268 FOUNDATIONS OF BIOLOGY 



2. Combinations 



Turning from modifications, which are useless to the geneti- 

 cist, and concentrating attention on characters which repre- 

 sent an expression of germinal factors, we see that, in the 

 final analysis, heredity is germinal resemblance among 

 organisms related by descent a consequence of the con- 

 tinuity of cells by division. Hereditary differences which 

 appear in offspring are either COMBINATIONS of ancestral char- 

 acters apparently new characters which owe their origin to 

 recombinations of the germinal factors of old characters or 

 MUTATIONS due to fundamental changes in the germinal con- 

 stitution, possibly in the factors, or genes, themselves. 



For didactic purposes we may somewhat arbitrarily classify 

 the obvious hereditary differences following fertilization 

 which are the result of recombinations of parental characters 

 represented in the egg and sperm: that is, cases in which 

 nothing is apparent which is not clearly related to the condi- 

 tions expressed in the ascendants. In the first place the off- 

 spring 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 may be termed ALTERNATIVE 

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

 of the characters of its progenitors. Here each parent con- 

 tributes a certain character but without the exclusion of that 

 of the other and without blending the offspring exhibits 

 MOSAIC inheritance. 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 mulat- 

 toes. Such a result is known as BLENDING inheritance. Or 

 again, certain characters are transmitted from males solely 

 to female offspring. This is an example of SEX-LINKED in- 

 heritance. In still other instances characters of grandparents 

 which are invisible, or 'latent/ in the parents appear again in 



