Heredity - 483 



sis and fertilization — as may be seen in Fig- 

 ure 26-7. As to feather color genes, each of 

 the Pj fowls can produce only one kind of 

 egg, or sperm, depending on the sex. Conse- 

 quently all F, individuals must be alike; all 

 are heterozygous (Bb) as to the feather color 

 genes. But when the Fj hybrids form gametes, 

 each fowl produces two kinds of gametes in 

 equal numbers. Half the eggs and half the 

 sperm must carry the B gene, and half must 

 carry the b gene. Consequently, depending 

 entirely on the chance union of the gametes, 

 the F 2 offspring must be homozygous black 

 (BB), heterozygous (Bb) blue, and homo- 

 zygous white (bb) in a ratio of 1.2.1 as is in- 

 dicated in the following Punnett Square: 



Fj eggs B b 



F 2 zygotes 



All other matings among Andalusians can 

 be explained by the same mechanism. A blue 

 hen mated to a black cock produces blues and 

 blacks on a 1:1 basis; and similarly a cross 

 between a blue and a white fowl gives blues 

 and whites in a 1:1 ratio (Fig. 26-8). In fact 

 the chromosome hypothesis has been vali- 

 dated by experiments on a countless number 

 of unit hereditary differences, taken singly 

 and in combination, in practically every kind 

 of plant and animal. 



The results so far indicate that a parent 

 transmits hereditary qualities to the offspring 

 through the agency of discrete material 

 units, called genes, which are borne in the 

 chromosomes of the germ cells. All the di- 

 ploid cells of the organism are equipped 

 with at least two genes that influence the 

 development of any one characteristic. In 

 exerting its effect, each gene maintains a 

 discrete individuality. Its effect upon the or- 

 ganism may be blended with the effects of 

 other genes, but the gene itself is not changed 

 by this association. During meiosis in the 

 succeeding generation, the members of each 



Fig. 26-8. Above, cross of blue and black Andalu- 

 sians; Below, cross of blue and white. 



pair of genes become segregated into sepa- 

 rate gametes, so that each gamete can trans- 

 mit only one of any given pair of genes. 



All of the foregoing ideas are implicit in 

 the work of Mendel, except that Mendel re- 



