New advances in biology XIX. century

1994/05/01 Bandres Unanue, Luis Iturria: Elhuyar aldizkaria

• Historia

As we have seen before, XIX. By the end of the 20th century nature lovers considered Darwin's work definitive, since evolution through natural selection was considered an established principle. Then embryological research to achieve greater accuracy was seen for prosperous roads, since according to the hypotheses of Meckel and Haeckel, the history of an embryological being would be to miniature remake the history of its species.

We may think there were exceptions. On the one hand, De Vries investigated the changes and reached enormous prestige due to its hereditary consequences. On the other hand, Bateson in 1890 criticized the evidence of Haeckel’s “law” and its logical foundations and proposed to return to Darwin’s ways. Darwin's ideas about the origins of the species that were then in the pile had some obstacles. The most important were, according to Bateson:

“The first difficulty refers to the magnitude of the changes that give rise to new forms. It is considered that the changes produced by species in all ancient beings about evolution are small, although often not explicitly stated. But if they are small, what advantages or advantages do they bring to those who have them over their peers? This difficulty is known as “small changes or initials.”

The second difficulty is similar. Considering that these changes occur, if maintained and perpetuated, they would generate new species. How can they keep them? Can't these changes be eliminated when they mingle with those who don't have beings they have? This second difficulty is called the "destructive result of crossing".

To this Bateson added that, as anyone who is engaged in the breeding of plants or animals knows, although in many cases small variations appear with respect to what is considered normal in beings, in many others are large. Research on this subject by the end of the century was sufficient to verify that abrupt mutations were not very occasional and that many times these mutations passed to their descendants. This way new changes may appear, but not species.

However, the reason and cause of these changes were not known and, as they appeared, should be taken simply. But in those years new works were known, rather, they forgot about ancient works of interest such as those of Mendel.

Johann Mendel was born in Henomorf (Germany) in 1822 in a peasant family. After elementary studies, he entered the convent of the Augustinians and at the age of twenty-one he was ordained a priest, later becoming known as Gregor, the world of science. His desire to pursue science education led him to undertake special studies at the University of Vienna. Then at Br\nn High School he taught throughout fourteen physical years and natural sciences. However, he did his greatest work and his most prolific research at the monastery of Brünn itself.

In 1856 Mendel began his research by crossing peas of different races in the garden of his monastery. Although at first it seems that he considered it as a hobby or a game, soon, in view of the results, a research passion with his own personality was unleashed. With repeated trials and changes he discovered the general laws of hybridization. After that, for eight years, alone, he devoted himself to checking the achievements obtained. During this time, as it is said, he thoroughly explored twelve thousand plants.

All this work was collected in the report entitled “Hybrid Plant Trials” and explained to the Brognn Natural History Association in 1865. That association, apparently, was not one of the highest and did not give him any importance.

In view of this, Mendel sent his work to some scientists (among others, Von Nágeli, professor of botany of Munich, very high at that time), but from him he received only a courtesy response, perhaps considering it as a result of an entertainment game of a botanical friar.

Faced with the ill reception he suffered, and seeing the difficulty of approving his works, Mendel gradually lost his passion for science. In 1868 he was appointed prelate and because of the work that this office generated in the diocese, he devoted his efforts to science in this regard. Therefore, until his death, on January 6, 1884. He devoted himself to ecclesiastical tasks.

The report titled “Hybrid Plant Trials” contains a statement of the laws that support current genetics and are named Mendel. In the introduction of the work, Mendel recalled the work of scientists who had previously worked on hybrid research, and explained what it is and how it will address the problem.

In this work two fundamental laws appear: the first is that of the dysfunction of characters (which was previously known) and the second is that of the independence of those characters. Mendel's research began by exploring a pea, the Pisum sativum. He studied the crossing of smooth (L) and rough (Z) seed peas. In the first generation, called F1, only soft seeds appear. But if the latter cross each other, in the second generation (F2) 75% of peas type L and 25% type Z appear. In the F1 hybrid the character Z (underlying) was hidden and only the character L (main) appeared, but in the hybrid F2 Z will appear again.

Mendel's laws allow it. The F1 hybrid is of type LZ, where the gametes L and Z are the same. Therefore, once crossed these hybrids, in the hybrid F2, we will have the following four types of hybrids: LT, LZ, LZ and ZZ. In them the LL will be soft. Also those of LZ, since the character L is the main one regarding Z, but those of ZZ will be rough. Therefore, the previous proportions will be 75% smooth and 25% rough.

Mendel also used peas with several characters: smooth and yellow seeds on one side and wrinkles and greens on the other. In this case there were four types of gametes: yellow-rough polishing, Russian-yellowish, polished green and rough green, so sixteen combinations in the proportions 9, 3, 3, 1, being smooth and yellow the main character and green and the inferior roughness.

Due to the results obtained by Mendel, it was observed that hereditary characters were related to the elements that could be distributed, thus indicating a discontinuity in the heritage of the inheritance.

In Mendel all elements of modern genetics appear: the laws of hybridization and its uses. Mendel's conception was very concrete. It is not a cloudy intuition as it happens in the initial works, but something mature and humiliated. Leaving aside terminology, Mendel's work can be considered written today. Therefore, in the field of inheritance science, Mendel cannot be considered a pioneer, but a creator of that science.

As we have said before, all this work was discarded and that friar, with great patience, had to leave all his achievements to the thirty-five years of silence, that is, until his death. In 1900 De Vries, Correns and Tscherma discovered, confirmed and disclosed Mendel's work.

Due to the accepted trend in the world of today's physics, it is quite appropriate to express biological qualities, so to speak, through atomic units under probability legislation. If it is not possible to predict the movement of a specific atom or electron, the same happens with the type of inheritance that will appear in a special organism, but in both cases we can calculate probabilities and if we take a very high number we can be sure that the predictions will be fulfilled.

As we have seen, the specificity of the characters, that is, whether they are predominant or sub-decisive, must be taken into account in heritage research. One can transmit the main character to his descendants, as long as it is clear that he has it.

On the contrary, in some cases some underlying character may appear without having previously noticed one of the following. In the event that two beings carrying an underlying character (even if they do not appear) in their cells fertilize each other, that underlying character will clearly appear in 25% of their descendants.

However, in most cases hereditary problems are much more complicated than seen with two characters. In addition, some qualities may be predominant or sub-sexual. Other times the characters appear in pairs, that is, one does not appear if the other does not appear, and vice versa. In other cases they are incompatible and will never appear together.

Many Mendelian characters have been identified in plants and animals and this method has been used to improve races, increase certain peculiarities and rule out others. Therefore, these techniques abandoned weighting methods, leaving way for scientific methods. For example, Biffen got a very rich type of wheat in which, after a long series of trials based on Mendel's laws, mold immunities, high fertility and other positive particularities appeared.

When Mendel's work was discovered, the structure of the cell was known and it was observed that within the nucleus of each cell there are filamentous bodies, called chromosomes. The number of chromosomes in the fertilized egg by the combination of two germ cells (the simplest case) is double, two per class, one per parent. Dividing the oculus also divides each chromosome, passing each part to each child cell. Thus, each new cell receives a chromosome from each initial chromosome. The same goes for each next division. Consequently, each plant and animal cell has a double series of chromosomes from each parent.

In the beginning, germ cells also have a double series of chromosomes, but in the last transformation, that is, the union of spermatozoa and eggs is performed in pairs. So the type of division is different: instead of dividing the chromosomes themselves, the chromosomes of each pair are divided and each passes to the cells of their children. Thus, the adult germinal cell receives one or another of each chromosomal couple.

XIX. At the end of the 20th century and the beginning of the 20th century, some experts discovered the affinity between the effects of Mendel's heritage and cellular phenomena. But what has been given scientific formulation was Sutton. Sutton noted that chromosomes have a distribution as hereditary factor and that in each case the pairs of factors and chromosomes are distributed independently of the other. However, since the number of hereditary factors relative to the walls of the chromosome is very high, there are different factors that are combined in the same chromosome and, therefore, that appear together is perfectly normal.

Starting in 1910 Morgan and his colleagues analyzed these relationships more thoroughly. A numerical relationship was found between the number of groups of heredity qualities and the number of pairs of chromosomes. This amount is seven for the pea, eight for the wheat, twenty for the rat, and thirty-three for the man. Therefore, although the number of pairs of chromosomes is not very high, the possible types of germ cells are very numerous (more than a million) and the possible combinations of two of them are much wider. Therefore, it is easy to understand why two beings of the same race do not form exactly the same.

While research on inheritance was on Mendel's line, another type of research was also developed, the path of statistics. To this end, large quantities had to be taken into account, and the theory of probability and the statistical margin of combustion were adapted to human changes.

The normal combustion curve is obtained by a very large number of elements, but De Vries clarified the risks that can be presented when applying this theory in the field of inheritance.

The top figure shows the length variations of the three types of fruit. While the lengths are given in horizontal line, in vertical the number of elements is marked with a fixed length. The curves A and C are very similar to the normal ones and in each of them there is a clear average length. On the contrary, we can observe that in curve B there are at least two subgroups. If we have taken all three fruits at once, instead of having three curves, we would only have one, and this would have the appearance of normal. Therefore, taking data often blank, you cannot know if all the elements received are of the same type or of different subgroups.

Some researchers analyzed the behavior of empty elements. They started from a single element to form an empty group (for example, a baberrun), and through self-fertilization they obtained the descendants. The changes presented by these empty elements conformed to the law of combustion. But these changes did not go through inheritance, that is, taking the larger elements of that group achieved, their descendants were not older, but were directed to a normal size of the group.

Apart from the research of the purely enunciated elements, the mixture of ancient characters in any natural race results in changes that are transmitted from parents to children from antiquity. If the two parents choose to convey a peculiarity, there is a greater probability that the children acquire that peculiarity. For example, if we take height to investigate in humans, if the parents are long, that is, greater than the average population, the children will tend to be like that.

But due to Mr. Vilmorin, which in its day passed as those of Mendel in the bed, the best results in the growth of plants are not achieved by choosing as parents some special elements, but selecting elements of good average yield.

For a long time biometrists and jeans had strong debates. Now the two ways for proper inheritance research are considered complementary, so both must be taken into account.