INHERITANCE 287 



2. Recombinations 



Turning from modifications, which appear to be useless to the 

 geneticist, we find that the most common inherited differences 

 which appear in offspring are recombinations which owe their 

 origin to new groupings of the germinal factors, or genes. 



Everyone is familiar with some of the more obvious hereditary 

 differences following fertilization which are the result of new 

 combinations of parental characters represented in the egg and 

 sperm: that is, cases in which nothing is apparent which is not 

 clearly related to the conditions expressed in the ancestors. In 

 the first place the offspring may exhibit a character, eye color, let 

 us say, of one parent to the exclusion of that of the other - - the 

 character appearing unmodified. This is termed alternative 

 inheritance. Or the offspring may seem to be a sort of mosaic 

 of the characters of its progenitors, each parent contributing a 

 certain character but not to the exclusion of that of the other. 

 Sometimes the parental traits seem to fuse so that the progeny 

 exhibit a more or less intermediate and different condition, as 

 in the color of the skin of mulattoes, frequently called blend- 

 ing inheritance. And as a final example, characters of grand- 

 parents or more remote ancestors may crop out, and constitute 



REVERSION. 



3. Mutations 



But quite different results now and then occur. Characters 

 which have no place in the ancestry appear and are transmitted 

 to the descendants. Sometimes these new inherited variations, 

 or mutations, are only slight departures from the parental con- 

 dition, while in other instances they are quite abrupt. But the 

 significant fact is that mutations result from relatively radical al- 

 terations in the gene complex and so afford new opportunities for 

 variation. 



Thus combinations and mutations contrast sharply with modi- 

 fications which are not transmitted to the offspring; the latter 

 being merely the results of environing conditions on the soma 

 during embryonic development or later. The importance of this 

 distinction can hardly be over emphasized because it makes 

 comprehensible many of the inconsistencies of earlier work on 

 genetics. 



