Chromosome

“ The noteworthy thing about this “chromosome x” is the fact that in every essential detail it corresponds to the accessory chromosome of the Orthoptera. It is a spermatogonial chromosome that comes over intact into the spermatocyte; it retains its form and staining power unchanged through the prophase of the spermatocyte; it divides in only one of the spermatocyte mitoses; and is a large and conspicuous element of the cell at all times. ”

“ The evidence that the chromosomes are structural elements of the cell that perpetuate themselves at every division has continually grown stronger. That factors have the same distribution as the chromosomes is clearly seen in the case of sex-linked characters, where it can be shown that any character of this type appears in those individuals which from the known distribution of the X chromosomes must also contain the chromosome in question. For example, in Drosophila, as in many other insects, there are two X chromosomes in the cells of the female and one X chromosome in the cells of the male. ”

“ If the accessory chromosome is a sex-determinant, then when an egg is fertilized by a sperm lacking this element, the egg itself must carry the factor that determines that the sex of the resulting organism of 35 chromosomes shall be a male. In the event of the other possible combination the egg selected must have been one carrying the female determinant and, since the accessory chromosome is a male-determinant and the union of the two produces a sex-hybrid, femaleness must dominate in order to yield a female organism of 36 chromosomes. ”

“ In consequence there are two classes of sperms in man; female producing containing 24 chromosomes, and male producing containing 23 chromosomes. If the factor for color blindness is carried by the X chromosome its inheritance in man works out on the same chromosome scheme and in the same way as does white eye color (or any other sex linked character) in the fly, for the O sperm in man is equivalent to the Y sperm in the fly. ”

“ The Sex Chromosome.—The evidence centers about a special chromosome or chromosome-group commonly designated as the sex-chromosome or X-element, which has been found in various species of animals, including man. ”

“ The single-chromosome generation, which I will call the HAPLOID, must have been the primitive generation in all organisms; it might also persist as the only generation. Every sexual differentiation in organisms, which occurred in the course of phylogenetic development, was followed by fertilisation and therefore by the creation of a diploid or double-chromosome product. ”

“ If we suppose that each chromosome bears the determiner of a Mendelian character and that chromosomes bearing allelomorphic characters make up the various pairs which are seen in the early germ-cells of an individual before reduction occurs, then the segregation of the individuals of an allelomorphic pair into different gametes must result in consequence of the passing of the corresponding chromosomes into separate gametes. ”

“ When we come to consider the second spermatocytes, spermatids, and spermatozoa, it is necessary to divide them into two classes, because of the unequal apportionment of the accessory chromosome consequent upon its remaining undivided in the first spermatocyte mitosis. There are formed, accordingly, two numerically equal classes of second spermatocytes—those containing sixteen chromosomes plus the accessory chromosome, and those with merely the sixteen chromosomes. ”

“ Therefore we have made use of a trick we call “marking the chromosome,” which means that we use a male whose sex chromosome carries a known gene near the tumor locus. By using this type of male in successive generations we get two types of daughters: one type like their surviving brothers in eye color that do not carry the tumor-gene and the other daughter with normal eyes that carries it. ”

“ The next step is the most significant of all. It consists in the splitting of each chromosome into two equal halves. The threads do not divide in their middle but split lengthwise, so that there are formed two halves identical in every respect. In this way are produced twice the original number of chromosomes, but all in pairs. ”

“ The essence of fertilization is, therefore, the union of two germ nuclei only differing from one another in that they are derived from separate individuals. [43] Since the number of chromosomes appearing in mitosis is solely dependent on the number which originally entered into the composition of the nucleus (Boveri’s Law of Chromosome-Constancy) , it follows that, in the mitotic figures of the developing embryo, the chromosomes will be half maternal, half paternal in origin ”

“ Other evidence has shown that the Y chromosome of the male is indifferent, so far as these Mendelian factors are concerned.
Fig. 59. Cross between red eyed male and white eyed female; reciprocal cross of Fig. 58.
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The reciprocal experiment is illustrated in figure 59. A white eyed female is mated to a red eyed male (top row) . All the mature eggs of such a female contain one white-producing X chromosome represented by the open bar in the diagram. The red eyed male contains female-producing X-bearing sperm that carry the factor for red eye color, and male-producing Y chromosomes. ”

“ In some cases the female-producing spermatozoa contain one more chromosome than the male-producing ones, in others both classes have the same number of chromosomes but one of them is much smaller in the male-producing class. These initial differences in the spermatozoa lead to corresponding differences in the nuclei of the two sexes, the cells of females either containing one more chromosome than those of males or showing a greater quantity of chromatin in the greater size of one of the chromosomes. ”

“ During the stages preparatory to the mitotic, or indirect, division of a cell into two cells (one of the methods of reproduction) the chromatin segregates in typical cases into two groups of loops, and each group has equal portions of the chromatin. When the chromatin is in this shape, a loop is called a chromosome.
The chromosomes are very important. They occur in constant definite numbers in the somatic cells of the various species of many animals and plants, and it is probable that each species of plant and animal has its own characteristic number of chromosomes. ”

“ This is the fundamental cause of the fixity of species, which means the production of offspring having the same specific characteristics as their parents. How, under these conditions, the mutability of species is brought about must be discussed later. It is first of all necessary to inquire more closely into the composition of the chromatin, and to study the special phenomena of cell-growth in connexion with conjugation, where new and extraordinary features come to light.
A chromosome is not, or is not usually, a simple body. ”

“ If we think of all the factors that lie in one chromosome as linked because ordinarily they go together, in the sense that they are likely to remain in that chromosome, this linkage will be disturbed, or broken, at one time only in the history of the chromosomes, viz., at the time of conjugation of the pairs, when an interchange between the members of an homologous pair becomes possible. ”

“ The only observable unit-bodies that fulfil the necessities of such cases are the chromosomes. This tendency of characters to exist in groups which are inherited independently of one another is coming more and more into evidence as we penetrate farther into the intricacies of inheritance, and it is exactly what we would expect on the supposition that each chromosome carries the determiners of a number of characters instead of a single one. ”

“ C, reduction division in which the synaptic pairs separate while the sex chromosome does not divide, consequently the second spermatocytes D and D′ contain respectively 23 X and 23 chromosomes. E and E′, second maturation division in which every chromosome divides, giving rise to two equal classes of spermatids and spermatozoa, one of which has 24 chromosomes and the other 23. If an egg containing 24 chromosomes is fertilized by a sperm with 24, a female with 48 chromosomes is produced; if an egg with 24 chromosomes is fertilized by a sperm with 23, a male results. ”

“ 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 ”

“ What happens is that each of the reproductive cells, male and female, prepares itself for conjugation by getting rid of half its chromosomes. Two divisions of the nucleus take place, not as in the ordinary fashion of cell-division, when the chromosomes split longitudinally, but in such a way that, in each division, four of the sixteen chromosomes (making eight in all) are bodily expelled from the nucleus and from the cell, when they either perish, or, in some cases, appear to help in forming an envelope of nutritive matter round the germ-cell. ”

“ In some animals these cells can be identified early in the cleavage (fig. 48) .
The reproductive cells are at first like all the other cells in the body in that they contain a full complement of chromosomes, half paternal and half maternal in origin (fig. 49) . They divide as do the other cells of the body for a long time (fig. 49, upper row) . At each division each chromosome splits lengthwise and its halves migrate to opposite poles of the spindle (fig. 49 c) .
But there comes a time when a new process appears in the germ cells (fig 49 e-h) . ”

“ Judging from all available evidence, the granules are self-propagating units; that is, they can grow and reproduce themselves. So that what really happens in mitosis in the splitting of the chromosomes is a precise halving of the series of individual granules of which each chromosome is constituted, or in other words each granule has reproduced itself. Thus each of the two daughter cells presumably gets a sample of every kind of chromosomal particle, hence, the two cells are qualitatively alike. ”

“ From numerous investigations which have been made to trace the chromosomes through the various stages of the nuclear ontogeny of plant cells, it appears that the individuality and continuity of the chromosomes can only be conceived as possible if we assume the existence of something like chromosome centres in the resting nucleus around which the chromosomes become organized for purposes of division. ”

“ Form, size, and number are different in different animals, but there is reason to believe that they are normally the same in all the individuals of a species. The fertilised nucleus, thus consisting of chromosomes from male and female, then divides by a complicated process known as karyokinesis, in which each chromosome splits longitudinally, one half passing to each daughter-nucleus. Throughout the whole process of embryonic and post-embryonic growth, the chromatin is gradually increasing in bulk, and being distributed by karyokinesis. ”

“ I think the most striking and significant result in this field is the proof that a particular chromosome or chromatin mass determines a particular character of an adult organism, which is quite a different matter from the reference of all the hereditary characters to all of the chromatin. Professor Wilson has brought forward the convincing data showing that the complex character of sex in insects actually resides in or is determined by particular and definite masses of this wonderful basis of inheritance. ”

“ Individuality of the Chromosomes.—The most striking feature in the behaviour of the chromatin in mitosis is its resolution, at each division, into a—for any particular species—constant number of chromosomes. This constant recurrence of the specific number of chromosomes at every cell-division is capable of explanation in two radically different ways. ”

“ To employ Sutton's graphic illustration: if the number of chromosomes be taken as 8 and designated as A, a, B, b, C, e, D, d (large letters denoting paternal chromosomes and small ones the corresponding maternal) , the chromosome-pairs in the equatorial plate might, so far as the cytological evidence shows, present any or all the groupings ABCD/abcd' abCD/ABcd' aBcD/AbCd' and so on, which gives a possibility of 16 different combinations in the gametes and of 256 in the zygotes or offspring. ”

“ This mechanism resides in the so-called chromosomes of the nucleus. These exist for the most part in similar pairs in both sexes: but one pair is dissimilar in one sex. In mammals and man this sex is the male. Man possesses one chromosome less than woman. He possesses only one member of this pair of special sex-chromosomes, whereas she possesses two. All her ova are alike in possessing one, whereas half his sperms possess one, half possess none. Therefore, when the former kind of sperms fertilize an ovum, two sex-chromosomes [Pg 138] are present in the fertilized egg and a female results ”

“ Other tests may also be made of the other classes of offspring. Bridges has shown that they fulfill all the requirements predicted. Thus a result that seemed in contradiction with the chromosome hypothesis has turned out to give a brilliant confirmation of that theory both genetically and cytologically. ”

“ The chromatin stands towards the other contents of the cell in the same relation that a nerve-element stands to any element of an organism which it excites." "We may infer that cell-evolution was, under one of its aspects, a change from a stage in which the exciting substance and the substance excited were mingled with approximate uniformity, to a stage in which the exciting substance was gathered together into the nucleus and finally into the chromosomes, leaving behind the substance excited, now distinguished as cytoplasm." ”