Penetrance and Expressivity 33 



which have a low or intermediate penetrance. If we are to 

 learn how inherited traits are transmitted in hmiian beings, these 

 genes cannot be ignored. The analysis of poliomyelitis in Mc- 

 Dowell County in West Virginia by Addair and Snyder points 

 to the conclusion that this gene for susceptibility has a reduced 

 penetrance as only 29 individuals contracted infantile paralysis 

 whereas the relationships of the families studied would suggest 

 that 40 individuals were homozygous for the recessive gene for 

 susceptibility. In a case in fowl, reported by Hutt and Child, a 

 recessive gene for inherited tremor is present in which the af- 

 fected individuals continually shake to a greater or lesser extent. 

 This gene has an unusually low penetrance. Their breeding 

 studies indicated that about 112 individuals were homozygous 

 recessive for this gene, but actually only 39 chicks showed the 

 character. Therefore, the penetrance of this gene is about 35 

 per cent. Genes are known with a lower penetrance than that, 

 such as the gene for abnormal abdomen in Drosophila funehris, in 

 which the penetrance is only 10 to 15 per cent. 



It is not always clear why a given gene has a low or high 

 penetrance. It is probably due to the nature of the gene itself. 

 If a certain gene has low penetrance, apparently its action dur- 

 ing development is weak and can easily be disturbed by the 

 action of other genes and also by external factors. These genes 

 with low penetrance are so easily affected that in most indi- 

 viduals their action is negatived completely and the individual 

 develops the character of an allele. The action of genes with 

 high or complete penetrance is so strong that in few or no cases 

 can it be upset or blocked by any other combination of genes 

 or by environmental conditions. 



Hutt and Child interpret the low penetrance of the gene for 

 tremor as the result of modifying genes. In some recessive in- 

 dividuals these modifying genes are powerful enough to prevent 

 the recessive gene from being expressed in any degree; in other 

 individuals some or all of these modifying genes are absent and 

 the individuals are recessive phenotypically. In Drosophila 

 melanogaster, the gene giant has a low penetrance, but environ- 

 mental conditions are responsible. If the food is so scanty that 

 there is extreme competition among the larvae, the action of the 

 recessive gene for giant is inhibited in the homozygotes and the 

 adults are normal in size. 



