iioo LIFE : OUTLINES OF GENERAL BIOLOGY 



(b) a blend between the two parents, if they differ in the amount 



of a fluctuating character, such as the length of the pendent 

 ears of two (varieties of) lop-eared rabbits ; or the characters 

 of half-bred sheep resulting from crossing Leicesters and 

 Cheviots ; 



(c) a coarse-grained mixture, when a paternal characteristic is 



expressed in one part of the body, and a corresponding 

 maternal characteristic in another part, as in a piebald pony; 



(d) a reversionary rehabilitation of an ancestral characteristic 



which has been for several generations latent, while in other 

 features the offspring may show {a) or {b) or (c) ; 



{e) a characteristic feature of one side of the house only, unin- 

 fluenced by its absence or difference in the other parent — 

 the first step in Mendelian inheritance, which is of very 

 frequent occurrence; or 



(/) something quite novel and unpredictable — a mutation. 

 For the hereditary relation is such that while it reduces 

 the risks of variability, tending in the main to the persistent 

 . . continuance of a specific organisation, it by no means pre- 



cludes the outcrop of variations. Variability is antithetic 

 to complete hereditary resemblance, yet its possibility is 

 implied in heredity. 



22. Typical Mendelian Inheritance may be briefly described as 

 follows : 



If a cross is effected between two organisms belonging to true- 

 breeding strains of a species, differing in a crisp, weU-defined unit 

 character, such as gigantism and dwarfism in garden peas, or banded 

 and bandless shells in wood-snails, or horns and no horns in cattle, 

 the offspring will take after one parent only; and the character that 

 is developed is for that reason called dominant, while the one that 

 is not developed is for that reason called recessive. 



Parent with D x Parent with R* 



Result the offspring 



show only the 



D character 



D{R) 



The reason for putting the R character in brackets is that it has 

 not really disappeared, as subsequent breeding shows, although it 

 is unexpressed. 



If the hybrid D(R) offspring are inbred, or bred with others of 



the same history, the second filial generation will show both parental 



types, as regards the character being tested. Out of every four, 



three on an average will show the dominant character, and one the 



* Or it may sometimes be absence of D. 



