. Readings in evolution, genetics, and eugenics. Evolution; Heredity; Eugenics. 388 READINGS IN EVOLUTION, GENETICS, AND EUGENICS something is called by various names in the literature of genetics, the commonest one being determiner. In our illustration, therefore, two of the four chromosomes carry the determiner for tallness. At this point two questions may be asked. I. Why do just two of the four chromosomes carry the determiner for tallness rather than all of them or only one of them ? Just here it would be difficult to explain why no more than two of the four chromosomes are represented as
. Readings in evolution, genetics, and eugenics. Evolution; Heredity; Eugenics. 388 READINGS IN EVOLUTION, GENETICS, AND EUGENICS something is called by various names in the literature of genetics, the commonest one being determiner. In our illustration, therefore, two of the four chromosomes carry the determiner for tallness. At this point two questions may be asked. I. Why do just two of the four chromosomes carry the determiner for tallness rather than all of them or only one of them ? Just here it would be difficult to explain why no more than two of the four chromosomes are represented as carrying the same determiner. This will be explained later. It is easy to answer, however, why the deter- miner is being carried by more than one chromosome. When gametes are formed the chromosome number is reduced one-half. Since every gamete from a pure tall plant carries the determiner for tallness there. F, Hybrid Dwarf Parent Gametes Fig. 67.—Diagram illustrating behavior of chromosomes in Mendel's cross of tall and dwarf peas. Large rectangular figures, nuclei of zygotes or mature individuals; large circles, gametes; small circles within zygotes and gametes, chromosomes; letters on chromosomes, determiners {T, tallness; D, dwarfness). {From Coulter and Coulter.) must have been at least two chromosomes carrying the determiner before the gametes were formed. 2. Do these two chromosomes carry any other determiner than that for tallness ? In a tentative way this question may be answered in the affirmative, but a fuller discussion of the situation must be deferred. There is much experimental evidence that indicates that more than one determiner is carried on a single chromosome. In some cases also there are more Mendelian determiners than there are chromosomes. The situation is represented in Fig. 67. This shows a somatic cell with the diploid or 2x number of chromosomes. In the formation of gametes this number is reduced to the haploid, or a; number, which in this case is
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