Hypostasis

HYPOSTASIS (in biology), a term opposed to epistasis, introduced into genetics by Bateson to denote a type of gene interaction in an organism in which the action of one dominant gene is covered by the action of another dominant gene. The covered gene is called hypostatic, the covering one—epistatic or epistatic. In terms of external effect, H. is similar to recessiveness, and epistasis—to dominance, but with dominance the relationship between two allelomorphic genes of the same pair is considered, whereas here it is between completely different pairs. This difference finds reflection in the fact that hypostasis distorts typical Mendelian relationships: for example, the dihybrid ratio for the second generation (see Mendelism) of 9:3:3:1 changes into 12:3:1, and for the backcross of 1:1:1:1 into 2:1:1. Epistasis is known in animals and plants. The best example is white Leghorn chickens, in which the gene for white coloration covers the dominant gene for black coloration. Denoting the gene for black coloration by B and the epistatic gene for white coloration by A, we obtain for Leghorns the formula AABb. When crossed with red chickens, lacking the A and B genes, we obtain hybrids AaBb, which in the second generation will give 9 AB:3 Ab:3 aB:1 ab. But 9 AB and 3 Ab are indistinguishable in appearance due to the epistasis of A over B and will be white. aB will be black, having the B gene, and ab will be red, i.e., the characteristic ratio 12:3:1 will result. Just as dominance can be complete and incomplete, unnoticed passing into cases of absence of dominance, in which dominant and recessive allelomorphs turn out to be equivalent, so epistasis can be incomplete, unnoticed passing into cases of equivalence of genes, as in the example of chicken combs, where the gene for pea comb and the gene for rose comb, present simultaneously, give a new form of comb—the walnut form. The relationship of epistasis and H. is denoted by the symbol e>, and the expression Ae> B is read: 'gene A epistatizes over B'.

A. Serebrovsky.

HYPOSTASIS (in pathology) (from Greek hypo- under and stasis- stop), hypostatic congestion, a term used to denote diffuse stasis of blood in the capillary network of the low-lying parts of a given organ or of the whole body. Hypostasis usually develops on the ground of general stasis hyperemia due to a disturbance of those factors that determine the correctness of the general flow of venous blood. Among the causes disturbing venous circulation, the weakening of cardiac activity, which is the result of certain changes in the cardiovascular apparatus, stands first, and besides this the weakening of the suction action of the chest cavity. Stasis hyperemia in the lungs can also be caused by an obstruction to the outflow of blood from the lungs due to a pathological change in the valve apparatus of the heart. In the presence of these factors, there is a predominant filling of the lower parts of the organs with blood, depending on the fact that the venous blood, detained in its advance and obeying the action of gravity, fills the capillary network mainly of the low parts of the organs. Thus, in persons with weakened cardiac activity, H. develops in the lower extremities, which in the future gives edema of these parts. In weak patients lying in bed, H. of the lungs develops, especially sharply expressed in the posterior and lower parts of them, which is sometimes accompanied by edema of the latter. At autopsy in these cases the indicated parts of the lungs are felt to be somewhat dense, on section are dark red in color, and when compressed from the section a bloody fluid flows out. Due to the displacement of edematous fluid (transudate) from the alveoli, these areas of the lungs can become airless and sink in water. Microscopically in H. of the lungs, a marked dilation and filling of the capillary network of the alveolar septa is detected, and in the alveolar cavities—edematous fluid. Prolonged H. of the lungs favors the development of pneumonia (hypostatic pneumonia). Besides the cases indicated above, H. of the lungs occurs in patients in the atonic period, which is why patho-anatomists often discover them at autopsies. Along with in vivo or atonically developing hypostases of the lungs, one always observes the development of posthumous, or cadaveric, H. at autopsies. The latter occurs due to the fact that under the influence of the action of gravity, blood from the veins of the high-lying parts of the corpse flows into the small vessels of the lower-lying parts of it. Usually these cadaveric hypostases (livor mortis) are localized in the lower-lying parts of the skin corresponding to the position of the corpse, while the places subjected to pressure remain pale. The localization of hypostases on the corpse makes it possible for forensic experts to resolve questions as to in what position the corpse lay during the first day after death (see Corpse). Similar hypostases are detected in the corpse in internal organs. They develop approximately 3-6 hours after death. When the position of the corpse is changed in the first 12-15 hours after death, the livor mortis of the integuments can move, which also has a certain forensic-medical significance in some cases. - In various poisonings, the color of H. changes depending on the change in the blood Hb. Thus, in death from carbon monoxide poisoning, livor mortis is bright red, in potassium cyanide poisoning—cherry-colored, in barium chloride poisoning—brown, in asphyxial death, livor mortis is sharply expressed, blue-purple in color, and often with a large amount of ecchymoses.