46 Genes and Characters 



dominant gene with almost complete penetrance. Affected in- 

 dividuals show an uncontrollable twitching of head, body, and 

 limbs which develops in the adult and becomes progressively 

 worse. In spinal ataxia, which is due to the interaction of one 

 dominant gene and a homozygous condition at another locus, 

 the afferent nerve tracts become degenerated and the individual 

 loses his sense of equilibrium. Many other inherited conditions 

 of mental disorders might be listed, but those that are mentioned 

 here should probably be sufficient to give a general picture of 

 the need for taking the genotype into account when studying 

 the numerous types of low-grade mentality that are known. 



QUESTIONS AND PROBLEMS 



1. Breeding results indicate that 103 plants are homozygous for the 

 dominant gene. A, but only 79 of them are A phenotypically. What is 

 the penetrance of that gene? 



2. An individual is born with six fingers on one hand, but, because 

 this would make him a curiosity and handicap him socially, the extra 

 finger is removed shortly after birth. Would this affect his genetic con- 

 stitution? Would it prevent his offspring from having six fingers? 



