300 



Cell as Basis of Organic Activity 



establish these ratios. The manner in which the genes are distributed 

 in the Fa generation may be illustrated by a Punnett square. All pos- 

 sible combinations must be considered. 



This same type of inheritance, involving but a single pair of char- 

 acters, has been studied among innumerable plants and animals. One 

 common trait among cattle is that in which the horns are absent. This 

 condition known as polled is inherited as a dominant trait over the 

 horned condition. 



It can be seen here that the phenotypic ratio in the second genera- 

 tion is the expected three polled to one horned individual. 



PARENTS 1^ R 



POLLED HOANEO 



PP 

 P 



PP 

 P 



POLLED 



Pp 

 P P 



Pp 



POLLED 



Pp 



HOANEO 



pp 



Fig. 90. — A monohybrid cross in cattle. In this cross, P—a. gene for polled, and 

 p—a gene for horned. Polled is dominant over the horned condition. 



Monohybrid Crosses Without Dominance. — In the examples of 

 monohybrid crosses so far considered, the expression of one gene is 

 completely dominant over that of its contrasting gene or allelomorph. 

 Such is not always the case. At times, one gene does not exercise com- 

 plete dominance over the expression of the other. Such a condition may 

 result in the heterozygote being intermediate between the two or in 

 some instances entirely different. 



In snapdragons, for example, a plant homozygous for the factor 

 for red flowers when crossed with a plant homozygous for white flowers 

 produces plants with pink flowers. A cross between two Fi pink flowers 

 yields one red flower, two pink, and one white. The phenotypic and 

 genotypic ratios are identical in the F2. 



This illustrates a case where the heterozygous condition is inter- 

 mediate between the two extremes. Actually there may be all degrees 

 of blending between two contrasting characters. In some instances, the 



