by
Y.S. DEMIN
Institute of Developmental Biology
Academy of Sciences
Moscow, U.S.S.R.
The fundamental principle of reproduction is that one or two organisms give life to a new one. Now we know how the process goes on, but only in 1875 was it definitely proved that the basis of the fertilization process in higher organisms lies in the fusion of one female and one male cell (gametes). Pairing of the carriers of hereditary information (chromosomes) takes place in each nucleus and the fusion of these cell nuclei takes place at fertilization. The fertilized egg-cell (zygote) gives rise to a new organism. Sometime ago, findings concerning certain peculiarities of chromosomes brought to light their role in inheritance. While studying Echinus eggs, Boveri (1902–1907) proved experimentally that certain disorders in their development were due to irregular distribution of chromosomes - for normal development the whole set of chromosomes characteristic of the species is required.
It has been established now that all the cells of multi-cellular organisms, with the exception of generative ones, contain an identical set of chromosomes (Figs.1a and 1b). As a rule, generative cells (sexual) bear half as many chromosomes as the somatic cells. Somatic cells are reproduced by duplication which is called mitosis or karyokinesis, while generative cells originate by meiotic division or meiosis.
In proliferating tissues and organs (embryonic tissues, blood forming tissues and so on) the cells are found to be in constant division. The interval between two cell divisions or two mitoses is called interphase. The period covering an interphase and a mitosis is called the mitotic cycle. Interphase and mitosis in their turn are sub-divided into a number of consecutive stages, each playing an important role in the process of cell division. In the course of the interphase the hereditary material of the chromosome is doubled. The chromosome that had been a single thread structure (consisting of one chromatid) has become a two-thread structure. Then at the end of the interphase, just before the mitosis, there occurs a change in the physico-chemical structure of the protoplasm and chromosomes. There appears a protein body, a spindle of division, while the chromosomes are contracting as a result of spiralization. The interphase of the cell transfers into the first stage of mitosis-prophase, at which time the chromosomes can be seen by means of the optical microscope.1 During mitosis chromosomes coil into a close spiral and at the metaphase-anaphase doubled chromosomes duplicate and each daughter-cell gets one chromatid (daughter chromosome). Each chromatid is a replica of the parent chromosome and as genes are localized in the chromosomes, the daughter cells acquire not only karyotype of the parent cell but its genotype as well (a set of genes). Mitotic division is the basis of asexual reproduction and is characteristic of certain stages in the life cycle of lower plants and animals. (Fig.2)
The process of mitosis is very complicated, but this is justified by the resultant fact that both the daughter cells are identical to the parent cell. There is no loss of hereditary information whatsoever as well as no disappearance in a series of cell generations. In the course of mitotic cycle the hereditary information is replicated each time and is transported from cell to cell unchanged. 2
2 A more detailed information on the question can be found in the paper on Mutations.
As already mentioned, each organism develops from the fertilized egg (zygote), which is the result of fusion of the parent gametes.
Fig. 1a Normal chromosome set (karyotype) of Crepis capillaris (2n = 6).
Fig. 1b Normal chromosome set of mice (2n = 40)
 |  | |
| Fig.2 Diagram of mitosis | Fig.3 Diagram of meiosis | |
| I. | Early prophase (chromosomes are observed in the nucleus) | |
| II. | Late prophase early metaphase (chromosomes are doubled) | |
| III. | Early anaphase | |
| IV. | Late anaphase | |
| V. | Telophase and the end of mitosis (cytoplasm division) | |
Fig.4 Crossover between chromatids of a pair of homologous chromosomes
In regular cell division (mitosis) daughter cells get the same set of chromosomes as of the parent cell. In the course of meiosis or reduction division, which results in the formation of sex cells, the number of chromosomes is reduced twice.
Higher organisms are mostly diploid, having two homologous sets of chromosomes in each cell (2n). Cytological findings have proved that the cells forming gametes within a diploid organism experience two consecutive divisions (1st and 2nd division) as a result of which the number of chromosomes is reduced twice and gametes having haploid (half) number of chromosomes (1n) are formed. Meiosis in animals producing male sex cells is called spermatogenesis, while the one producing female sex cells is called ovogenesis. Fig.3 gives a diagram of meiosis in an organism whose diploid set of chromosome is 4(2n=4). Meiosis is characteristic of all animals and plants that reproduce sexually.
The first meiotic division is characterized by a long, prophase during which homologous chromosomes come closer and enter a close, elongated conjugation (synapsis stage) forming bivalents or tetraploids.1 At the time of conjugation, homologous chromosomes, each having two chromatids can exchange their sites; in other words, exchange of hereditary material between homologous chromosomes takes place.
This kind of interchange is called crossing-over and the sites where chromosomes interchange are called chiasmata (Fig.4). The meiosis prophase is transformed into metaphase and then follow anaphase and telophase. Contrary to mitosis, however, at the first meiotic division chromatids of the same chromosome remain paired while homologous pairs resulting after the first division have half the number of chromosomes as compared to the mother cell (2n after the first meiotic division), and the second one starts in the course of which the cells formed by the first division duplicate mitotically yielding four cells (gametes), each having a haploid set of chromosomes.
Here is a diagram:
After spermatogenesis all four cells become gametes, while in ovogenesis only one of the four cells becomes an egg cell; the rest perish.
Another important feature of meiosis is that the duplication of chromosomes in daughter cells occurs in accordance with the theory of probability, i.e. by chance. When the chromosomes brought into the zygate by male and female gametes develop into newly formed gametes (during the formation of sex cells in the organism derived from this zygote) they recombine and do not retain their original male and female (parental) characteristics. (Fig.3). Therefore, gametes receive both parents' chromosomes in equal number. Pairing (conjugation) and separation of homologous chromosomes is responsible for the mechanism of segregation of allele genes (see Mendel's “Law of Segregation”). Independent assortment of genes occurs as a result of an occasional orientation of each chromosome to one or another pole during meiosis, providing for purity of gametes (see “Law of Gamete Purity”). Thus daughter cells during meiosis (unlike in mitosis) receive a different set of genes, i.e. they have different genotype
One more point is worth mentioning. Double reduction in the number of chromosomes when gametes are derived is due to the fact that in the process of fertilization there occurs chromosome pairing of fusing gametes. If this pairing is not compensated by double reduction of chromosomes in gamete formation, each subsequent generation would have double the number of chromosomes, that would lead eventually to an unlimited increase of chromosomes within several generations. Double reduction of chromosomes at meiosis ensures a constant number of chromosomes in organisms.
Rapid development of research at the beginning of the 20th century intended to verify the regularities stated by Mendel brought to light a number of facts which seemed to contradict each other as well as put to doubt the universal application of Mendel's laws. Thus on the one hand profound theoretical investigations made by cytologist W.S. Sutton (1903) convincingly proved that Mendel's regularities of character inheritance were absolutely parallel to those of chromosome inheritance in the process of sex cell formation. Sutton-Boveri hypothesis which declared chromosomes to be the material bearers of hereditary disposition was repeatedly proved and ceased to be a hypothesis, but became an established scientific phenomenon, incorporating genetics and cytology to form a new branch of science - cytogenetics.
On the other hand considerable experimental data had been obtained that contradicted the law of segregation and independent assortment of characters. There were many cases proving that the characters brought about by one parent did not always lead to independent assortment at segregation, but revealed a tendency to be handed on together in successive generations demonstrating linkage.
Thus Bateson and Punnett (1906) reported that in crossing of different varieties of sweetpeas dihybrid splitting differed from the normal: genes were not inherited independently from each other as in Mendel's tests with peas, but had a tendency to be inherited in pairs. It was further found that the genes introduced together in crossing remained linked in successive generations as well. It appeared that at the initial crossing of type AABB × aabb in F2 there occurred a surplus of parent combinations of type AB and ab and the corresponding lack of Ab and aB combinations. Experiments showed that gametes AB, Ab, aB and ab were not formed in equal numbers (which should have been the case in independent assortment of genes), but according to the ratio 10AB : 1Ab : 10aB : 10ab. Facts similar to these undermined the acceptance of Mendel's laws as universal. The chromosome theory of inheritance that was completely developed in the second decade of the 20th century by the American school of genetics headed by T.H. Morgan 1 managed to explain the phenomenon of linkage. This theory did not only explain and connect all the facts of the so-calles exceptions from Mendel's laws, but it also appeared to be (as much as Mendel's laws) a sound basis for the whole structure of modern genetics.
Most investigations of that time were carried out by the Morgan school on the fruit-fry, Drosophila melanogaster, which proved to be a most favourable genetic material. It has only a few chromosomes (4 pairs 2n=8), well distinguishable microscopically. It is highly fertile under laboratory conditions. By the time the chromo some theory of inheritance was formulated, there had been about 400 Mendelian factors or genes described.
Research workers dealing with Drosophila were also able to observe the tendency of a number of genes to be inherited as linked. When flies having a common (wild) colour of the body and long wings are crossed, the first generation will be of the wild type. If one of the male offsprings F1 crosses with a black vestigial-winged female, the generation F2 may be of only two types: half of them black having vestigial wings, and the other half of the wild type. It means that two mutant 1 characters inherited together (here black and vestigial) will be inherited in the same pattern further on, and the two alleles of the wild type (wild colour of the body and normal wings) when inherited appear to be combined. Black flies with long wings as well as grey ones with vestigial wings do not appear in the second generation as they should have if an independent assortment of genes had occurred in gamete formation.
To explain the phenomenon of coupling, Morgan put forward-a suggestion that only those genes that are in the same chromosome are inherited together. Further experiments helped to establish the fact that there are groups of genes (linkage groups) tending to be inherited together and that the number of these groups of genes never exceeds the number of pairs of homologous chromosomes in a given organism, i.e. it is equal to the haploid set of chromosomes. Thus, the idea of linkage groups is the first point of the chromosome theory of inheritance.
In Drosophila (having 2n=8) 4 linkage groups were defined: 4 groups of genes with a tendency to be inherited together. Fig.5 shows the genes, body colouring and wing shape, in relation to the chromosome theory of inheritance. The genes of black body (b) and vestigial wings (v) are shown to be in the same chromosome. Homologous chromosome carries normal (wild type) alleles of these genes (BV). In F1 there is one chromosome of each type, and the fly is normal because both normal alleles are dominant. In embryonic cells these two chromosomes break up at the first meiotic division in the male F1, each getting into one of the gametes. Crossing F1 male and a black female having vestigial wings, all egg cells contain genes of black body and vestigial wings, and the progeny has the characters of the male F1 gametes since the female gametes carry recessive genes. The idea of linkage groups leads to the important conclusion that independent assortment is typical only for genes located in different chromosomes.
The next point of the chromosome theory of inheritance concerns the linear order of genes in the chromosomes. Figuratively speaking, a chromosome is like a thread of beads where every bead is a gene. A thorough study of the degree of different gene linkage within the same chromosome made it possible to determine the position of genes within one chromosome, as well as their sequence in relation to each other.
Experiments showed that the degree of linkage between two given genes was always the same - the frequency of recombinations between them appeared to be always the same, but for the different pairs of genes the frequencies were different (Fig.5 is a case of complete linkage when the adjoining genes do not recombine at all). To explain this phenomenon Morgan made a supposition that genes found in one chromosome close to each other are tightly linked. Linkage of genes placed at some distance from each other is less manifest. In other words, he believed that the degree of linkage is inversely proportional to the distance between the genes.
Crossing-over is another hypothesis put forward by Morgan to explain the linkage failure. According to this hypothesis there takes place an exchange of homologous sites between the chromosomes of homologous pairs in bivalents during the first meiotic division (Fig.6). The break up in gene linkage occurs due to the break up of chromosomes at the point between the given loci (a locus is the site of gene location in the chromosome which is always the same for the given gene). It is also due to the recombination of chromosome sites occurring there. In the course of the process the site of one chromosome contacts with the site of the other. Two new chromosomes are derived, each having only one of the two genes previously found in the same chromosome. A number of crossing-over at different sites may occur between one pair of homologs; in case of crossing-over, however, the possibility of the second crossing-over close to this site is limited (such phenomenon is called interference). Fig.7 gives a diagram of a double crossing-over occurring at the stage of four chromatids. Thus the third point of the chromosome theory of inheritance is crossing-over, the mechanism leading to the exchange of genes or homologous sites between chromatids of homologous chromosomes at meiosis.
Fig.5 Backcrossing of F1 male (grey-vestigial × black-normal) with black-vestigial females
Fig.6 Crossing over and segment exchange between the non-sibs chromatids in the prophase of the first meiotic division and recombination
It should be mentioned again that independent assortment of genes and splitting in hybrid progeny according to Mendel is secured by chromosome behaviour at meiosis, independent assortment being possible only for those genes that are located in nonhomologous chromosomes. The mechanism of meiotic assortment (reassortment) is related to the behaviour of homologous chromosomes during meiosis.
Only 20 years after the chromosome theory of inheritance had been formulated did it become possible to see the process of crossing-over under the microscope and to prove cytologically what had been established by the Morgan genetic school. The observations were made by C. Stern (1929) on Drosophila and by H.B.Creighton and B.McClintock on maize (Zea mays).
Dealing with maize, Crayton and McClintock isolated a strain containing one pair of chromosomes with morphologic and genetic markers that help to locate it in genetic experiments and to be seen through the microscope so that they could follow the behaviour of the genes contained in this pair of chromosomes. One of the chromosomes of this pair was normal and the other had a thickening at the end of one arm 1 besides being markedly elongated as compared to that of a normal chromosome-homolog.
In the experiment a normal chromosome contained a recessive gene c (unstained
endosperm) and a dominant gene WX+ (starchy endosperm). The changed chromosome
contained a dominant gene C+ (stained endosperm) and a recessive gene WX+ (waxy
endosperm). Heterozygote 
was crossed with the strain having morphologically
normal chromosomes with recessive genes c and wx:
Crossover and non-crossover seeds were found in progeny. Cytological tests showed that crossover seeds (grains) always contained the given chromosome with exchanged sites - the chromosome of normal length but with a thickening, or the chromosome without it but elongated. Fig.8 shows the diagram of this test.
Thus the studies carried out by C.Stern (1929) and McClintock both cytologically and genetically showed that assortment of linked genes is accompanied by a site exchange of homologous chromosomes. Crossing-over has ceased to be a hypothesis and has become a practically confirmed phenomenon. The period of a site exchange between chromatids at crossing-over is now associated with the time of chiasma appearance at meiosis. It is now considered that chiasma occurs where exchange of sites takes place. The mechanism of crossing-over, however, is still obscure.
Fig.7 Diagram illustrating crossing-over effect between two anterioposteriorly segregated chromosomes.
Fig.8 Cytological proof of the crossing-over
in Zea mays
C+ - stained endosperm
c - unstained
wx+ - starchy
wx - waxy
The study of crossing-over brought about the idea of a linear order of genes in a chromosome, which in its turn made it possible to have genetic maps of chromosomes showing interposition and distance between the genes of a given linkage group.
It has already been mentioned that according to Morgan's supposition the distance between the genes in a chromosome is proportional to the number of crossings between them. This served as a basis for the determination of gene position inside the chromosome.
If in two genes A and B crossing-over is rare, say, in 50 percent of the cases, one can suppose that they are located rather close to each other in the chromosome which results in a small probability of a chromosome break up at the site and consequently of the site exchange. Thus if the number of gene exchanges is proportional to the distance between them, the measurement of the crossing-over frequency between A and B as well as between B and C (Fig.9) is to indicate the distance between the genes A and C, because the distance between A and B plus the distance between B and C is to be equal to the distance AC (the distance unit between the genes was considered to be the chromosome site where 1 percent of crossing-over occurs). 1 Most cases prove occurrence of such dependence. Thus in crossings of red-eyed, yellow-bodied Drosophila with white-eyed grey-bodied forms, the crossing-over between the yellow-body gene (yellow) and the gene of white eyes (white) can be observed in 1.5 percent of the cases. The crossings between white-eyed flies with normal wings and red-eyed flies with bijid wings (bijid) cause the crossing-over between the gene of white eyes and bijid wings in 5.5 percent of the cases. If the gene of white eyes lies between the genes of yellow body and bijid wings, one should expect that the distance between W and Y (1.5) plus the distance between Y and b (5.5) should be equal to the distance between y and b (7.0).
1 In Russian genetic literature this unit is called morganida after Morgan T.H.
It should also be noted that since the genes occupy a definite position inside the chromosome the percentage of their crossing-over is always the same. An analyzing crossing, i.e. crossing with a recessive homozygote, is nearly always used to determine the frequency of crossing-over. As we know, an analyzing crossing helps to detect all types of gametes that are found in the analyzed hybrid species.
Here is another example: It is well known that gene Rr in a hen (R-short-legged, r-ordinary leg length) and gene Wn (rose-like comb; wn-leaf-like comb) are placed at the distance of 9 crossing-over units. In what way is it possible to determine according to these genes the number and kind of the gametes produced by a double heterozygote (diheterozygote) rWn and RWn? If the distance between the genes is equal to 9 crossing-over units, it means that in the gametes produced there were 9 percent of crossovers and 91 percent of non-crossovers. Since the amount of non-crossover classes rwn and Rwn is the same, then the amount within each class will be
For the same reason the amount of crossover classes will be:
Hence the distance between the genes is defined by the formula:
where 1 = distance in crossing-over units; N = total number of offsprings; n = number of offsprings developed from crossover gametes.
While considering the method of determining the distance between the genes in the chromosome, one cannot help dwelling on such a phenomenon as double crossing-over. (Fig.9). In Fig.9 (4) one can see that double crossing-over results in missing double crossover classes in the counts; thus according to the above-mentioned formula, the distance between the genes set far apart appears to be shorter than it really is. Therefore, to determine the distance between the genes set far apart, one should summarize the distances between intermediate genes, as it is discussed above.
It is easy to realize that the distance between the genes determined by means of crossover percentage will never exceed 50 units, in which case it will be the class frequency in independent assortment. (IAB:IaB:Iab).
On genetic maps, however, one can often come across distances between genes of 100 units or more. These distances are calculated by summing the distances between intermediate genes. The following diagram shows A to F = 107 units.
When making a genetic map, a linkage group is the first to be determined; i.e. the largest possible number of genes found in the given pair of homologous chromosomes. Then by means of crossing-over, a relative interposition of genes in the linkage group is determined. In genetic mapping a definite system of designation of genes accepted for each organism is used. In such maps the linkage group is sure to be shown as well as the distance from one of the chromosome ends taken as a zero point, the former being expressed in morganida, as well as the site of the centromere. Fig.10 shows a genetic map of Drosophila.
By means of a special method T. Dobzhansky succeeded in determining cytologically the position of genes in gigantic chromosomes of the salivary glands of Drosophila. Full coincidence of interposition of genes determined by the two methods was found later on in comparing the sequence of genes on both cytological and genetic maps.
The most common type of sex determination is syngamic, when sex is determined at the moment of gamete fusion in fertilization (mammals, birds, fish and so on).
Sex as well as any other character of the organism is hereditarily determined. The most important role in genetic determination of sex and the maintenance of equal correlation of sexes is played by the chromosome mechanism. In other words, sex is determined in the process of fertilization by the chromosome set of the fusing gametes. Most animals give birth to an equal number of male and female individuals which means that sex distribution is close to 1 : 1, which can be observed when analyzing crossing. In progeny resulting from this kind of crossing the genes are split in the ratio lAa : laa. Genes A and a should be located in one pair of chromosomes. If sex is inherited following the same principle as other characters, it may be supposed that one sex is homozygote while the other is heterozygote. Then the sex distribution in the progeny will be 1 : 1.
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Fig.9 Diagram of the mechanism and crossing-over effect in a pair of homologous chromosomes 1. Lack of crossing-over 2. Crossing-over between A and B 3. Crossing-over between B and C 4. Double crossing-over between A and B, B and C | |
| Fig.10 Genetic mapping of Drosophila chromosomes |
The above supposition was first made by Mendel. Later on genetic investigations of C. Correns and L. Doncaster, as well as cytologic studies made early in the 20th century, proved the existence of the chromosome sex-determining mechanism for most animals and a number of plants. Females of many animals were found to have all chromosomes paired and to produce only one kind of gametes at gametogenesis. Males produced two different kinds of gametes, one similar to those found in the female and another differing in the structure of one of the chromosomes. A homologous pair of chromosomes (determined as such by their behaviour at meiosis) but dissimilar in size and shape, appeared to be connected with sex determination. Such chromosomes have been called sex chromosomes. It has been shown that the fusion of an egg-cell carrying a sex chromosome (X-chromosome) with a sperm carrying the same chromosome will give rise to a XX zygote and to the development of a female individual. The fusion of an egg-cell with the sperm carrying another sex chromosome (Y-chromosome) will give rise to an XY zygote and to the development of a male. This can be seen on a Pennet grate as follows:
| male gametes | |||||
| X | Y | ||||
| female gametes | X | XX | XY | sex ratio: | |
| X | XX | XY | 1XX (♀) | : 1 XY (♂) | |
Further investigations revealed different ways of chromosome sex determination. It appeared that homogamy (homogametic sex is the one producing only one kind of gametes, heterogametic sex producing two kinds) may be the attribute of not only the female sex, but also of the male - some fish species, carp for example probably have homogametic sex determination.
Another way of sex determination is by the mechanism XO, when the tendency for the development of a sex depends on whether the zygote has got one or two X-chromosomes. Some ways of sex determination are shown in Figs.11 and 12 as well as in Table I.
There are also a number of modifications of the modes of sex determination described above, depending on the balance of sex chromosomes and autosomes in the zygote. All these methods of chromosomal sex determination have one thing in common, i.e. the sex ratio in the progeny is conditioned by a random combination of homogametic and heterogametic sex gametes that results in the primary zygote yield of male and female types in the ratio 1 : 1.
The genes located in sex chromosomes are called sex-linked and the inheritance of such genes (and hence their related characters) is called sex-linked inheritance. Which sex brings about dominant and recessive characters, has no relation to the splitting according to the given characters in hybrid offsprings. This is true when genes are located in autosomes equally reflected in both sexes. When genes are located in sex chromosomes the nature of inheritance and splitting depends on the chromosome behaviour at meiosis and their combination in fertilization. Genetic investigations of a number of subjects (Drosophila, man) showed that Y chromosome of heterogametic sex does not carry any gene; i.e. it is inert in respect to inheritance. There follows an important practical conclusion: recessive genes of X chromosome are developed in heterogametic sex, since no dominant alleles in X chromosome are opposed to them.
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Fig.11 Scheme of different types of chromosome determination of sex | Fig.12 Diagram of sex determination with male heterogamy (xx-xy) Drosophila | |
| I. | Diploid tissue | |
| II. | Spermatozoa | |
| II'. | Egg-cell | |
| III. | Offsprings | |
Fig.13 The inheritance of sex-linked characters (plumage colouring) in hens. B-factor of striped colouring; b-factor of black colouring
As sex-linked characters are of great practical importance, it is worth discussing the phenomenon of plumage colouring in hens. It should be remembered that homogametic sex in hens is the male.
If striped hens XY are crossed with cocks XX of total black colour (australorp breed) having a recessive allele of a stripe-controlling gene characteristic of a homozygote condition (Fig.13), the former leading to an even distribution of the colour, then the sex of the hatched chicks can be distinguished after a few days. All the hens having a gene of even colour not protected by an inert Y chromosome will be black. Reciprocal crossing of a black hen (having a recessive gene of even colour) with a cock, which is homozygous in respect of the gene with striped characteristics (Fig.13), will yield only striped offsprings in F1.
It may be observed that genes located in sex chromosomes are inherited criss-cross. Sex chromosomes of homogametic organisms are passed on to both sons and daughters, while only the X-chromosome of heterogametic sex is handed on, either to daughter (in the case of a male heterogamy) or to sons (in female heterogamy). If there is a definite tendency of crossing and the characters depending on X chromosome are handed on from mother to sons and from father to daughters, then the inheritance is called crisscross. In sex-linked characters the appearance of hybrids F1 depends on which character has been contributed by the father and which one by the mother. In some lines of crossing, we come across apparent contradictions to Mendel's first law of dominance or uniformity of progeny F1 (Fig.13). In fact, elucidation of the reasons for such exceptions of Mendelian rules brought about a number of few facts confirming and stressing the authenticity of the laws stated by Mendel.
An important conclusion based on the study of the sex-linked characters is that these characters may serve as markers which can help to distinguish animal sex as early as in F1. In silkworm breeding, in particular, it is preferable to make use of caterpillars of male sex, since their cocoon yield of silk is 20–30 percent higher. Fig.14 gives a diagram of the inheritance of the sex-linked character of green colour. At present there is colour diffentiation of green according to intensity, which makes it possible to get a desirable sex ratio in offsprings.
Sex-limited or sex-depending characters are those expressed exclusively or mainly in one sex. Secondary sex characters in men and animals, as well as a number of other characters, may be referred to as such. Manifestation of these characters is apparently connected with the effect of male and female hormones.
The effect of certain genes as well as hormones in the process of the organism's development may result in sex reversal, i.e. the development of the other sex not predicted by genetic constitution. Sex reversal is known to exist in plants, Drosophila and man. Hormonal sex reversal, however, is of great practical value for cattle breeding as a means of artificial regulation of sex ratio. The following experiment made by T. Yamamoto in the 1950's is of interest to fish breeders.
Yamamoto experimented on aquarium fish, Oryzias latipes, whose heterogametic sex is male. White as well as red Oryzias occurred in the experiment, the red gene being carried by y chromosome, while its recessive allele r- is carried by the X chromosome. The males were red (xryR); the females were black (XrXr). In this case males were always red, since they carried the dominant allele R. The sons of this type of inheritance will always have the father's character (if no crossing-over occurs between x and y, which is very rare). Crossing of xrxr and xryR always resulted in white females and red males (Fig.15). Hatched young fish, even before sex differentiation, were divided into 2 groups. One group had a routine food ration; the other received female sex hormones (estrone and stilbestrol) added to the food ration. As a result, all red fish in the second group, determined genotypically as males xryR (red), turned out to be females according to phenotype having normal ovaries and female secondary characters. They were able to cross with normal red males. The crossing of such females with normal males (XrXR × XryR) resulted in the sex ratio of 1 (XrXr): 30 (2xryR and 1yRyR), not 1 : 1.
 |  |
Fig.14 Scheme for inheritance of sex-linked character of silkworm (Bombyx) egg colouring. A-factor of light colouring of silkworm eggs, a-factor of dark colouring. | |
Fig.15 Phenotypical sex determination in fish | |
 | |
Fig.16 Structure model of DNA according to Watson and Crick. Combination of two polynucleotide chains by hydrogen bonds, forming the so-called steps of a rope ladder. |
Yamamoto's investigation clearly demonstrates the possibility of sex differentiation in ontogenesis. Similar results were obtained with hens.
Thus, the study of sex-determination mechanisms and the behaviour of sex-linked characters enables us to conclude that:
sex of the organism is inherited as any other character determined by a gene;
the sex ratio 1 : 1 results from the formation of two kinds of gametes having equal frequency in heterogametic sex at meiosis;
heterogametic sex may be both male and female;
inheritance of the sex-linked characters is predetermined by the genes located in sex chromosomes.
A number of significant generalizations on heredity and inheritance were established as far back as the twenties of this century. The chemical nature of hereditary structures and the relation of chemical structure to the transport of hereditary information, however, remained unknown. It was evident that hereditary material must carry maximum hereditary information in the minimal size, and there must exist a biochemical mechanism of transport of this information in cell generations. Among the models of this information transport existing then, A.R.Koltsov's model of a chromosome molecule suits the modern conception best. According to Koltsov, the carrier of information is a protein structure, having the capacity of self reproduction and replication. Although protein appears not to be the carrier of information, as will be shown below, the main prediction of Koltsov, the principle of self-replication, has been completely confirmed. At present it may be considered for certain that the carrier of genetic information in higher organisms is deoxyribonucleic acid (DNA) whose molecules are capable of self-replication or reduplication.
Basic information concerning chemical structure of hereditary material (chromosomes and genes) and of gene activity were obtained during the 50-ties after J.D.Watson and F.H.C. Crick (1953) put forward the model of DNA reduplication. There has since been a rapid development of molecular genetics, the science concerning biochemical mechanisms of storing, transportation and incorporation of genetic information.
There are two types of nucleic acids in the cell, DNA and ribonucleic acid (RNA). The transfer of hereditary information proceeds according to the scheme: DNA-RNA-protein. Finally it is the protein that is responsible for the development of a character.
As it can be seen from the scheme DNA does not take part in the protein synthesis and is mediated by different types of RNA.
DNA is a gigantic two chained molecule resembling a twisted rope ladder. The sides of the ladder are composed of alternative units of sugar (deoxyribose) and a phosphate group (Figs. 16 and 17). The steps of the ladder represent a pair of organic nitrous bases: adenine, always chained with thymine or guanine coupled with cytosine. The sequence of the bases in DNA is the code for protein synthesis, four letters (A, T, G and C) being the letters of the code. Twenty well-known amino acids are the building blocks of the proteins. The site of each amino acid in the protein chain is accounted for by specific sequence made up of three bases in the DNA molecule, which is inherited by an organism from its ancestors. The protein is synthesized outside the nucleus, in the cell particles called ribosomes. Genetic information is transported from DNA to ribosomes by information or matrix RNA (M-RNA). The DNA chain serves as a matrix for the formation of complementary molecules m-RNA 1 (Fig.18). Protein is synthesized on the matrix of m-RNA and the sequence of bases in DNA is responsible for that of m-RNA, while the sequence of the three bases (codone) in the matrix of m-RNA is responsible for the sequence of amino acids in protein. One or a few codones correspond to each amino acid (Table II). After m-RNA breaks off the gene (the site of DNA molecule) and joins the ribosome, another type of RNA, transport (t-RNA) brings to it several amino acids and places them along m-RNA molecule according to the code which is transcribed in it, i.e., according to the sequence of nucleotide triplets. Different amino acids are lined up in the way indicated by the molecule m-RNA. After peptide bonds have been established a polypeptide is formed. This is a general outline of the gene activity, the scheme of the transcription process. In other words, it is the transfer of the sequence of the gene nucleotides to that of m-RNA nucleotides, the latter being the starting product of the gene. It is also the scheme of further transfer of this information into the sequence of amino acids of the protein, i.e. translation (Fig.19).
At present the idea of a gene is correlated with the site of a DNA molecule which is responsible for the synthesis of any peptide. 2 There exist about 103 genes for a bacterial cell, while for a human cell the number is 106.
Finally, according to modern concepts, hereditary information is coded by the sequence of nucleotides in the DNA molecule. The code for protein synthesis is enclosed in one gene, being a part of the DNA molecule. Thus a gene represents a specific sequence of bases in DNA. Another important feature, namely reduplication of DNA, or its capacity to self-replication, results from a two-chained nature of the molecule and the specific pairing ability of the bases (Fig.20). At the point of reduplication, the bonds between the bases in the starting molecule are broken and each chain provokes the formation of the second chain with a complementary set of bases. Therefore, along the whole DNA molecule there occurs a sort of attachment of complementary nucleotides resulting in the duplication of the starting molecule, yielding two completely identical daughter molecules. Such a replicating mechanism provides for the proper reduplication (an exact replication of hereditary information) of genes in the process of cell division.
A chromosome contains DNA. Chromatid segregation in the course of cell division is responsible for an even distribution of hereditary information (DNA) between daughter cells.
Genes are located in chromosomes in a linear order. The genes located in one pair of homologous chromosomes represent one linkage group, and the number of linkage groups in the organism is equal to the haploid set of chromosomes. The adjoining genes of the same linkage group do not follow Mendel's law of independent assortment but tend to be handed on in groups, recombining at a definite frequency only due to the process of crossing-over. The frequency of recombinations between linked genes is an indication for determining the distance between them.
Fig.17 DNA molecule resembles a rope ladder coiled into a spiral, where pairs of the bases connect two parallel chains consisting of deoxyribose and phosphate units. In all pairs, adenine (A) is connected with thymine (T); and guanine (G) with cytosine (C).
Fig.18 DNA molecule
Fig.19 Genetic information coded in the DNA molecule (1) is the basis of protein biosynthesis. It is reproduced in RNA, the messenger (2). RNA as a messenger passes on to ribosomes (3). Aminoacids, designated and numbered as rectangles, are transferred to corresponding sites on RNA-messenger by the molecules of RNA-carrier (4). When connected, certain aminoacids form a protein molecule.
Fig.20 Double DNA molecule; the formation of a new thread follows molecule segregation.
We could also see that the sex determination mechanism depends on the balance of certain sex chromosomes in the zygote; sex balance 1 : 1 results from a random combination of gametes of heterogametic and homogametic sexes in accordance with the type of gene combination at analysing crossing.
Finally, according to modern concept, hereditary information is coded by the sequence of nucleotides in the DNA molecule. The code for protein synthesis is enclosed in one gene, being a part of the DNA molecule.
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