Regardless of the complexity of the structure of a biological object, be it a bacterium, an algae, an invertebrate animal or a person, the total number of all signs of any organism is much greater than its chromosome composition. Since the middle of the 20th century, science has known that such characteristics of a plant, animal or microorganism as the color and shape of the body, the size of the limbs, the characteristics of metabolism, 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? These fundamentally important questions were answered by Morgan's law, which we will study in our article.
Why are some traits inherited together?
Observational genetic scientists, using in their research the classical patterns discovered in the 19th century by Mendel, faced intractable problems. So, applying the law of independent inheritance of traits, the researchers could not explain the fact that the plant has a lion'spharynx dark red color of the corolla almost always accompanies the dark green color of the stem. In nature, the burgundy corolla and lettuce color of the stem of this plant from the labiaceae family are extremely rare.
Get a correct explanation of this phenomenon was helped by the law of linkage of Morgan, an American geneticist who made a breakthrough in understanding the mechanisms of gene inheritance.
Chromosomal theory of heredity
After the relative nature of the application of Mendelian patterns was recognized by most scientists, the question arose of how to explain the fact of simultaneous inheritance in descendants of two or more traits received from parents. Thomas Gent Morgan proposed the idea of a linear arrangement of hereditary inclinations in the chromosome. He proved that adjacent DNA segments in the process of meiosis pass into the same gamete together, and do not diverge into different germ cells. The scientist called this phenomenon gene linkage, and Morgan's law has since been called the law of linked inheritance.
The geneticist combined the numerous experimental data collected into a coherent scientific theory. It reflects the results of experiments, namely: it was proved that genes are located in the chromosome like beads, linearly one after the other. Thanks to Morgan's law, biology has received evidence that each non-homologous chromosome contains its own unique hereditary composition. In addition, the scientist's idea thatseveral genes located in neighboring loci are inherited together, and the number of such complexes is equal to the haploid set of chromosomes. So, in the human karyotype there are 23 gene linkage groups.
The story of the discovery of Morgan's law
Biology knows many examples of how a properly selected living object for experiments in the future completely determined the successful course of scientific research. Like Mendel, Morgan conducted thousands of experiments in his laboratory. However, for them, he chose not a plant containing hundreds of genes in its bulky karyotype, but an insect - a fruit fly Drosophila.
Only four pairs of her chromosomes were perfectly visible under a microscope, and their uncomplicated gene composition was easily amenable to statistics and study. The experiments of the American geneticist are based on the crossing of parent organisms of Drosophila, which differed from each other in body color and wing shape. All the resulting descendants were subsequently crossed only with flies that had a black color and short, underdeveloped wings, i.e., an analyzing cross was carried out. What were the results? They did not coincide with any of the known genetic postulates, since some flies appeared in the offspring with combinations of traits: 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 the wings are located nearby in insects of this species - they are linked in the same chromosome. This idea was further expressed in Morgan's law.
Crossing Over
In the prophase of the first division of meiosis, an unusual picture can be observed: the internal chromatids of sister chromosomes exchange loci - sections with each other. The closer the genes are located, the less exchange - crossing over - occurs. Therefore, one of the provisions of Morgan's law says that the frequency of exchange between genes is inversely proportional to the distance between them, measured in morganides. Crossing over explains such an important phenomenon as hereditary variability. Indeed, the offspring of any parental pair does not look like a clone that completely copies the characteristics of the father or mother. It has its own unique properties that determine its individuality.
The Meaning of Thomas Morgan's Works
The formulation of Morgan's law, which includes the basic postulates we have considered, is widely used in theoretical genetics. All breeding work is based on it. It is now impossible to develop a new breed of animal or plant variety without predicting in advance the possible combinations of their expected useful traits or properties.
Creating 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 calculate the risks of pathologies in an unborn child with high accuracy.
