Why do children look like their parents? Why are certain diseases common in some families, such as color blindness, polydactyly, joint hypermobility, cystic fibrosis? Why are there a number of diseases that only women suffer from, and only men that suffer from others? Today we all know that the answers to these questions must be sought in heredity, that is, in the chromosomes that the child receives from each of the parents. And modern science owes this knowledge to Thomas Hunt Morgan , an American geneticist. He described the process of transmitting genetic information and, together with his colleagues, developed the chromosome theory of inheritance (it is often called the Morgan chromosome theory), which has become the cornerstone of modern genetics.
Discovery story
It would be wrong to say that Thomas Morgan was the first to become 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 works of Chistyakov, Beneden, Rabl in the 70-80s of the XIX century.
Then there were no microscopes so powerful that it was possible to make out chromosome structures. And the term "chromosome" itself was not there either. It was introduced by the German scientist Heinrich Waldeir in 1888.
The German biologist Theodor Boveri as a result of his experiments proved that for the normal development of the body he needs a normal number of chromosomes for his appearance, and their excess or lack leads to severe malformations. Over time, his theory was brilliantly confirmed. We can say that the chromosome theory of T. Morgan got its starting point precisely thanks to the studies of Boveri.
Start of research
Thomas Morgan was able to generalize the existing knowledge about the theory of heredity, complement and develop it. He chose a fruit fly as an object for his experiments, and not by chance. It was an ideal object for research on the transfer of genetic information - only four chromosomes, fertility, short life span. Morgan began research using clean fly lines. Soon, he discovered that the sex cells have 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 chromosome as Y.
Sex-Related Inheritance
Morgan's chromosomal theory has shown that there are certain traits linked to the floor. The fly with which the scientist conducted his experiments normally has red eyes, but as a result of the mutation of this gene white-eyed individuals appeared in the population, and among them there were much more males. The gene that is responsible for the color of the eyes of the front sight is localized on the X chromosome, but it is not on the U chromosome. That is, when a female is crossed, on one X-chromosome of which there is a mutated gene, and a white-eyed male, the probability of this trait in the offspring will be related to sex. The easiest way to show this is in the diagram:
- P: XX 'x X'U;
- F 1 : XX ', XU, X'X', X'U.
X - sex chromosome of a female or male without a white eye gene; X'-chromosome with the gene for white eyes.
Decipher the results of the crossing:
- XX '- a red-eyed female, a carrier of the gene for old eyes. Due to the presence of the second X-chromosome, this mutated gene "overlaps" healthy, and the sign does not appear in the phenotype.
- X'U is a white-eyed male who received an X chromosome with a mutated gene from his mother. Due to the presence of only one X chromosome, the mutant trait has nothing to overlap, and it appears in the phenotype.
- X'X 'is a white-eyed female inherited from the mother and father on the chromosome with a mutant gene. In a female, only if both X chromosomes carry the white eye gene, it will manifest itself in the phenotype.
The chromosomal theory of heredity of Thomas Morgan explained the mechanism of inheritance of many genetic diseases. Since there are much more genes on the X chromosome than on the Y chromosome, it is clear that it is responsible for most of the signs of the body. The X chromosome from the mother is transmitted to both sons and daughters, together with the genes responsible for the body's properties, external signs, and diseases. Along with X-linked, there is U-linked inheritance. But the U-chromosome is only in men, because if any mutation occurs in it, it can be transmitted only by a male descendant.
Morgan's chromosomal theory of heredity helped to understand the patterns of transmission of genetic diseases, but the difficulties associated with their treatment have not yet been resolved.
Crossingover
In the course of research by Thomas Morgan's student Alfred Stertevant, the phenomenon of crossing over was discovered. As further experiments showed, due to crossing over, new combinations of genes appear. It is he who violates the process of linked inheritance.
Thus, T. Morgan's chromosome theory received another important position - crossing over occurs between homologous chromosomes , and its frequency is determined by the distance between the genes.
Key Points
To systematize the results of the scientist’s experiments, we present the main points of the Morgan chromosome theory:
- Signs of an organism depend on genes embedded in chromosomes.
- Genes of one chromosome are transmitted to offspring in a linked manner. The strength of this linkage is greater, the smaller the distance between the genes.
- In homologous chromosomes, the phenomenon of crossing over is observed.
- Knowing the crossover frequency of a particular chromosome, one can calculate the distance between genes.
The second point of the Morgan chromosome theory is also called the Morgan rule.
Confession
The research results were perceived brilliantly. Morgan's chromosomal theory was a breakthrough in twentieth-century biology. 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.
Morgan's chromosome theory of heredity is currently being studied in schools. She is devoted to many articles and books.
Examples of sex-linked inheritance
Morgan's chromosomal theory has shown that the properties of an organism are determined by the genes embedded in it. The fundamental results obtained by Thomas Morgan answered the question about the transmission of diseases such as hemophilia, Low syndrome, color blindness, Brutton's disease.
It turned out that the genes of all these diseases are located on the X chromosome, and in women these diseases appear much less frequently, since a healthy chromosome can overlap the chromosome with the disease gene. Women, not knowing about it, can be carriers of genetic diseases, which then appear in children.
In men, X-linked diseases, or phenotypic signs, appear because there is no healthy X chromosome.
The chromosomal theory of heredity of T. Morgan is used in the analysis of family anamnesis for genetic diseases.