Fig. 20
Diagram showing union of factors from the two separate parents in fertilization and their segregation in the formation of germ-cells (after Wilson). With four pairs of factors (Aa, Bb, Cc, Dd), sixteen types of gametes are possible, as shown in the series of small circles at the right. The same diagram equally well represents the pairings and segregations of chromosomes.
A Single Chromosome not Restricted to Carrying a Single Determiner.—It has been objected that there may be more pairs of independently heritable allelomorphic characters than there are pairs of chromosomes. It is true that there are more pairs of characters than pairs of chromosomes but there is no reason for supposing that a given chromosome is restricted to carrying a single unit-determiner. On the contrary it probably carries several or many. Some workers have pointed out that certain units might be interchanged during the pairing of chromosomes before the reduction division, others that inasmuch as the chromosomes become diffuse and granulated during the intervals between divisions it is not improbable that the individual units may become separated from their original system during such times and that it is a matter of chance into which of the homologous chromosomes, A or a, they enter with the re-establishment of the chromosomes. On the other hand, cases are known where two or more separate characters are permanently associated in inheritance, that is, if they enter a crossed form together they come out together in the grandchildren as if they were carried in the same unit-body in the germ-cell. The only observable unit-bodies that fulfil the necessities of such cases are the chromosomes. This tendency of characters to exist in groups which are inherited independently of one another is coming more and more into evidence as we penetrate farther into the intricacies of inheritance, and it is exactly what we would expect on the supposition that each chromosome carries the determiners of a number of characters instead of a single one.
CHAPTER IV
MENDELISM IN MAN
The Mendelian Principles Probably Applicable to Many Characters of Man.—We are really just beginning to make the proper observations and collect the necessary data with reference to the application of Mendelian principles to the traits of man. Yet brief as has been our study we have disclosed much significant evidence which makes it seem highly probable that many of his characters, good and bad, of mind and body are as subservient to these laws as are the traits and features of lower forms. Davenport and Plate record over sixty human characters or defects which are seemingly inherited in Mendelian fashion. Although about fifty of these are pathological or abnormal conditions, this does not mean that such conditions are more prone to follow Mendelian inheritance but merely that being relatively conspicuous or isolated they are easier to follow and tabulate.