62 CELL HEREDITY 



In the course of asexual reproduction, mutations may occur giving rise, 

 for example, to colorless mutants which lack chlorophyll and can grow 

 on sodium acetate without photosynthesis. When a normal green strain 

 is mated with the colorless mutant of opposite mating type, a very simple 

 result is obtained. Each zygote gives rise to two green and two color- 

 less offspring. This 1 : 1 ratio is almost invariant and indicates that the 

 hereditary determinants for greenness pass through the zygote unim- 

 paired. Pairs of genes of this sort, called alleles, .segregate from one 

 another in the formation of gametes. This is Mendel s first law. 



We can think of other ways in which the unit factors could behave 

 when together in the zygote. They might interact by altering one another 

 — but they do not. They retain their identity and segregate inviolate 

 into the four daughter gametes. They might be distributed at random 

 into these cells. Thus, if only a few factors of each kind were in the 

 zygote, some of the resulting gametes would have none. But this is not 

 the case. Rather, a regular mechanism of segregation insures that each 

 daughter cell will receive one parental factor, either the paternal or the 

 maternal one. 



Mendel's second law states that the segregation of one pair of alleles 

 is independent of the segregation of a second pair. Thus if the letters 

 A and a represent one pair, and B and b another, we may write the 

 genotypes of the parents in a cross as AB x ab. Half of the gametes 

 will receive A and half a; half of the gametes will receive B and half b. 

 If the possession of a specific member of the A/a set has no influence on 

 which of the B/b set will be present, then one quarter (0.5 X 0.5 = 0.25) 

 will have each of the four possible combinations: 



(34A + Ha) X (HB + V2b) = MAB + ]iAb + ^flB + V4ab 



In Chlamydomonas the factor pairs for color (green = y'*', yellow = y ) 

 and for mating type (mt ) segregate independently. For example, from a 

 cross of y^ mt^ x y~ mt~ 183 gametes were collected at random. Their 

 genotypes were as follows: 



y^_mt^_ 43 | ^^ Parental types 



y mt 44 j ■' 



+ .^ > 96 Recombination types 



y mt 42 J ■' 



Total 183 



The numbers of different types obtained are approximately equal to 

 the 45.75 of each anticipated if they were formed with a frequency of 



