Physiology: The Science of the Body

by Ernest G. Martin

In practical animal breeding the blended inheritance just described is not very useful, for even though a blended character might appear which is just what the breeder has been looking for, it will not occur in more than half the offspring and can not ever be depended on to show itself in any particular individual. This explains largely why pure-bred stock is always more desirable than hybrid, and why breeders strive so eagerly to obtain desired traits in pure-bred animals. In plants, blended characters are much more valuable, for two reasons; first, because the offspring are so numerous that even though half of them come out pure, and so lack the desired blend, there are enough left that have it to make the crop worth while; and second, because propagation by cuttings is possible in very many kinds of plants, which means that the same plant is kept going in hundreds of places, and for tens or even hundreds of years. A trait that is desirable can be perpetuated indefinitely by this means, even though it may be a blending of several hereditary traits, which would separate out in a few generations by ordinary means of propagation.

There are several more things in heredity that must be taken up while we are on the subject, so we shall have to return to the chromosomes for a while. We have seen that there are several determiners to each chromosome; for convenience, we assigned three apiece to our chromosomes, except the ninth, which has to get along with two; but in reality the number to each chromosome is often much greater. This grouping of the determiners, several to a chromosome, carries an interesting consequence with it, in that all the hereditary traits controlled by one chromosome have to go together in reproduction. In the example we have already used A, B, and C are together; therefore any individual that shows character A must show B and C as well. The most striking instances of this are certain traits that are bound up with sex, but we cannot describe these further until we have looked into the heredity of sex, which we shall do in a minute. First a word must be said about occasional exceptions that turn up to the rule that we have just stated. In the study of thousands of specimens now and then one has been found in which there has evidently been a swapping about of determiners. We can illustrate the situation by supposing chromosome one is found to contain determiners A, b, and C, instead of A, B, and C; one small-letter determiner has traded places with a large. Of course, the effect of this is to permit different combinations of hereditary traits than ordinarily occur, and at the present time students of heredity are actively engaged in following this up to see how it happens, and what advantage can be taken of it. This crossing over of determiners from one chromosome to another takes place only among such as are actually in contact at times within the nucleus as seen under the microscope, which confines it to the members of corresponding pairs.

In man, and in many of the lower animals, sex is a hereditary character. That means that there is a determiner for it which is grouped with other determiners in one of the chromosomes. In man the determiner is for femaleness; there is no special determiner for producing the male sex; it is produced whenever the female determiner is missing from one chromosome of the pair, and this is brought about by having the whole chromosome that should make up this pair absent. At the beginning of this chapter the fact was mentioned incidentally that a good many of us have only 47 chromosomes, instead of the 48 that are characteristic of human beings. The distribution is really almost exactly half and half, for all males have 47 and all females 48. This means that the cells of the germinal tissue of females have 24 complete pairs, while the corresponding cells in males have only 23 complete pairs and one chromosome over. This extra chromosome is the one that contains the determiner for femaleness; each of the chromosomes of pair 24 in females contains this determiner also. These are often spoken of as sex chromosomes.

Now when in the course of the production of egg cells within the mother’s germinal tissue the pairs of chromosomes are pulled apart, each separate cell, and so each egg, will contain the full number of chromosomes, 24, including the sex chromosome. But when the same thing happens in the course of the production of sperm only every other one will have the full number; the remaining half having only 23, and all of this half lacking the chromosome that contains the determiner for femaleness. There are, then, always equal numbers of two kinds of sperm, one with 24 and the other with only 23 chromosomes. If the egg is fertilized by a sperm containing 24, including the sex chromosome, the pairing of chromosomes is complete in the egg, and the offspring will be a female; if, on the other hand, the fertilizing sperm is one that contains only 23 chromosomes the pairing in the egg will be incomplete; the single sex chromosome of the egg will not be paired with a corresponding one from the sperm and the egg will develop into a male. Since it is a pure chance whether fertilization will be accomplished by a sperm of 24 or one of 23 chromosomes, we should expect the sexes to appear in exactly equal numbers, taking the world as a whole. As a matter of fact, whenever extensive birth data have been accumulated they have shown a very slight excess of male births over female. We are not able to explain this at the present time. It is possible that the 23 chromosome sperms are a little more vigorous for some reason than those that have 24, and so are able to fertilize slightly more than their share of eggs.

We spoke a moment ago of hereditary traits whose determiners are bound up in the sex chromosomes. All such behave interestingly in heredity for the simple reason that they can never be transmitted from father to son, but only from father to daughter. This is because, as we have just seen, the sex chromosome in the sperm always causes the egg which that sperm fertilizes to develop into a female. The single-sex chromosome which males possess invariably comes from the mother. An interesting example is the common type of color blindness known as Daltonism. Normal color vision is hereditary and the determiners which establish it are in the sex chromosomes. Occasionally a person is found in whom these determiners are defective. If this person is a male, he will be color blind, but if a female not, unless both sex chromosomes are defective in this regard, since normal color vision is dominant over color blindness; so if one sex chromosome is normal the vision will be also. The woman, in this case, will be a hybrid with respect to color vision; one of her sex chromosomes containing a normal determiner, the other a defective.

This works out in heredity as follows: If a color-blind man is married to a woman who has no color blindness in her heredity, none of his children will be color-blind because he cannot transmit the sex chromosomes which carry the determiners for color blindness to his sons, but only to his daughters; all the latter will be hybrid with respect to the character, since all of them come from fertilized eggs which received sex chromosomes from the sperm. If these daughters, in turn, marry men who are free from color blindness, some of their sons may be color-blind, but none of their daughters can be. The only way in which women can be color blind through inheritance is by descent from color-blind fathers and from mothers who are either themselves color-blind or are hybrid with respect to the trait. The result of this difference in the heredity of the two sexes is to make color blindness many times as frequent among men as among women. In round numbers four men out of every hundred have this type of color blindness, while only six or seven women in ten thousand show it.

We have left for discussion only one topic dealing with heredity, but this is the most baffling of all, since it deals with the problem of how the various kinds of determiners came into existence. It is evident that if given one parent with all large-letter determiners and the other with all small-letter, we might, in the course of many generations, get a great variety of combinations and so a great many different-appearing individuals. But unless we have various kinds of determiners to start with, there is no way in which this can be done. We do not pretend to know very much about how the innumerable determiners that are in existence came about, but we have one clue that is thought to point the way. In some animals, and many plants, descendants put in their appearance from time to time that are so different from their ancestors as not to be accountable according to ordinary laws of heredity. These have long been known, and the name of “sport” has been applied to them by breeders. Since the facts about determiners have been learned, it has been clear that these “sports” cannot have all their determiners like those present in their parents, and it has come to be believed that occasionally spontaneous changes take place within individual determiners. Since the determiners are undoubtedly complex chemical structures, we know of no reason why this might not happen. Probably it is much more common an occurrence in some kinds of plants and animals than in others. The name of “mutant” has been applied to the plant or animal in which this has happened, and the process is called “mutation.” It is important since it is the most likely way in which the innumerable kinds of determiners that are now in existence came into being.

We suppose that since life first put in its appearance on the earth there have been uncounted mutations, a vastly greater number than are now represented by determiners. Many of the mutants could not compete with their brothers and sisters of ordinary descent and so promptly died, but occasionally it might happen that a mutant would be as well fitted for life as its relatives, in which case it would establish itself, and in course of time become ancestor to a whole line like itself. If this happened often enough, and time were allowed for it to work out, all the kinds of plants and animals that are now in existence might have come by descent from a very few ancestors. The geological history of the earth shows that there has been plenty of time, even though valuable mutations did not occur oftener than once in a thousand years.