Physiology: The Science of the Body

by Ernest G. Martin

The first step in development is the coming together of the egg cell with one sperm in the process that we call fertilization. The sperm penetrates the egg and its chromosomes line themselves up with the egg’s, restoring the pairing that is present regularly. Immediately afterward the cell divisions begin that make up development, and in all of these the usual lengthwise splitting of determiners takes place. Every cell in our body contains its pairs of chromosomes, one member of each pair tracing back directly to the egg cell while the other traces directly to the sperm. Thus half of our determiners came from the maternal germ tissue and half from the paternal.

Figure 1, chromosome splitting in ordinary cell division, in which each determiner splits in half, contrasts with Figure 2, reduction division, in which the chromosomes of the pair are simply pulled apart.

We shall now begin to see how heredity works out. Suppose chromosome one in the egg has large-letter determiners A, B, and C, while the corresponding chromosome in the sperm has small-letter determiners a, b, and c. When these line up after fertilization, restoring pair one, we have A opposite a, B opposite b, and C opposite c. Since we have supposed the large and small letters to stand for contrary hereditary traits we introduce here a conflict and must ask at once how it is settled. One of the things the monk Mendel worked out in his studies of heredity in peas was this particular problem. He found that where there is conflicting heredity one of the determiners usually dominates over the other, and when this happens the trait in the offspring will be like that of the parent which contributed the dominant determiner. To illustrate: suppose A is dominant over a, then in the case in question the offspring will be like the mother in feature A. In some kinds of animals and plants conflicting characters blend in the offspring, producing an intermediate appearance. A good example of this latter case is in the common flower, the four-o’clock. In this plant white blossoms and red blossoms are due to determiners that occupy the same positions in the chromosomes; therefore, if white and red flowered plants are crossed, these conflicting determiners come together when the sperm and egg chromosomes pair. Since neither determiner dominates over the other, the color of the flowers in the offspring is neither white nor red, but pink.

Any individual whose chromosome pairs contain conflicting determiners is called a “hybrid”; there may be every degree of hybridism, from the simplest, in which all the chromosome pairs are alike except one, up to the most complete, in which all the pairs are unlike. It is easy to tell a hybrid by its appearance in the cases in which there is blending inheritance, but not so easy when one trait dominates over the other, for then the hybrid will look like the parent that furnished the dominant determiner. The sure way to detect hybrids is by the study of their offspring. Suppose we have two parents, both of whom are hybrids in respect to chromosome pair one. This pair in both will contain determiners A, B, and C lined up opposite a, b, and c. Since we have supposed the large letters to be dominant over the small, both will have the same appearance, dependent on the presence in their chromosomes of A, B, and C. Since in the formation of eggs and sperm the chromosome pairs are pulled apart, half the eggs produced by the mother will contain determiners A, B, and C, and the other half a, b, and c, and half the sperm of the father will similarly contain one set and half the other. Since it is an absolute matter of chance which sperm encounters which egg in fertilization we can safely conclude that in the long run all the chances will be realized equally. Calling, for convenience, the large-letter eggs E and the small-letter e, and, similarly, the large-letter sperm S and the small-letter s, this means that E can be fertilized either by S or s, and e also by either S or s. The possible combinations are ES, Es, eS, and es. In terms of actual determiners these combinations are ABCABC, ABCabc, abcABC, and abcabc.

The combinations just given represent the possible offspring from a pair of hybrid parents. If we look them over, we see at once that only half of them are hybrid, namely, combinations ABCabc and abcABC; the other half are pure breed, the chromosome pairs being exactly alike; but these pure breeds are of two kinds, one having only large-letter determiners, the other only small-letter. If it is a case where the large letters dominate over the small, the large letter pure breeds and the hybrids will look alike, but the small-letter pure breeds will look different, since in them the traits governed by the small-letter determiners have a chance to show themselves. A very good illustration of this is seen in eye color in human beings. Brown eyes are dominant over blue; in other words, the determiner that causes eyes to be brown dominates over that responsible for blueness in cases where both come together in hybrids. A person who has brown eyes may be either a pure breed in that respect or may be a hybrid; there is no way to tell the difference from the appearance; but if the brown-eyed person has offspring, and any of them turn up with blue eyes, it is proof positive that the parent is a hybrid so far as eye color is concerned. Moreover, the blue-eyed child is not a hybrid in this respect; his brown-eyed brethren may or may not be; in the long run two-thirds of them will prove so; the other third will be pure breed, having in their chromosome pairs only brown-eye determiners.

Where the hybrids differ from the pure breeds, as in the case of the four-o’clocks, given earlier, it is easy, of course, to tell which are pure and which are hybrid. When pink four-o’clocks are interbred, the chromosomes will combine just as described above for hybrids, since the plants that have pink flowers are hybrid. One-half the offspring will have pink flowers, showing that they are hybrid; one-fourth will have white flowers, proving that in them the white-flower determiners have separated out, and the other fourth will have red flowers, because in them the red-flower determiners are the only kind present. This and similar experiments have been tried hundreds of times, and whenever the numbers of offspring have been great enough to allow the chances to equalize, the proportion of different kinds of offspring has always agreed almost exactly with expectation. Of course in human beings the families are not large enough for this always to work out accurately, but even so the agreement is often striking.