way he obtained a further generation which on the average was composed of three talls to one dwarf. Subsequent experiment showed that the dwarfs always bred true, as did also one out of every three talls; the two remaining talls behaved as the original hybrids in giving three talls to one dwarf. Having regard to the characters, tallness and dwarfness, three and only three kinds of peas exist, viz. dwarfs which breed true, talls which breed true, and talls which give a fixed proportion of talls and dwarfs. The relation between these three forms may be briefly summarized in the subjoined scheme, in which pure tall and dwarf are represented by T and D respectively, while [T] denotes the talls which do not breed true. Experiments were also made with several other pairs of characters, and the same mode of inheritance was shown to hold good throughout.
Fig. 1.
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Fig. 2. |
Unit-Characters.—As Mendel clearly perceived, these definite results lead inevitably to a precise conception of the constitution of the reproductive cells, or gametes; and to appreciate fully the change wrought in our point of view necessitates a brief digression into the essential features of the reproductive process. A sexual process (see Sex) is almost universal among animals and plants, and consists essentially of the union of two gametes, of which one is produced by either parent. Every gamete contains small definite bodies known as chromosomes, and the number of these is, with few known exceptions, constant for the gametes of a given species. On the fusion of two gametes the resulting cell or zygote has therefore a double structure, for it contains an equal number of chromosomes brought in by the paternal and by the maternal gamete—in the case of a plant by the pollen grain as well as by the ovule. By a process of repeated division the zygote gives rise to a plant (or an animal) whose cells apparently retain the double structure throughout. Certain of the cells of such a zygote become the germ cells and are set apart, as it were, for the formation of gametes. Histology has shown that when this occurs the cells lose the double structure which they had hitherto possessed, and that as the result of a process known as the reduction division gametes are formed in which the number of chromosomes is one half of that which characterizes the cells of the zygote. It is generally acknowledged that the chromosomes play an important part in the hereditary process, and it is possible that the divisions which they undergo in gametogenesis are connected with the observed inheritance of characters. We shall refer later to the few observations which seem to connect the two sets of phenomena.
Our conception of what occurs when a cross is made between two individuals may be illustrated by the diagram which forms fig. 2. Zygotes are here represented by squares and gametes by circles. The dominant and recessive characters are indicated by small plain and black rectangles. Each zygote must contain two and each gamete but one of these unit-characters. Zygotes such as the original parents which breed true to a given character are said to be homozygous for that character, and from their nature such homozygotes must produce identical gametes. Consequently when a cross is made only one kind of zygote can be formed, viz. that containing both the dominant and recessive unit-characters. When the germ-cells of such a heterozygote split to form gametes, these, as indicated in fig. 2, will be of two sorts containing the dominant and recessive characters respectively, and will be produced in equal numbers. If we are dealing with a hermaphrodite plant such as the pea the ovules will consist of one half bearing only the dominant character and one half bearing only the recessive character; and this will be true also of the pollen grains. Consequently each dominant ovule has an equal chance of being fertilized by a dominant or by a recessive pollen grain, and the dominant ovules must therefore give rise to equal numbers of dominant homozygous and of heterozygous plants. Similarly the recessive ovules must give rise to equal numbers of recessive homozygotes and of heterozygotes. Hence of the total offspring of such a plant one quarter will be pure dominants, one quarter recessives, and one half heterozygotes as indicated in fig. 2. Where one character is completely dominant over the other, heterozygotes will be indistinguishable in appearance from the homozygous dominant, and the F2 generation will be composed of three plants of the dominant form to each recessive. These are the proportions actually found by Mendel in the pea and by many other more recent observers in a number of plants and animals. The experimental facts are in accordance with the conception of unit characters and their transmission from zygote to gamete in the way outlined above; and the numerical results of breeding experiments are to be regarded as proving that in the formation of gametes from the heterozygote the unit-characters are treated as unblending entities separating cleanly, or segregating, from one another. From this it follows that any gamete can carry but one of a pair of unit-characters and must therefore be pure for that character. The principle of the segregation of characters in gametogenesis with its natural corollary, the purity of the gametes, is the essential part of Mendel’s discoveries. The quite distinct phenomenon of dominance observed by him in Pisum occurs in many other cases, but, as will appear below, is by no means universal.
Illustrations.—Mendelian inheritance in its simplest form, i.e. for a single pair of characters, has already been shown to occur in many species of animals and plants, and for many very diverse characters. In some cases complete dominance of one of the pair of unit-characters occurs; in others the form of heterozygote is more or less intermediate. Fresh cases are continually being recorded and the following short list can but serve to give some idea of the variety of characters in which Mendelian inheritance has been demonstrated.
A. Dominance nearly or quite complete. (The dominant character is given first).
Tall and dwarf habit (pea, sweet pea).
Round seed and wrinkled seed (pea).
Long pollen and round pollen (sweet pea).
Starch and sugar endosperm (maize).
Hoariness and absence of hairs (stocks, Lychnis).
Beardless and bearded condition (wheat).
Prickliness and smoothness of fruits (Datura).
Palm and fern leaf (Primula).
Purple and red flowers (sweet pea, stocks, &c.).
Fertility and sterility of anthers (sweet pea).
Susceptibility and immunity to rust (wheat).
Rose comb and single comb (fowls).
Black and white plumage (Rosecomb bantams).
Grey and black coat colour (rabbits, mice).
Bay and chestnut coat colour (horses).
Pigmentation and albinism (rabbits, rats, mice).
Polled and horned condition (cattle).
Short and long “Angora” coat (rabbits).
Normal and waltzing habit (mice).
Deformed hand with but two phalanges in digits and normal hand (man).
