484 - Heredity and Evolution 



ferred to "factors" rather than genes, and 

 Mendel did not realize that the factors are 

 localized in the chromosomes. The central 

 and pre-eminently important concept of 

 Mendelian inheritance is the realization 

 that the genes are stable self-perpetuating 

 units that are transmitted, without change, 

 throughout many generations. This means 

 that the characteristics of a stock are not lost 

 by dilution, but tend to appear without al- 

 teration generation after generation. Hered- 

 ity is, indeed, more a matter of "shuffling" 

 (not mixing) the genes during the processes 

 of meiosis and fertilization. Or, in other 

 words, heredity is particulate — a process of 

 transmitting discrete particles, which are the 

 genes. This is in sharp contrast to many 

 older concepts of heredity. These older views 

 considered heredity to depend upon a sort 

 of blending process in which inevitably the 

 characteristics of a stock woidd be further 

 and further attenuated with each succeeding 

 generation of outbreeding. 



Allelic Genes. Each gene occupies a spe- 

 cific locus in a specific chromosome of the 

 species, and all genes occupying this locus in 

 a given pair of chromosomes are said to be 

 allelic to each other. Among Andalusians, 

 for example, the "black" gene (B) is the 

 allele of the "white" gene (b), since these 

 genes occupy corresponding loci in the same 

 chromosome pair. In other words, just as the 

 chromosomes in the diploid cells of the or- 

 ganism occur in homologous pairs, so the 

 genes are present as allelic pairs; and if one 

 of the alleles of a certain pair has been de- 

 rived from one of the parents, the other 

 inevitably was derived from the opposite 

 parent. 



Dominant Genes. The case of the Anda- 

 lusian fowl was taken as a first example in 

 Mendelian inheritance because neither of the 

 allelic genes determining feather color ob- 

 scures the effects of the other. In many cases, 

 however, one member of an allelic pair may 

 totally hide the effects of the other. Such 

 complete dominance is shown in the follow- 

 ing experiment, which deals with the inherit- 



ance of seed color in garden peas, a material 

 used by Mendel in his original studies. 



Two stocks of the garden pea have been 

 cultivated: one with yellow seeds, the other 

 with green. Both stocks breed true when 

 self-pollinated; that is, no greens are obtained 

 from the yellows, and vice versa. When such 

 stocks are cross-pollinated, in either direc- 

 tion, all the Fj seeds display a yellow color. 

 Proceeding to the F 2 generation, obtained by 

 self-pollinating the plants grown from the 

 yellow seeds, 75 percent of the F 2 seeds are 

 yellow and 25 percent are green. 



Figure 26-9 shows that this case is similar to 

 the experiment with the Andalusians, ex- 

 cept that one of the gene pair determining 

 seed color, being dominant, suppresses the 

 effects of the other (recessive) gene. The P t 

 green-colored seeds must have been homo- 

 zygous (having the two genes of the allelic 

 pair identical) as to the "green gene" (g) and 

 the yellow-seeded stock must likewise have 

 been homozygous for the "yellow gene" (G); 

 otherwise neither of these stocks would breed 

 completely true with successive self-pollina- 

 tions. Consequently only one kind of gamete 

 is produced by each of the P : parents, and all 

 of the Fj hybrids are inevitably alike (Fig. 

 26-9). Due to the dominance of G gene, how- 

 ever, all the Fj seeds display a yellow color. 

 But during meiosis, when segregation occurs, 

 the F x plants form two kinds of spores, and 

 subsequently, the same two kinds of gametes 

 (Fig. 26-9). Consequently when the ¥ 1 eggs 

 and sperm combine at random, the F 2 seeds 

 must come out "yellow" and "green" in a 

 ratio of 3:1, as is shown by the following 

 Punnett Square: 



Fi eggs G g 



F zygotes 



Genotype vs. Phenotype. Although all 

 look alike, not all the yellow-colored F.,seeds 

 are identical in their genetic constitution 



