Regardless of the complexity of the structure of a biological object, whether it is a bacterium, algae, invertebrate animal or human being, the total number of all signs of any organism is much larger than its composition of chromosomes. Since the mid-20th century, science has known that such characteristics of a plant, animal or microorganism as color and shape of the body, size of limbs, and metabolic characteristics are encoded in chromosome regions - genes. How many genes does each chromosome have, in what sequence are they located in it, how are they inherited? Morgan’s law, which we will study in our article, answered these fundamentally important questions.
Why are some traits inherited together?
Observational geneticists, using in their studies the classical laws discovered by Mendel in the 19th century, faced insoluble problems. So, applying the law of independent inheritance of characters, the researchers could not explain the fact that the plant snapdragon has a dark red color of the corolla almost always accompanies the dark green color of the stem. In nature, the burgundy whisk and the salad color of the stem of this plant from the labiaceae family are extremely rare.
The law of Morgan’s linkage, an American geneticist who made a breakthrough in understanding the mechanisms of gene inheritance, helped to get the right explanation for this phenomenon.
Chromosomal theory of heredity
After the relative nature of the application of Mendelian laws was recognized by most scholars, the question arose of how to explain the fact of the simultaneous inheritance of two or more signs received from parents by the descendants. Thomas Gent Morgan proposed the idea of a linear arrangement of hereditary inclinations in the chromosome. He proved that the adjacent DNA sections in the process of meiosis pass into the same gamete together, and do not diverge into different germ cells. The scientist called this phenomenon the linkage of genes, and the law of Morgan has since come to be called the law of linked inheritance.
The geneticist combined the numerous experimental data collected into a harmonious scientific theory. It reflected the results of experiments, namely: it was proved that genes are located on the chromosome like beads, linearly behind each other. Thanks to Morgan's law, biology has received evidence that each chromosome that is not homologous contains its own unique hereditary composition. In addition, the scientist’s idea was confirmed that several genes located in neighboring loci are inherited together, and the number of such complexes is equal to a haploid set of chromosomes. So, in the human karyotype, there are 23 groups of gene linkage.
Morgan Law Discovery History
Biology knows many examples of how a correctly selected living object for experiments in the future fully determined the successful course of scientific research. Like Mendel, Morgan conducted thousands of experiments in his laboratory. However, for them he did not choose a plant containing hundreds of genes as part of his bulky karyotype, but an insect - the fruit fly, Drosophila.
Only four pairs of its chromosomes were perfectly visible under a microscope, and their simple gene composition easily yielded to statistics and study. The experiments of American genetics are based on the crossing of Drosophila parental organisms, which differed from each other in body color and wing shape. All descendants received were subsequently crossed only with flies that were black in color and had short, underdeveloped wings, i.e., an analysis of crosses was carried out. What were the results? They did not coincide with any of the known genetic postulates, since a part of flies with combinations of characters appeared in the offspring: a gray abdomen - underdeveloped wings and a black body - normal wings. The scientist suggested that the DNA sections that control the signs of color and shape of wings in insects of this species are located nearby - linked on the same chromosome. This idea was further expressed in the law of Morgan.
Crossingover
In the prophase of the first division of meiosis, an unusual picture can be observed: the internal chromatids of the sister chromosomes exchange loci - sites with each other. The closer the genes are located, the less often the crossing-crossingover occurs. Therefore, one of the provisions of Morgan’s law states that the frequency of exchange between genes is inversely proportional to the distance between them, measured in organisms. Crossover explains such an important phenomenon as hereditary variability. Indeed, the offspring of any parental couple is not like a clone that completely copies the characteristics of a father or mother. It has its own unique properties that determine its individuality.
The Importance of Thomas Morgan
The wording of Morgan's law, including the basic postulates that we examined, is widely used in theoretical genetics. All breeding work is based on it. Now it is impossible to breed a new breed of animals or plant variety without first predicting possible combinations of their expected useful signs or properties.
Creation of chromosomal maps of organisms, taking into account the provisions of the theory of heredity, helps doctors working in the field of medical genetics to identify defective genes in advance and accurately calculate the risks of pathologies in an unborn child.