 

 only mt~ cells as their mates, and vice versa. When thev mate, as shown 

 60 



I 



