12 GENETIC VARIATIONS 



produce the vigorous or normal condition usually prevails 

 over the other; the individual is not weak or defective. In 

 general, it may perhaps be said that the gene that carries 

 development farthest usually prevails over the other, though 

 whether this is the invariable rule is doubtful. The gene that 

 prevails is said to be dominant; the one that does not prevail, 

 that produces no obvious effect, is said to be recessive. It is 

 only when two of the weak or recessive genes get together in 

 a pair — one such coming from each of the two parents — that 

 the weak or defective or recessive characteristic is produced 

 and appears in the individual. 



Thus each individual is double with respect to his genes. 

 Each contains a number of genes that have no manifest 

 effect; a lot of recessive genes, whose effect is suppressed by 

 their dominant companion genes in the same pairs. Thus one 

 gene of a pair can become modified, become defective per- 

 haps, without producing any obvious effect on the character- 

 istics. But if two parents each contain such a defective gene, 

 in the same pair, then when they mate, some of their offspring 

 may get a pair of genes in which both members are recessive 

 — weak or defective — and these offspring will manifest the 

 weak or defective characteristics, though their parents did not. 



Such then is the picture of the germ plasm, or of what we 

 may call the genetic system, a picture that we must keep in 

 mind in trying to understand genetic variation. In any indi- 

 vidual a double set of genes, frequently only one of each pair 

 of genes manifested, because dominant; the other recessive, 

 producing no effect (see Figure 2). 



If we examine different individuals we find that the genes 

 of a certain pair — having a certain general function — may 

 exist in many different modifications in the different individ- 

 uals. So a certain gene pair, located at a particular point in 

 the chain, that affects the eye color in the fruit fly, is known 



