CHAPTER IX

MENDELIAN HEREDITY AND ITS MECHANISM[200]

I

1. The scientific era of the investiga­tion of heredity begins with Mendel’s paper on plant hybridiza­tion which was not appreciated by his contemporaries. Mendel invented a method for the quantitative study of heredity which consisted essentially in crossing two forms of peas differing only in one well-defined hereditary character; and in following statistically and separately the results of this crossing and that of the inbreeding of the second and third genera­tions of hybrids. This led him to the recogni­tion of one essential feature of heredity; namely, that while the hybrids of the first genera­tion are all alike, each hybrid produces two types of sex cells in equal numbers, one for each of the pure breeds which has been used for the crossing. This takes place not only when the forms used for the crossing differ in regard to one character only but also if they differ for two or more characters. The statement made is Mendel’s law of heredity, or, more correctly, Mendel’s law of the segrega­tion of the hereditary characters of the parents in the sex cells of the hybrids.[201] Mendel’s law allows us to tabulate and calculate beforehand the relative number of different forms which appear if the offspring of a mating of two varieties are bred among themselves.

In order to do this it must be remembered also that while in some cases the hybrid is an intermediate between the two parent forms, in other cases it cannot be discriminated from one of the two parent forms. In such cases the character which appears in the hybrid was called by Mendel the dominant character and the one which disappeared the recessive character. According to Bateson, who was the first to systematize the phenomena of Mendelian heredity, recessiveness means generally the absence of a character which is present in the dominant type. When, e. g., the cross between a tall and a dwarf form of pea gives in the first genera­tion only tall peas, on the basis of the presence and absence theory the dominant form contains a factor for growth which is lacking in the dwarf form. While this theory fits many cases it meets with difficulties in others. Thus the presence of a factor for pigment should be dominant over the absence of such a factor, which is usually the case, inasmuch as the cross of a coloured rat or rabbit with an albino is black or coloured. There is, however, also a case where whiteness is dominant over colour, as we shall see later. This fact does not necessarily contradict the presence and absence theory.[202]

When two pure breeds of parents differ in one character, e. g., two varieties of beans, one with a violet the other with a white flower, the cross between the two species (the F1 genera­tion) has pale violet flowers, approximately intermediate between the two parents. If these hybrids are bred among themselves the offspring is called the F2 genera­tion. According to Mendel’s law the hybrids of the first F1 genera­tion all have two kinds of eggs in equal numbers, one kind representing the pure breed of the parents with violet, the other of the pure breed with white flowers. The same is true for the pollen cells. Hence the following possible combina­tions must appear in the offspring when the pale violet hybrids are inbred:

The four possible combina­tions are: (1) violet—violet; (2) violet—white; (3) violet—white; (4) white—white. The first will result in pure violet flowers, the fourth in pure white, and the second and third in pale violet flowers. Since all four combina­tions will appear in equal numbers when the number of crossings is sufficiently large the numerical result will be:

violet : pale violet : white = 1 : 2 : 1

Fifty per cent. of the F2 genera­tion will be pale violet, 25 per cent. violet, and 25 per cent. white. The violets and whites each will breed true when bred among themselves since they are pure, and produce only one type of eggs and pollen. The pale violets are hybrids and will again produce the two types of eggs and pollen, that is, if bred among themselves will again give violets, pale violets, and whites in the ratio 1:2:1. This the experi­ment confirms.