Spottedness

SPOTTEDNESS, a phenomenon of uneven coloring of the skin or its derivatives (hair, feathers, scales, etc.) in animals and of leaves and stems in plants with more or less sharp boundaries between areas of different shades. Spottedness can form as a result of causes of three different orders: 1) pathological processes, 2) the action of hereditary predispositions, and 3) the non-uniformity of hereditary material in different areas of the body's surface tissues. I. The formation of spots can be caused purely by mechanical damage, for example, horses often develop white spots on healed scars in areas previously rubbed by harness. Areas of hair of a different shade usually remain in the place of severe burns, which can be observed in animals that have been branded. Spottedness can also occur as a result of general body disorders—see Vitiligo. II. Hereditary spottedness (piebaldism in mammals) is determined by the action of hereditary predispositions in embryonic development and in some cases depends on the external environment. The most common cases are white spottedness, where white spots of various sizes are scattered on a pigmented background in various places. Usually, the skin coloration correlates with the hair coloration, although discrepancies are often encountered, i.e., non-pigmented hair grows on pigmented skin and vice versa. White spots can cover almost the entire surface of the body, leaving only small islands of pigmented hair by which the basic color inherent to the individual can be determined. White spots sometimes cover the entire surface of the body; this phenomenon is called leucism and differs from albinism in that the eyes and usually the skin have pigment (examples of leucism include the white spitz, polar bear, etc.). The patterns of inheritance of spottedness were first studied from a purely quantitative side, determining the percentage ratio of the area covered by hair of one color to the entire surface of the body. As a result of this approach, the view of the inheritance of spottedness according to the principle of univalent factors emerged. This approach did not yield real results, as spots on different parts of the body are inherited and develop to a large extent independently of each other. In the study of the distribution of spots on the body, there were basically two approaches. In 1903-1904, Ellen showed that pigmented spots in piebald mice are located on more or less certain places on the body, which he called pigment centers. Subsequently, he and other researchers extended this view to a number of other mammals. This hypothesis was criticized in 1928 by Ilyin, who pointed out mainly the following: a) often in the place of a pigment center there is not one, but several isolated pigmented spots, which contradicts the hypothesis of pigment formation from one center; b) even if one disregards the previous fact, it is still impossible to accurately determine the location of the pigment center, since in the case of the presence of one small colored spot in the area of action of the pigment center, the position of the latter is not definite; c) a number of studies indicate the unresearchability of pigment centers. As a result of studying spots and their inheritance in guinea pigs by Ilyin, the theory of depigmentation centers was proposed. According to this theory, in ontogenesis, areas devoid of pigment, i.e., white or having a lighter color than the rest of the body, for example, yellow on black fur, can spread from certain points on the surface of the body called initial depigmentation points. With weak action of the initial depigmentation point, only a few white (correspondingly light) hairs appear at a strictly defined point on the body (the initial depigmentation point); with strong action, a spot covers a certain area of the body surface characteristic of this initial depigmentation point. The largest depigmentation area that can form from one point is called the depigmentation field. The depigmentation fields of individual points overlap each other at their boundaries. With strong action of all initial depigmentation points, all hair becomes devoid of pigment (leucism). The inheritance of spottedness varies. There are both recessive and dominant genes for spottedness. The basic distribution of spots can vary depending on genes that enhance and weaken spottedness. For example, cattle have a dominant gene for white head. The area of its action can be reduced by a dominant modifier that gives a colored muzzle on the white head. Genes for spottedness are very common in domestic animals. An example of spottedness of the hair cover in humans is a dominant gene that causes white coloring of a strand of hair near the forehead. Among plants, cases of leaf spottedness, variegation, are common, depending on the unequal distribution of certain pigments that hide the basic pigment chlorophyll from our eyes. In some ornamental plants, there are white spots or stripes on the leaves as a result of the absence of chlorophyll in these places. III. The formation of spots can be caused by the non-uniformity of hereditary material in different cells of the body. The occurrence of such non-uniformity happens due to: a) somatic mutation, for example, in Leghorn chickens, which have a dominant gene for white color in the heterozygous state instead of the usual homozygous, a few feathers on the body have black coloration, since in the cell from which the cells making up this feather originated, a mutation of the type of loss of a chromosome (or part of a chromosome) carrying the said dominant gene occurred. As a result, the recessive coloration could manifest. The same is observed in the case of spotted potatoes, where obviously initially a mutation of one gene occurred in one cell; b) Dispermy, i.e., fertilization by two spermatozoa of two nuclei (usually the nucleus of the egg cell and the nucleus of the directive body), as a result of which the organism carries cells of two different