MEANS AND METHODS OF EVOLUTIONARY CHANGE 333 



eyes is also present. Hence we say that the gene for blue eyes is recessive, 

 the gene for brown eyes dominant. A recessive gene, then, is one that does 

 not produce a visible effect in an individual who also possesses the corre- 

 sponding dominant gene. As a consequence, an individual must inherit a 

 recessive gene from both of his parents if he is actually to exhibit the char- 

 acteristic which that recessive gene produces. 



It follows that there are two kinds of brown-eyed people in the world. If 

 we let B represent the gene for brown eyes, b the gene for blue eyes, we can 

 represent the two kinds of brown-eyed people as follows: (1 ) BB, (2) Bb. 

 Both types are said to have the same phenotype: the same visible, bodily ex- 

 pression of the gene B. But they differ in genotype: in genetic constitution. 

 Members of the first type have inherited the gene from both of their parents. 

 Members of the second type have inherited a brown-eye gene from one 

 parent, a blue-eye gene from the other. (Actually there are more than these 

 two kinds of brown-eyed people, since more than one pair of genes are in- 

 volved in producing the varying shades of brown eyes observable. But for 

 present purposes concentration of attention on the one pair of genes will 

 suffice. ) Following this scheme, blue-eyed people have the formula bb, the 

 gene for blue eyes having been received from both parents. Blue-eyed 

 people, and brown-eyed people of the first type (i.e., BB), having both 

 members of the pair of genes alike, are said to be homozygous. Brown-eyed 

 people of the second type (Bb), having one gene for brown eye, one gene 

 for blue eye, are said to be heterozygous. 



Now suppose that two heterozygous brown-eyed people marry (Bb x Bb; 

 Fig. 15.1). The mother produces some ova which contain B, some ova which 

 contain b. Similarly, the father produces sperm cells which contain B, others 

 which contain b. What kinds of children are possible? A 5-containing ovum 

 may be fertilized by a fi-containing sperm cell; the resulting child will have 

 the BB combination and will be brown-eyed. Or a fi-containing ovum may 

 be fertilized by a /^-containing sperm cell; the result will be a brown-eyed 

 child (Bb). A heterozygous brown-eyed child will also result from the com- 

 bination of a /^-containing ovum with a 5-containing sperm cell. Finally, a 

 /j-containing ovum may be fertilized by a /^-containing sperm cell, the result 

 being a blue-eyed child. This latter result is the most interesting from our 

 present standpoint since it illustrates the mechanism by which children 

 frequently differ from their parents. In general terms, heterozygous parents 

 can transmit to their offspring recessive genes which have no visible effect 

 in the parents themselves. Some children, receiving two such genes from 

 their parents, will consequently differ from the latter in the characteristic 

 concerned. 



So far we have concentrated upon but one pair of genes, B and b. Another 



