by
Y.S. Demin
Institute of Developmental Biology
Academy of Science
Moscow, U.S.S.R.
Hybridological analysis is one of the main methods applied in modern genetics. The honour of having created this branch of study belongs to Gregor Mendel, who published the results of his investigations in the field of hybrids of different varieties of peas (Pisum sativum) in 1865 (Mendel, 1866). Despite its brevity, this work played an outstanding part in the development of genetics; however, it outpaced the level of development of contemporary biology and remained scarcely noticed by the leading scientists.
Though his contemporaries did not appreciate the importance of the regularities established by Mendel, 35 years later, in 1900, the dialectics of these factors unquestionably brought the science to the stage of re-establishment of the regularities discovered by him.
When planning his experiments, Mendel was especially careful in the selection of the parental pairs for the hybrids. Plants for the experiment had to meet the following requirements:
Invariably differing characters;
Their hybrids should be immune to the influence of foreign pollen, or capable of being protected; and
Hybrids and their descendants should not suffer from marked violation of fertility.
Unlike earlier investigators who studied simultaneously many hereditary characters, Mendel limited his studies to the behaviour of only one character at a time. This simplified considerably the problem of analyzing the behaviour of several characters in generations of hybrids. Thus, when tracing the behaviour of a character such as colour in flowers, he, for the time being, did not take into account the shape of seed, height of the plant, etc. The method Mendel used in his work consisted of crossing two plants differing in one pair of contrasting characters. Later, such crossing, in which parental forms differ only in one pair of contrasting or alternative characters, became known as monohybrid. Then the seeds so obtained were planted, and the appearance of the first hybrid generation F11 was analyzed. Then Mendel crossed two hybrid plants (or permitted them to self-pollinate) and raised as many as he could of the second generation of descendants - F2. These descendants showed higher or lower variability in respect to the character under study, and Mendel classified them accordingly, counting the number of plants which possessed each of the two contrasting characters.
Later, Mendel crossed not only the plants differing in one, but in two or more characters (dihybrid and polyhybrid crossing). He traced the distribution of these characters in the first and second hybrid generation.
Based on the practical information obtained, Mendel made certain generalizations, which were later confirmed by numerous experiments with other plants and animals. Today these generalizations have been fully confirmed and are known in genetics as Mendel's Laws of Heredity.
(F1)
One of the first factors noticed by Mendel (although the phenomenon had been known even earlier) was that out of two alternative characters of the first generation hybrids (F1) only one usually developed. The second feature seemed to disappear or be suppressed. Thus, studying smooth and rugose forms of beans, Mendel found out that all beans of F1 plant were smooth.
Mendel called the characters, such as revealed in beans with a smooth form, dominant; and those with rugose form, recessive. For all seven characters studied by Mendel, the domination of one over another was complete for each pair of characters, and this fact facilitated considerably his further work.
In subsequent investigations carried out by other scientists, many characteristics were found which showed complete and incomplete dominance. However, the importance of the phenomenon discovered by Mendel consisted of the fact that the appearance of a specimen could no longer be considered as an exact indicator of its hereditary constitution, because domination resulted in a hybrid or cross having the appearance of a pure-bred specimen. This is of importance in the application of the principle of dominance in cultivation of plants and in stock breeding. Later this principle of domination found its expression in such terms as phenotype (appearance) and genotype (hereditary constitution).
Mendel continued his experiments further, beyond F1, and produced the second generation, F2, by means of crossing F1 plants (or by self-pollination). By subjecting all experiments to thorough qualitative analysis of the developing features, Mendel established that while in F1 only one of the two contrasting characters was observed, in F2 the characters appeared together in definite numerical ratio of 3 dominant to 1 recessive.
Thus, 253 smooth beans of plant F1 gave 7 324 beans in F2, out of which 5 474 were smooth and 1 850 rugose, i.e., a ratio very close to 3:1 (2.96:1).
In another series of experiments, where seed colouration was taken into account -yellow (dominant) and green (recessive) - out of 8 023 beans produced 6 022 appeared to be yellow and 2 001 green, a ratio again close to 3:1 (3.01:1). Further, out of 1 181 plants, 882 were ordinary convex beans and 299 were beans with cervix (a ratio 2.95:1).
Mendel analyzed the behaviour of characters in the F3 hybrid generation and discovered that if a recessive character remained constant, then seeds having dominant characters could be separated into two groups. One remained constant and imparted to the descendants only this feature, and another group behaved like F2 hybrids, i.e., split into dominating and recessive forms in the ratio 3:1. This was a very important conclusion concerning heterogeneity of the F2 dominating group. Mendel also discovered that the ratio of constant segregating seed was 1:2. Thus, Mendel arrived at the conclusion that the 3:1 ratio pertaining to the distribution of dominant and recessive characters divides, therefore, for all experiments to a ratio of 2:1:1 ..... hybrids with two-character seed form differing plants, half of which remain constant and produce equal dominant and recessive characters.
The behaviour of each pair of differing characters in hybrid combination does not depend on other differences in both original plants (Mendel, 1866). This conclusion means that the distribution of hereditary factors, introduced by male and female organisms during the formation of gametes, occurs sometimes in accordance with the law of probability; i.e., the distribution by the gametes of one allelic pair is not connected (does not correlate) with the distribution of the other.
Mendel suggested an explanation for the phenomenon of segregation, and basically this explanation was correct, so that further investigations added only details, and the basic explanation itself remained unchanged. First of all, Mendel suggested that the elementary cells possessed properties or factors, the combination of which gave rise to one or another character. It must be stipulated that Mendel himself did not draw a clear line between a hereditary factor (according to modern terminology - gene) and a character. However, the logic of his experiment did distinguish between these two notions; the conclusion Mendel reached, on the basis of his own data, showed a clear difference between these two. Mendel reached the conclusion that hybrids form elementary pollen cells, which by their properties and to an equal extent, correspond to all the constant forms developed from the combinations of characteristics resulting from pollination (Mendel, 1866).
In other words, Mendel reached the conclusion that during the formation of gametes by a hybrid specimen, bearing, for example, factors of smooth and rugate seed, these gametes are not similar, and they give rise to two different types, half bearing only the smooth seed factor, and the other bearing the rugate seed factor.
This brings about two very important stipulations. First, each gamete is pure (not hybrid), i.e., contains one member of each pair of factors. 1 Second, during gamete formation, the hereditary substance decreases to half; thus each gamete contains only half of the factors which are present in the cells of the parent plant. Consequently, the main peculiarity of the mechanism of splitting relates to the two factors, one of which is brought by the gametes of one parent, and the other by the gametes of the second parent; these combine, and during the whole life time of a generation, exist together in the cells of a hybrid specimen, without crossing and losing their identity. When in such a hybrid its own sex cells are formed, these two factors again segregate or split, and each gamete contains only one of these factors and is pure (that is free from another alternative factor).
1 This relates only to diploid organisms
Genotype is a combination of all hereditary inclinations (Mendel's factors) or genes belonging to an organism. Double alternative factors (genes), similar to those studied by Mendel, are called alleles, and the very phenomenon of existence of such genes is allelism. At present, for a number of species, the genetics of which has been thoroughly investigated (e.g. corn, Drosophila, Neurospora, etc.), it is established that a feature can be determined not only by one pair of alternative genes, but by several genes; however, a diploid organism 2 invariably has only one pair of this group of genes. The phenomenon, when more than two allelic genes are responsible for the development of one feature, is called multiple allelism. By the terms “dominating” and “recessive” alleles we mean the alternative condition of the same gene.
Mendel was the first to utilize symbols to denote genotype, marking genes with letters of the Latin alphabet. By introducing letters - symbols - where the dominating allele is marked with a capital letter and the recessive with a small letter, it is possible to denote the heredity of genes in crossing (Figs. 1 and 2). This symbolic depiction of genes, which is used to indicate splitting in the offspring of hybrids and the interrelation of alleles, has now been generally adopted. Constant forms AA and aa, which in further generations do not give splitting but produce only organisms similar to themselves were called homozygous; and forms Aa, which give splitting, heterozygous. Thus, a homozygous organism is one which bears only one type of allele determining a given character; a heterozygous organism is one which bears two (for diploid) types of alleles determining a given feature.
2 A diploid organism has a double set of chromosomes (or allelic genes)
Fig.1. Diagram showing heredity of characters (in peas) in crossing.
The characters of a smooth form of seed (dominant) is conventionally marked A, the characters of the rugate form (recessive) is labelled a; genotypes and phenotypes of the parents of F1 are also given, as well as gametes formed by them. As a result of random combination of gametes, three genotypically and phenotypically different classes of F2 are formed.
The combination of male and female gametes in organism F1 occurs in accordance with the law of probability, namely, that the development of one or another combination (genotype) happens by chance. One of the methods of determining the combination of genotype as a result of accidental combination of gametes, consists in the drawing of a Penneth lattice (named after a geneticist who was the first to use this representation). The gametes, formed by both parents, are arranged along the upper (horizontal) and left (vertical) sides of a rectangle which is then divided into as many vertical columns as the number of gametes from one parent (upper), and into as many horizontal columns as the number of gametes from the other parent (left) (Fig.2).
Fig.2 Diagram showing the inheritance of yellow (Y) and green (y) colour in bean seed. From the combination given, it is possible to determine for generation F2 genotypes (2Yy : 1YY : 1yy) and phenotypes (3 yellow : 1 green).
Into the squares so formed, the structure of the corresponding gametes of both parents is inscribed, all the squares of the vertical column receiving the same gamete from one (upper) parent and the squares of a horizontal line receiving similar gamete from the other parent (left). Thus each square receives one pair of randomly combined gametes from both parents (in our case four combinations); they represent the genotypes of the specimens developed; on this basis it is not difficult to determine additionally the corresponding phenotypes.
Dihybrid crossing. In the foregoing the laws of heredity and segregation in the case of monohybrid crossing, namely, for one pair of allelic genes have been discussed. Crossing of parental organisms differing in more than one pair of contrasting characters is known as dihybrid crossing.
Mendel took homozygous bean plants which differed simultaneously in two pairs of characters. One plant gave smooth, yellow seed. The gene of smoothness was marked A, and the gene of yellow colouration Y; hence the genotype of the plant can be represented as AAYY. The other plant (as Mendel found in generation F1) had recessive features: rugate form of seeds (a) and green colouration of the seeds (y). The genotype of this plant can be represented as aayy.
Mendel observed that the resulting hybrid seed of generation F1 were smooth in form and yellow in colour. In the process of crossing such F1 hybrids he found F2 seeds with not only primary combinations - smooth yellow and rugate green, but seeds with new combinations of characteristics, i.e., smooth-green and rugate-yellow. These four types of seeds occurred neither in equal numbers nor in any specific numerical ratio. Altogether, Mendel obtained 556 seeds of the second generation, of which 315 were smooth-yellow, 108 smooth-green, 101 rugate-yellow and 32 rugate-green. The results thus obtained can be explained in the following way: it is known already that parental gametes should have AY and ay structure; in the process of crossing, they give an F1 hybrid with genetic structure AaYy. As a result of the phenomenon of dominance, such hybrids will have smooth yellow seeds, conforming completely to one of the parental pairs. The hybrids form gametes, one-half of which will contain gene A, another a. The same process will take place with genes Y and y. As each gamete must contain one gene of each allelic pair, four combinations of these genes are possible: gene A can join gene Y or y; and gene a has an equal probability to combine with Y and y genes. In this way four types of gametes (AY, Ay, aY, ay) develop. These types refer to the pollen as well as to the egg cells. The combination of gametes of different types in zygote formation occurs randomly; this random combination can be represented by a formula: (AY + Ay + aY +ay) × (AY + Ay + aY + ay). By multiplication we receive a genetic structure of organisms, which will develop as a result of combination of different gametes:
AAYY + AAYy + AaYY + AaYy + AAYy + AAyy + AaYy + Aayy + AaYY + AaYy + aaYY + aaYy + AaYy + Aayy + aaYy + aayy.
This is more clear on the Penneth lattice:
| AY | Ay | aY | ay | |
| AAYY | AAYy | AaYY | AaYy | |
| AY | smooth yellow | smooth yellow | smooth yellow | smooth yellow |
| AAYy | AAyy | AaYy | Aayy | |
| Ay | smooth yellow | smooth green | smooth yellow | smooth green |
| AaYY | AaYy | aaYY | aaYy | |
| aY | smooth yellow | smooth yellow | rugate yellow | rugate yellow |
| AaYy | Aayy | aaYy | aayy | |
| ay | smooth yellow | smooth green | rugate yellow | rugate green |
Thus in the second generation we have four different categories of seed. These categories occur with a frequency expressed by a ratio, which can easily be calculated from the Penneth lattice: 9 smooth-yellow : 3 yellow-rugate : 3 smooth-green : 1 rugate-green. This segregation ratio 9:3:3:1 is typical for dihybrid crossing. It should be borne in mind that for monohybrid crossing, the ratio was 3:1.
Apart from use of the Penneth lattice and the above formula, there is one other way which allows determination of all the possible combinations of male and female gametes during pollination, and also for the determination of phenotypes and genotypes of generation F2. This is a mathematical method based on the law of combination of two or more independent phenomena. In accordance with the theory of probability, the probability of simultaneous development of two independent phenomena is equal to the product of the probabilities of each. It can thus be observed that the splitting of each pair of alleles in dihybrid crossing develops as two independent phenomena. In monohybrid crossing, the development of varieties with dominant characteristics occurs in ¾ of all cases; and varieties with recessive characters in ¼ of all cases. Hence, the probability of simultaneous development of two dominant characters, such as smooth form and yellow colour, is equal to the product of the probabilities of each character, namely, ¾ × ¾ = 9/16. The probability of development of smooth form and recessive green colour is ¾ × ¼ = 3/16; the probability of recessive rugate form and yellow colour is ¼ × ¾ = 3/16; probability of simultaneous development of two recessive features, rugate-green seed, is ¼ × ¼ = 1/16. In other words, the product of separate probabilities gives the ratio of segregation by phenotype: 9/16:3/16:3/16:1/16 or 9:3:3:1. These ratios can be derived for other types of crossings, such as trihybrid when the probabilities of development of three and more pairs of alternative features should be multiplied respectively.
The ratio of splitting in F2 (9:3:3:1) is the splitting by phenotypes, by the appearance of the organisms. Analyzing the genotypes in the above Penneth lattice, the formula of splitting for genotypes is 1:2:2:4:1:2:1:2:1. Knowing that in monohybrid crossing the splitting of genotype gives 1AA:2Aa:1aa for one pair of alleles, and 1YY:2Yy:1yy for the other, it is possible to calculate the probability of genotype development in dihybrid crossing. Thus, if the probability of genotype AA and YY development is ¼, the probability of genotype AAYY development will be ¼ × ¼ = 1/16. The probability of genotypes Aa and Yy development is ½, and of genotypes aa and yy is ¼. By multiplying two probabilities, as with genotypes AA and YY, it is possible to predict all the classes of splitting by a genotype. Thus, the frequency of AAYY occurrence is 1/16; of AAYy ¼ × ½ = ⅛ = 2/16; of AaYY ½ × ¼ = ⅛ = 2/16; of AaYy can be derived from ½ × ½ = ¼ = 4/16 and so on and the same 9 classes of genotypes; 1:2:2:4:1:2:1:2:1 can be obtained. Thus it is seen that in the case of monohybrid splitting, a hybrid develops two types of gametes and phenotypes and three types of genotypes; in the case of dihybrid splitting, four types of gametes and phenotypes and 9 types of genotypes are produced. Mathematically the number of different gamete classes resulting from the hybrids F1, as well as the number of phenotypes in generation F2, is represented by 2n; the number of genotypes in generation F2 by 3n. The power n is equal to the number of gene pairs which participate in the process of crossing. Thus for monohybrid crossing, n = 1, and the number of classes of gametes and phenotypes is 21 = 2; the number of classes of genotypes is 31 = 3. For dihybrid crossing the number of classes of gametes and phenotypes is 22 = 4; and of genotypes, 32 = 9. Using these figures (2n and 3n), without drawing the Penneth lattice and without other complicated methods described above, we can determine at once the expected number of classes of splitting by genotype and phenotype, and also the number of gamete classes in hybrids F1 for each type of crossing (mono-, di-, and polyhybrid).
Analyzing crossing. To determine the genetic structure of gametes formed by a hybrid, the hybrid is crossed with an individual possessing genes to be analyzed in homozygous and recessive condition. This type of crossing is called “analyzing”. Consider the following example.
Mendel selected for crossing, hybrid plants F1 (seed: smooth and yellow) with plants homozygous with two recessive genes (seed: rugate and green, aayy). In the following generation four classes of seeds were produced, the numerical ratio of which was close to 1:1:1:1, namely: round yellow - 55 (AaYy), smooth green - 51 (Aayy), rugate yellow - 49 (aaYy) and rugate green - 53 (aayy). Thus by an experimental genetic method he proved that a dihybrid organism develops four kinds of gametes in equal ratio. The principle of such crossing is that it is necessary to use an organism which is known to have genes in the recessive state; this permits elimination of the effect of dominance, and reveals in the gametes of the organism under analysis both the dominant and the recessive genes. With knowledge of the genetic structure of gametes, it is quite easy to determine the genotype of an organism which form these gametes.
Immediately after the secondary discovery of Mendel's laws in 1900, very intensive scientific investigations began, which, on one hand, produced information confirming Mendel's statements; and on the other, traced certain cases which at first sight did not fit into these theories. However, the development of scientific work in the field of genetics eventually gave convincing explanations of these contradictory phenomena, with regard to their apparent deviation from the fundamentals of Mendel's theory. It was proved that all of them were connected with cases of interaction of allelic and non-allelic genes. Besides, in the course of investigations, organisms especially suitable for genetic analysis were found, e.g., Antirrhinum majus (snapdragon), corn, hens, rabbits, mice and the fruit fly, Drosophila melanogaster. Investigation of the genetics of these gave rise to further development of genetic analysis.
Dominance is very often incomplete, and sometimes not present at all. In the latter case, heterozygote Aa shows characters intermediate between AA and aa. Crossing of plants with red (AA) and white (aa) flowers can give F1 with pink flowers (Aa). Then, segregation in F2, calculated by the formula (A+a)×(A+a) gives a ratio of 1AA (red): 2Aa (pink) : 1aa (white). In dihybrid splitting, incomplete dominance gives a more complicated picture, and the formula 9:3:3:1 for complete dominance is substituted by 3:6:1:2:3:1 for incomplete domination of both allele pairs. Interaction of genes in incomplete dominance in one pair of alleles is shown in Fig. 3.
The ratio 9:7 in the case of dihybrid segregation in F2 is observed when an individual gene cannot act separately, but a certain character is developed in the simultaneous presence of both complimentary genes. On crossing plants with white flowers (AAbb) × (aaBB), plants F1 were produced with red flowers. In generation F2 segregation was observed in the ratio of 9 red : 7 white. Thus for the development of red colour, the presence of both dominating genes was necessary, as in F1 (AaBb) and in 9/16 of plants F2. Three other classes of the generation, developed in typical dihybrid segregation, produced only one type of plant, with white flowers. That can be explained with the help of the formula for dihybrid segregation, in which all the red plants are underlined:
 |  |
Fig.3 The inheritance of corolla colour and shape in Antirrhinum majus at incomplete domination in one pair of characters | Fig.4 Inheritance of seed colour in Triticum at the interaction of two pairs of genes (polymery) |
Fig. 5 Inheritance of scale patterns in Cyprinus carpio
| F1 | AaBb | ||
| Gametes (Ab+ab) | = | F1 AB, aB, Ab, ab, hence (AB+aB+Ab+ab) × AABB + AaBB + AABb + AaBb + AaBB + aaBB + AaBb + aaBb + AABb + AaBb + AAbb + Aabb + AaBb + aaBb+ Aabb + aabb. | |
The ratio 9:3:4 for dihybrid segregation occurs only when one of the types of the generation with a dominant gene cannot be distinguished from the double recessive. The following example of crossing mice (Muridae) illustrates this.
The crossing of a black mouse (CCaa) and an albino (ccAA) yielded F1 bearing both dominant genes, having genotype CcAa and hair colour of the wild type, the so-called aguti. On inter-crossing such hybrids, the following picture of segregation is received: 9 aguti : 3 black : 4 white.
There exists an interaction in which one gene can suppress the function of another, non-allelic to it, e.g. A B, B A, a B, b A, etc. This phenomenon of interaction between non-allelic genes is called epistasis. Epistasis may be dominant or recessive. By dominant is meant the phenomenon in which the dominant allele of one gene is able to suppress the function of the allelic pair in another gene. Recessive epistasis is an interaction in which a recessive allele of one gene, being in homozygous condition, does not allow the dominant or recessive alleles of other genes to develop (this phenomenon is also called cryptomery).
Dominant epistasis is characterized by the ratio at segregation of 12:3:1 or 13:3. Oats have dominant genes, one of which determines black, and the others grey colouring of the grains. If these genes are marked A and B respectively, then in certain crossings the parental types may have structure AAbb and aaBB. This means that the first parent has grains which are black, and the second has grey. F1 will have structure AaBb, i.e., it will contain simultaneously the genes of black and grey. As a grain cannot be grey and black at the same time, the grey colour cannot develop, since it is concealed under the black colour (in this case it is said that gene A is epistatic in relation to gene B). Splitting in F2 is: 12 black: 3 grey: 1 white. This is clear, proceeding from the normal ratio in dihybrid crossing.
| 12 | 9 AB = black + grey = black not distinguishable |
| 3 Ab = black | |
| 3 aB = grey | |
| 1 ab = white |
Double recessive aa presents, in this case, a new combination which was not traced among the parental forms, not in F1. The absence of gene A and gene B in the descendants of this group results in the formation of white grains.
Segregations such as 13:3 are rather rare (they are traced in some plants and in chickens). Such segregation occurs as a result of the influence of dominant factors (suppressors) which do not allow certain dominant genes to function normally.
It is necessary to stress once again that epistasis is an interaction between non-allelic genes, and thus it is different from the phenomenon of dominance, which reflects the interaction between the alleles.
Segregation in a 15:1 ratio is of general importance because it is connected with the presence of polymeric genes. This type of inheritance determines the inheritance of quantitative characters. In the case of polymery, one can see the combined action of several genes, causing a similar effect. Because these genes influence the same character in an identical way, it is customary to depict them with one latin letter and an index number for different alleles, A1, A2, A3 and so on. Such a polymeric character is demonstrated by the red colour of wheat grains. The inheritance of red colour in wheat grains is shown in Fig.4. It can be seen that 1/16 of all F2 plants have four dominant genes (A1A1A2A2) and have the most intensive grain colour; 4/16 of all grains have three dominant alleles (type A1A1A2a2); 6/16 have two (type A1a1A2a2) and 4/16 have one (type A1a1a2a2); all these genotypes determine grain colour of various intensity. Genotype a1a1a2a2, present in 1/16 of all grains, determines the white colour of grains. Fig.4 shows that the frequencies of the above five classes are expressed in the binominal series: 1+4+6+4+1 = 16, which reflects variability in grain colour, depending on the number of dominant alleles in the genotype. Thus the range of genotypes in the case of polymeric genes can be represented by a binominal curve of variations of a given character. The greater the occurrence of such genes in heterozygous condition, the more combinations of genotypes with various numbers of dominant alleles are obtained in F2.
Polymeric inheritance is intrinsic for many factors of economic value, namely: butterfat yield, egg yield, weight and growth of domestic animals, fertility and maturation rate, spike length, cob length, sugar content in the root crop of sugarbeets, etc. The skin pigmentation of man is also inherited through polymeric factors.
In conclusion it must be said that polymeric inheritance is extremely complicated, and, to date, the theory of polymeric genes provides the best explanation of the nature of inheritance of quantitative characters.
Pleiotrophy or polymeric function of genes is the influence of one gene upon several characters which have no apparent relationship.
One case of pleiotrophy is described by Mendel himself, who discovered that in pea plants the same gene determines the purple colour of flowers, red spots in leaf axils and grey or brown colour of the skin of the seed. Mendel was obviously dealing with a gene which determines the general pigmentation of peas.
Muller found another example in Drosophila where the same gene influences the length and the shape of wings, disposition of chaetae and viability.
The phenomenon of pleiotrophy indicates that a character, as a rule, is determined not solely by one gene, but is the result of the combined function of many genes.
Genotype modification is a phenomenon in which one or several genes determine the rate of development of a character, whereas the fact of the presence or absence of the character is determined by another gene. Genes which change the function of the main gene are called modifiers.
The gene which determines the hair colour of rabbits (gene d) is modified by gene H. If genes d and H are present together, rabbits with darker fur result (Viennese); if there is a combination of d and h, the fur will be lighter in colour.
The existence of gene modifiers is a widely spread phenomenon. Apparently, each gene in an organism interacts with others and therefore acts as their modifier.
Genes which cause destruction of an organism at a certain stage of development are called lethal genes. These are widely spread in nature. They are usually present in a concealed state in heterozygous organisms, and their lethal function cannot develop in the presence of a non-lethal allele. But when transferred to homozygous state, they exert a lethal effect. Examples are the genes which determine the character of scales in carp (Cyprinus carpio) and genes of hair colour in mice.
On crossing linear carp, segregation in the character of scales occurs in the ratio of 2:1 in the following generation. It can be seen that gene A in homozygous state brings about a lethal effect. This phenomenon is described also in the case of a gene which determines the yellow hair colour in mice.
In all these examples, attention should be paid to the pleiotropic effect of lethal genes in carp and mice. The pleiotropic effect manifests itself in the case where one gene determines viability and other characters.
The character of an organism depends not only on the combination and interaction of specific genes, but also on the conditions which influence the rate of development of a gene effect during the process of growth of the organism. These conditions can be divided into two groups: those being external to the organism, and those being internal, which are conditioned by the interaction of metabolic systems.
The review presented is the history of Mendel's discovery concerning stable inheritance factors or genes, and the role of this discovery in the development of genetics. Three main laws were formulated as a result of Mendel's experiments:
The law of dominance or uniformity of the first generation of hybrids.
The law of segregation and independent combination of genes, and
The law of purity of gametes.
The intensive development of genetics, which began after the re-discovery of Mendel's laws by H. de Vries, C. Correns and E. Tschermak resulted in the correlation of Mendel's abstract factors to actual material structures located in cell nuclei, namely the chromosomes.
Correns, C., 1902 Über Bastardirundsversuche mit Mirabilis-Sippen. Ber.Dtsch.bot.Ges., 20:540–608
de Vries, H., 1901 Die Mutationstheorie. Veit, Leipzig
Mendel, G., 1866 Versuche über Pflanzenhybriden. Verh.naturf.Vereines, Brünn, 4:3–47
Muller, H.J., 1940 An analysis of the process of structural change in chromosomes of Drosophila. J.Genet., 40:1–66
