Multiple Alleles and Lethals 



63 



hand, there is a gradation effect from full 

 pigmentation to white — agouti (C), chin- 

 chilla (c'') and albino (c). On the other hand, 

 another allele, c'', does not produce a simple 

 dilution in color, but under normal tem- 

 perature conditions results in a change in 

 coat color pattern to one called Himalayan 

 (already discussed in Chapter 1). The C 

 allele is completely dominant to all the others 

 listed, d' is completely dominant to c, while 

 C'' is only partly dominant to d' or c. 



Self-sterility in Nicotiana 



Among sexually reproducing plants it is not 

 uncommon to find that self-fertilization does 

 not occur even though the male and female 

 gametes are produced at the same time on a 

 given plant. The reason for this has been 



studied in tobacco, Nicotiana, where it was 

 found that if pollen grains (which will furnish 

 the male gametes) fall on the stigma of the 

 same plant, they fail to grow down the style. 

 The inability of pollen to grow will mean 

 that the male gametes will fail to reach the 

 ovary (which furnishes the female gametes), 

 so that self-fertilization is prevented, and 

 self-sterility ensues. The clue to the explana- 

 tion for this phenomenon comes from the 

 observation that different percentages of 

 pollen from a completely self-sterile plant 

 may grow down the style of other plants. 



The results of certain crosses are shown in 

 Figure 9-1. Genetically identical pistils were 

 exposed to pollen from the same plant (A), 

 from a second plant (B), and from a third one 

 (C). No pollen, approximately half, and 



Stigma 



PISTIL < 



Style 



<^ 



.^ 



%A ^K^ 



Ovary 



S1S2 S1S2 



A B 



FIGURE 9-1. Multiple alleles for cross- or self-Sterility. 



S1S2 



c 



