Physical Basis of Heredity with Theories | Genetics

In this article we will discuss about the physical basis of heredity with theories investigated by a great number of workers, some of whose results are as follows:

Encasement Theory:

One of the older, but now abandoned, ideas was set forth in the preformation or encasement theory, which supposed that the germs of all future genera­tion were encased one within the other like Chinese boxes, in the progenitor of the race and were successively developed into perfect individuals with each arising generation.

Thus Mother Eve, taken as the ancestress of all mankind, was estimated to contain no fewer than two hundred million homunculi, an assumption that spoke well for the imagina­tion of the older writers.

Darwin’s Pangenesis Theory:

Darwin realized that his doctrine of natural selection lacked completion if he merely accepted the fact of heredity at its face value without trying to learn in what way mental or physical traits or new variations got into the egg or sperm and thus were borne on to the next genera­tion.

He, therefore, devised the theory of pan­genesis according to which every cell of every tissue and organ of the entire body produces minute particles called gemmules, which, in each instance, partake of the nature of the cells producing them. These gemmules may circulate throughout the entire organism but finally congregate in the reproductive products or in buds.

So each germ cell or each bud (asexual) capable of giving rise to a new indi­vidual would be, in a sense, a miniature repli­ca of the parents’ body—and would thus be capable of developing into the same kind of a body even in minute detail.

Darwin thought that sometimes certain gemmules might be dormant for several gene­ration and then develop, bringing out in an individual its ancestral traits. If this theory of gemmules were true, it would lead to the acceptance of the teaching of Lamarck or even of Buffon, because, under such circumstances, the inheritance of acquired characters gained by the parent during its lifetime would be entirely feasible, for such modification could be impressed upon the germ cell as readily as could the variations which we call germinal.

Later research, however, proved that such a theory has no basis in fact and biologists soon began to search for a substance which could be a physical basis for heredity. A paral­lel is to be found in the search of the proto­plasm—which makes up the bulk of all organ­isms and is the physical basis of life.

While sev­eral biologists were convinced that there was such a substance, it remained for August Weismann, professor in the University of Freiburg, for nearly fifty years to identify and demonstrate its existence.

Weismann’s Germplasm Theory:

Weismann’s germplasm theory was published in 1892 in a volume called Daskeimplasma, wherein he brought together the accumulated observations of the numerous contemporary students of cell biology and utilized them for the development of his somewhat speculative theories. This volume crampton cells “on immortal foundation for all later work on inheritance”.

The essential principles of the germplasm theory are as follows : Weismann divided all the substances of an organism into two parts—the germplasm and somatoplasm; the former, the protoplasm of the germ cells, being the actual vehicle of heredity, while the latter—which constitutes the remainder of the plant or animal body—was not.

In reproduc­tion, a portion of germplasm is derived from each parent, the paternal sperm and maternal ovum, as the case may be, each of which has an equivalent share in the inheritance. These combine to form the fertilized egg. When the egg divides in the first or subsequent cleavages each daughter cell receives an equal share of germplasm, and this holds true for all cells which go to form the adult body.

None of the somatic cells, however, has any share in sub­sequent generation but only the germ cells which are derived directly from the original egg. It follows, therefore, that there is a con­tinuous stream of germplasm from generation to generation, to which the somatic cells— which are sister products from the original egg—contribute nothing.

Hence only those mutations which are germinal in their origin can possibly be handed down, and as the hereditary stream of germplasm is already set apart before the adult body is in use, one can not see how any modifications—impressed upon the latter through use or disuse, or extrinsic adaptations to the environment—can possibly become a part of the organism’s heritage.

In certain of the Protozoa not only is the germplasm handed down, but the entire body of the organism may be divided between the off-spring. Yet other Protozoa, which repro­duce by budding, give up only a portion of their nucleus and cytoplasm to their children and the same is true of all organisms which reproduce asexually.

On the other hand, there are plants and animals—the hot house Bigonia and the earthworm for example—in which purely somatic cells have the power to regenerate a complete organism, including reproductive organs and germplasm.

In the earthworm the reproductive organs and germplasm reside in the anterior segments and yet no matter where the worm is cut as under, if enough of the tail remains, it will reproduce the entire organism, including the reproductive bodies, with all of its specific hereditary details.

This shows that “We must postulate at least a potential supply of the germ residing in the somatic tissue which can make good the definitive germ cells when they are lost. It is a fact that there is a conti­nuity of germplasm, whether the germ cells are set aside early in individual development or later by the transformation of relatively undifferentiated typical somatic cells: this is really the crux of the question.” (Woodruff).

True freedom from all outside influence in these circumstances would be little short of marvellous and yet that is what we are led to believe.

Prenatal Influence:

There is a widely held belief that in mammals, a special addi­tional formative influence is everted by the mother in the period between conception and birth. Maternal impression, so-called, docs not refer to the results of good or poor nutri­tion, which are discussed below, but to the development in the unborn young of definite anomalies—birthmarks and the like.

While instances are numerous, they have never been given accurate scientific research and are still open to question; nevertheless, many people believe in prenatal influence and the belief is very old.

The first recorded instance known to us is in the Bible—Genesis XXX, 32-42, dating back to the time of the patriarch Jacob, about 1730 B.C. Jacob was to receive as his share of Laban’s flocks and herds all the speckled and spotted cattle, and all the brown cattle among the sheep, and the spotted and speck­led among the goats while Laban was to retain such as were not so marked.

So Jacob “took him rods of green poplar, and of the hazel and chestnut tree, and pilled white streaks in them, and made the white appear which was in the rods. And he set the rods which the hand pilled before the flocks in the gutters in the watering tours when they come to drink.

And the flocks conceived before the rods, and brought forth cattle ring-streaked, speckled and spotted”. Jacob was shrewd enough, however, to put the rods before the eyes of the stronger cattle, “but when the cattle were feeble, he put them not in: so the feebler were Laban’s and the stronger Jacob’s”.

Vestigial Organs:

Vestigial organs are such as are not fully developed, and are to be contrasted with rudimentary structures which are in process of evolutionary growth and, thus, are progressive.

The vestigial organs are retrogressive and are tending toward diminu­tion and ultimate loss. Rudimentary horns are often observed on the head of fossil ungulates such as titanotheres and rhinoceroses, where­as the splints on either side of the cannon- bone of a horse’s foot are vestiges of former­ly useful lateral toes. The vesitigial organs are, therefore, of historical importance and would not exist—especially where their old-time function has entirely ceased—were it not for heredity.

Of such are the vestigial hind limbs seen in the python and other related snakes. Externally they are mere spurs on either side of the vent, internally they are seen to contain several of the bones normal to a fully formed limb, ilium, femur, tibia, and claw hair of their ages—vanished forebears, and also, as Kukenthal has shown, the relics of hind limbs as well-developed buds.

At birth, however, the hair has been shed except for a few bristles about the lips and all external trace of hind legs has gone, there being only a few bones as in the python. Buried deep within the mass of mankind, according to Wiedersheim, are many such relics.

Inheritance of the Results of Func­tional Disuse:

This was apparently the sim­plest and most logical way to account for the atrophy of parts in evolution until Weismann’s epoch-making work cast doubt upon it. It is a well-known fact that, with the individual, use strengthens an organ while disuse causes it to weaken and partially atrophy, witness the Hindoo fakirs who hope to acquire a state of singular holiness through mortifying the flesh.

Some of them keep the arms raised perma­nently above the head, with a consequent shrinkage of muscle and stiffening of joint until the limb could not be used if they would. But, as Weismann has shown, such modifications are those of the mortal somatoplasm and can­not, apparently, impress themselves upon the race. How then can this explanation account for the evolutionary atrophy of parts unless there is still some factor of heredity which we know not of?