284 GENETICS 



bination of genes, which makes up its genotype or genetic 

 constitution. This combination produces, under the given life 

 conditions, a certain set of characteristics, structural and 

 physiological. The gene combination is made up of two cor- 

 responding sets of genes, one set from the individual's 

 mother, the other from his father. The genes are thus in 

 pairs, the two members of each pair having analogous func- 

 tions. Often one member of the pair is dominant, the other 

 recessive. 



When the individuals reproduce, this combination of 

 genes is broken up and a new one is made by putting together 

 half of the genes from each of the two parents (see figure 

 59). The great rule for the production of the new combina- 

 tion in the offspring is this: Each parent gives to any in- 

 dividual offspring one member of each of his own pairs of 

 genes. If the parent has a thousand pairs of genes, he gives 

 to each of the offspring one member of each pair, a thousand 

 single genes in all. The other parent gives also one member 

 of each of his pairs, so that each of the offspring receives 

 anew, from both together, the original number of pairs. 



Each parent may give to the child either member of any 

 of his pair of genes. In many cases, as we know, the two 

 members of a pair differ in their effects on development and 

 characteristics; commonly one is dominant, the other re- 

 cessive. If we designate the two members of a pair as A 

 and a, the child may receive either A or a, and his develop- 

 ment and characteristics may differ accordingly. Some of the 

 offspring receive one of the two, some the other. 



Furthermore, the different pairs of genes of the same 

 parent are In a measure Independent in the way they are 

 distributed to the offspring. If the child receives the domi- 

 nant gene of one pair, he may receive either the dominant 

 or the recessive gene of any other pair. It Is true that there 

 Is a tendency (known as linkage) for two genes that are 

 close together In either of the two parental strings of genes 



