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 others - only men? Today, we all know that the answers to these questions must be sought in heredity, that is, the chromosomes that a child receives from each parent. And modern science owes this knowledge to Thomas Hunt Morgan, an American geneticist. He described the process of transmission of genetic information and developed, together with colleagues, the chromosome theory of inheritance (often called the Morgan chromosome theory), which became the cornerstone of modern genetics.
Discovery history
It would be wrong to say that Thomas Morgan was the first who became interested in the transfer of genetic information. The first researchers who tried to understand the role of chromosomesin inheritance, one can consider the works of Chistyakov, Beneden, Rabl in the 70-80s of the XIX century.
Then there were no microscopes powerful enough to see chromosome structures. And the term "chromosome" itself did not exist then either. It was introduced by the German scientist Heinrich Waldeyr in 1888.
The German biologist Theodor Boveri, as a result of his experiments, proved that for the normal development of an organism, it needs a normal number of chromosomes for its species, and their excess or lack leads to severe malformations. Over time, his theory was brilliantly confirmed. We can say that T. Morgan's chromosome theory got its starting point precisely thanks to Boveri's research.
Start research
Thomas Morgan managed to generalize the existing knowledge about the theory of heredity, supplement and develop it. As an object for his experiments, he chose the fruit fly, and not by accident. It was an ideal object for research into the transmission of genetic information - only four chromosomes, fertility, short life expectancy. Morgan began to conduct research using clean lines of flies. Soon he discovered that in germ cells there is 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 as Y.
Sex-linked inheritance
Morgan's chromosome theory showed that there are certain sex-linked traits. The fly with which the scientist conducted his experiments normally has red eyes, but as a result of a mutation of this gene in the populationwhite-eyed individuals appeared, and among them there were much more males. The gene responsible for the color of the fly's eyes is localized on the X chromosome, it is not on the Y 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 having this trait in the offspring will be associated with sex. The easiest way to show this is on the diagram:
- P: XX' x X'Y;
- F1: XX', XY, X'X', X'Y.
X - the sex chromosome of a female or male without the gene for white eyes; X'-chromosome with the gene for white eyes.
Decipher the results of crossing:
- ХХ' - red-eyed female, carrier of the old eyes gene. Due to the presence of the second X chromosome, this mutated gene is "overlapped" by the he althy one, and the trait does not appear in the phenotype.
- X'Y 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, there is nothing to cover the mutant trait, and it appears in the phenotype.
- X'X' is a white-eyed female who inherited a chromosome with a mutant gene from her mother and father. In a female, only if both X chromosomes carry the gene for white eyes will it appear in the phenotype.
Chromosomal theory of heredity by 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 characteristics of an organism. The X chromosome is passed from the mother to both sons and daughters, along with the genes responsible for the properties of the body, external signs, and diseases. Alongwith X-linked, there is Y-linked inheritance. But only males have a Y chromosome, so if any mutation occurs in it, it can only be passed on to 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.
Crossing Over
In the course of research, a student of Thomas Morgan, Alfred Sturtevant, discovered the phenomenon of crossing over. As further experiments showed, thanks 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 genes.
Basics
To systematize the results of the scientist's experiments, we present the main provisions of Morgan's chromosome theory:
- The signs of an organism depend on the genes that are embedded in the chromosomes.
- Genes on the same chromosome are passed on to offspring linked. The strength of such linkage is greater, the smaller the distance between the genes.
- Crossing over occurs in homologous chromosomes.
- Knowing the frequency of crossing over a particular chromosome, we can calculate the distance between genes.
The second position of Morgan's chromosome theory is alsois called Morgan's rule.
Recognition
The results of the research were received brilliantly. Morgan's chromosome theory 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 excellence in genetics.
Now Morgan's chromosome theory of heredity is being studied in schools. Many articles and books are dedicated to her.
Examples of sex-linked inheritance
Morgan's chromosome theory showed that the properties of an organism are determined by its genes. The fundamental results obtained by Thomas Morgan answered the question of the transmission of diseases such as hemophilia, Lowe's syndrome, color blindness, Brutton's disease.
It turned out that the genes for all these diseases are located on the X chromosome, and in women these diseases appear much less often, since a he althy chromosome can overlap the chromosome with the disease gene. Women, not knowing about it, can be carriers of genetic diseases, which then manifest themselves in children.
In men, X-linked diseases, or phenotypic traits, appear because there is no he althy X chromosome.
T. Morgan's chromosome theory of heredity is used in the analysis of family histories for genetic diseases.
