Chromosomal theory of Morgan: definition, main provisions and features

Why do children look like their parents? Why do certain families have certain diseases, such as color blindness, polydactyly, hypermobility of joints, cystic fibrosis? Why are there a number of diseases that only women suffer from, and others - only men? Today we all know that the answers to these questions need to be sought in heredity, that is, the chromosomes that the child receives from each of the parents. And this knowledge of modern science owes to Thomas Hunt Morgan - American genetics. He described the process of transferring genetic information and developed together with colleagues the chromosomal inheritance theory (often called the Morgan chromosome theory), which became the cornerstone of modern genetics.

History of the discovery

It would be wrong to say that Thomas Morgan was the first to be interested in the issue of the transfer of genetic information. The first researchers who tried to understand the role of chromosomes in inheritance can be considered the work of Chistyakov, Beneden, Rable in the 70-80s of the XIX century.

Then there were no microscopes so powerful that one could see the chromosome structures. And the very term "chromosome" was not there either. He was introduced by the German scientist Heinrich Valdeir in 1888.

The German biologist Theodore Boveri, as a result of his experiments, proved that for the normal development of the organism he needs a normal number of chromosomes for his species, and their excess or lack leads to serious malformations. Over time, his theory was brilliantly confirmed. We can say that the chromosomal theory of T. Morgan got its starting point precisely thanks to the studies of Boveri.

Start of research

Thomas Morgan was able to generalize existing knowledge about the theory of heredity, to supplement and develop them. As an object for his experiments, he chose a fruit fly, and not by accident. It was an ideal object for research on the transfer of genetic information - only four chromosomes, fecundity, short life expectancy. Morgan began to conduct research using clean lines of flies. Soon he discovered that in the reproductive cells there was a single set of chromosomes, that is, 2 instead of 4. It was Morgan who designated the female sex chromosome as X, and the male sex as Y.

Sex-linked inheritance

The chromosome theory of Morgan showed that there are certain signs linked to the sex. The fly with which the scientist conducted his experiments normally has a red eye color, but as a result of the mutation of this gene, white-eyed individuals appeared in the population, and there were many more males among them. The gene that is responsible for the eye color of the fly is localized on the X chromosome, on the Y chromosome it is not. That is, when crossing a female, which has a mutated gene on one X chromosome, and the white-eyed male, the probability of having this sign in the offspring will be related to the sex. The easiest way to show this is in the diagram:

• P: XX 'x X'Y;
• F ₁ : XX ', XY, X'X', X'Y.

X is the sex chromosome of a female or male without a white eye gene; X'-chromosome with a gene of white eyes.

Let's decipher the results of crossing:

• XX 'is a red-eyed female, the carrier of the gene of the former eyes. Due to the presence of the second X-chromosome, this mutated gene "overlaps" healthy, and the phenotype does not manifest itself.
• X'U is a white-eyed male who received from his mother an X chromosome with a mutated gene. Due to the presence of only one X-chromosome, the mutant sign has no overlap, and it appears in the phenotype.
• X'X 'is a white-eyed female, inheriting on the chromosome with a mutant gene from the mother and father. In a female only if both X chromosomes carry a white eye gene, it will manifest itself in the phenotype.

Chromosomal theory of heredity Thomas Morgan explained the mechanism of inheritance of many genetic diseases. Since there are many more genes on the X chromosome than on the Y chromosome, it is clear that it is responsible for most of the body's signs. The X chromosome from the mother is transmitted to both sons and daughters, along with genes responsible for the properties of the organism, external signs, diseases. Along with X-linked, there is Y-linked inheritance. But the Y-chromosome is only in men, because if it has any kind of mutation, it can only be transmitted by a male descendant.

Chromosome theory of Morgan's heredity helped to understand the patterns of transmission of genetic diseases, but the difficulties associated with their treatment are not resolved until now.

Crossing-over

In the course of research, the phenomenon of crossing-over was discovered by the student of Thomas Morgan, Alfred Sturtevant. As shown by further experiments, crossing-over generates new combinations of genes. It is he who violates the process of concatenated inheritance.

Thus, the chromosomal theory of T. Morgan has received another important position: crossing-over occurs between homologous chromosomes , and its frequency is determined by the distance between the genes.

Basic Provisions

To systematize the results of the scientist's experiments, we give the main provisions of the Morgan chromosome theory:

1. The signs of the organism depend on the genes embedded in the chromosomes.
2. The genes of one chromosome are transmitted to the offspring in a linked way. The strength of such a bond is the greater, the shorter the distance between genes.
3. In the homologous chromosomes, crossing-over is observed.
4. Knowing the frequency of crossover of a particular chromosome, you can calculate the distance between the genes.

The second position of the chromosome theory of Morgan is also called Morgan's rule.

Confession

The results of the studies were well received. The chromosome theory of Morgan was a breakthrough in the biology of the twentieth century. In 1933, for the discovery of the role of chromosomes in heredity, the scientist was awarded the Nobel Prize.

A few years later, Thomas Morgan received the Copley medal for outstanding achievements in the field of genetics.

Now the chromosome theory of Morgan's heredity is being studied in schools. She devoted a lot of articles and books.

Examples of sex-linked inheritance

Chromosomal theory of Morgan showed that the properties of the organism are determined by the genes embedded in it. The fundamental results that Thomas Morgan received were answered on the transfer of diseases such as hemophilia, Low syndrome, color blindness, and Brutton's disease.

It turned out that the genes of all these diseases are located on the X chromosome, and in women these diseases are manifested much less often, since a healthy chromosome can block the chromosome with the disease gene. Women who are not aware of this can be carriers of genetic diseases, which are then manifested in children.

In men, X-linked diseases, or phenotypic signs, manifest themselves as there is no healthy X chromosome.

Chromosomal theory of heredity T. Morgan is used in the analysis of family history for genetic diseases.