Mendel's second law deals with cases where more than one pair of characters are involved. It has been found that tall and short races of peas are contrasted characters that segregate in the same way as do yellow and green. If a tall race with yellow peas is crossed to short race with green peas, the segregation of each pair is independent of that of the other, so that one-quarter of the egg cells of such a hybrid contains the elements for tall and yel low; another quarter the elements for tall and green; another quarter the elements for short and yellow, and another quarter for short and green. Similarly in the formation of the pol len of the same four kinds of gametes are pro duced. Chance meetings of ovules and pollen give 16 combinations as shown in the following diagram.
Since yellow dominates green and tall dominates short, there are in ..this second filial (F2) generation nine tall yellow; three short yellow; three tall green; one short green. Thus, while members of each pair of factors of the hybrid segregate at the time of ripening of the germ tells, the separation is independent for each pair. This is Mendel's second discovery that may be called the law of independent as sortment.
Mendel showed that three pairs of char acters behave in the same way, i.e., their genes assort independently, and there is reason to suppose that this law holds whenever the genes for the two or more pairs of characters are carried by separate pairs of chromosomes, but, as will be shown below, whenever the genes are carried by the same pair of chromosomes the distribution is controlled by a third law of heredity, viz., the law of linkage.
The elements that are supposed to represent, in a sense, the hereditary characters are com monly called genes and the word Genetics, or the study of behavior of the genes, has come to replace, in the modern work on inheritance, the older term Heredity with its numerous con notations. The Mendelian characters have been spoken of as unit characters, and it is sometimes implied that the gene directly pro duces each such character. The clearest evi dence, however, indicates that the so-called unit character is only one of the many effects which the gene may produce in conjunction always with many, perchance with all of the other genes. Thus, the germ plasm is looked upon as the sum total of the genes whose combined effect is responsible for each character of the body. While the body is built up through the interaction of the materials that the gene pro duces, yet in the formation of the germ cells the genes act as independent units that collect in pairs, then segregate, those pairs in separate pairs of chromosomes assorting independently; those pairs in the same chromosome pair being linked.
Modern work on the cell has pointed unmis takably to the mechanism by means of which both segregation of the genes and the assort ment of the chromosomes takes place. Every cell in the body or immature germ cell con tains a double set of chromosomes (except in the male of certain groups, which lacks one of the sex chromosomes) ; one member of each pair has come from the father and one from the mother. During the maturation process the
maternal and the paternal chromosomes mate with each other — like with like. Subsequently at the so-called reduction division one member of each pair goes to one daughter-cell and the other member to the other daughter-cell. If the chromosomes carry the Mendelian genes the maternal and the paternal genes will be segre gated when the chromosomes are reduced to produce the gametes; but at the reduction divi sion there is not a separation of all the maternal from all of the paternal chromosomes as a group, but each pair of chromosomes segre gates independently of the others, so that the daughter-cells may get any possible assortment of the chromosomes of paternal and maternal origin, but always one or the other member of each pair. This condition fulfills all the re quirements of Mendel's second law of free as sortment.
But obviously if, as assumed, the chromo some threads are the bearers of the genes and if, as generally held to-day, the thread is a structural element that remains intact even in the resting stages of the cell, then the genes must be inherited in groups corresponding in number to the number of the chromosomes. In a word, all the genes in a given chromosome will be linked together. The most recent evi dence shows that this is the case, and that there are as n;iany groups of linked genes as there are kinds of chromosomes. Since 1906 the number of demonstrated cases of linked genes has steadily increased so that it can no longer be questioned that this relation is a characteristic feature of Mendelian inheritance. In one in stance, that of the fruit fly, Drosophila ampelo phila, it has been shown that the 200 known hereditary differences are inherited in four groups corresponding to the four pairs of chromosomes. Thus, Mendel's law of segrega tion has found its justification in the cytological mechanism of reduction in the germ cells; while his law of free assortment has been confirmed in the method of assortment of the chromosome. Later the discovery of the meaning of linkage phenomena has brought • all the fundamental properties of heredity into complete harmony with the chromosomal mechanism. The individ uality of the chromosomes that is responsible for linkage has been found, however, not to be absolute, for the members of a pair have been shown to interchange equivalent parts at times, but the interchange has been found to follow a predictable course, and while it complicates the results, in no sense undermines the general principle involved. In some species the inter change (crossing over) takes place only in the female sex (Drosophila), in others in the male sex (silkworm moth), while in still others it takes place in both sexes, as in some herma phroditic plants.