GENETICS 671 



purple to 7 white (Fig. 33.1). Subsequent analysis has shown that two 

 pairs of genes are involved, one of which (C) regulates some essential 

 step in the production of a raw material and the other (E) controls the 

 formation of an enzyme which converts the raw material into purple 

 pigment. The homozygous recessive cc is unable to synthesize the raw 

 material and the homozygous recessive ee lacks the enzyme to convert 

 the raw material into purple pigment. One of the white-flowered varie- 

 ties was genotypically ccEE— lacked the gene for the synthesis of raw 

 material— and the other was CCee, without the gene for the enzyme re- 

 quired for pigment synthesis. Crossing CCee and ccEE produces an Fi 

 generation all of which are CcEe and have purple flowers because they 

 have both raw material and enzyme for the synthesis of the pigment. 

 The C and E genes are located in different chromosomes, hence their 

 inheritance follows Mendel's Law of Independent Assortment. There 

 are nine chances out of sixteen that any one of the Fo generation from 

 the mating of two F^ plants will have at least one C gene and one E 

 gene and therefore have purple flowers, and seven chances out of sixteen 

 that it will lack either a C gene or an E gene or both and hence have 

 white flowers. Two independent pairs of genes which interact to produce 

 a trait in such a way that neither dominant will produce its effect unless 

 the other dominant is also present are called complementary genes; the 

 action of each one "complements" the action of the other in the pro- 

 duction of the phenotype. This 9:7 ratio is characteristic of the Fg 

 generation of a cross involving two complementary genes. A pure-breed- 

 ing variety of purple-flowered sweet peas could be established by self- 

 fertilization of a plant with the genotype CCEE. 



Supplementary Genes. The term supplementary genes is applied 

 to two independent pairs of genes which interact in the production of a 

 trait in such a way that one dominant will produce its effect whether 

 or not the second is present, but the second gene can produce its effect 

 only in the presence of the first. The inheritance of coat color in guinea 

 pigs, studied by Sewall Wright of the University of Chicago, provides a 

 classic example of supplementary genes. In addition to the pair of genes 

 for black vs. brown coat color (B and b) the gene C controls the produc- 

 tion of an enzyme which converts a colorless precursor into the pigment, 

 melanin, and hence is required for the production of any pigment at all 

 in the coat. The homozygous recessive, cc, lacks the enzyme, no melanin 

 is produced and the animal is a white-coated, pink-eyed albino, no mat- 

 ter what combination of B and b genes may be present. The eyes have 

 no pigment in the iris and the pink color results from the color of the 

 blood in the tissues of the eye. The mating of an albino, ccBB, with a 

 brown guinea pig, CCbb, produces offspring all of which are geno- 

 typically CcBb and have black-colored coats! When two of these Fi 

 black guinea pigs are mated, offspring appear in the Fo in the ratio of 

 9 black : 3 brown : 4 albino. Make a Punnett square to prove this. 



Some combination of complementary and supplementary genes may 

 be involved in the inheritance of a single trait. The dominant genes 

 C and R are both necessary for the production of red kernels in maize, 

 and the absence of either dominant results in white-colored kernels. 



