Chapter 8 

 PROBABILITY 



Perhaps one of the most frequently asked questions is, "What 

 is the chance that my child will be a boy?" People also wonder 

 whether the chance that the fourth child will be a boy is greater 

 when the first three children are girls than it is when they are 

 boys. These and many other questions in genetics involve the 

 theory of probability. A student of genetics should become 

 acquainted with the elements of the theory of probability, as 

 the interpretation of all genetic ratios is based on that theory. 

 For example, in tobacco, gene F, which produces anthocyanin 

 pigments in the flowers of Nicotiana Sanderae, is dominant over 

 its allele, /, which produces no color. If a plant with colored 

 flowers is crossed with a white-flowered plant, and if the off- 

 spring consist of 672 colored plants, one would say that it is 

 highly probable that the colored parent was homozygous for F 

 and one would assume that as a working basis. 



The situation is different in cattle, which cannot produce such 

 a large number of offspring. The polled or hornless condition is 

 produced in cattle by gene H and is dominant over horned (h). 

 If a polled cow is crossed with a horned bull, all the offspring 

 should be polled if the cow is homozygous for H, but the off- 

 spring should segregate into a 1 : 1 ratio if the cow is heterozy- 

 gous. However, let us assume that only one calf is produced 

 from the mating. If the polled cow were of unkno\vn ancestry, 

 and a polled calf were produced, it would be impossible to tell 

 the genotype of the cow. If the cow were homozygous, only 

 polled calves would be possible. On the other hand, if she were 

 heterozygous, it would be equally probable that the one calf 

 would be either polled or horned. If the calf had been horned, 

 there would be no doubt that the cow was heterozygous for it 

 would be impossible for a homozygous polled cow to produce a 

 horned calf. A distinction must be made between certainty 

 and probability, and it must be realized, also, that the larger the 



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