5 Major theories regarding the mechanism of heredity, which shed considerable light on the subject. The theories bear the names of its protagonist’s viz. (0 Weisman, (ii) Galton, (iii) Mendal, (iv) Darwin and (v) Lamarck’s.
1. Weisman’s Theory:
According to Weisman it is the germ plasma that is transmitted from generation to generation. The human body grows from one germ cell by the process of MITOSIS (i. e., continuous bifurcation of cells).
At a very early stage some cells namely reproductive cells or germ cells are set apart. One of these germ cells of each parents is responsible for the new human body. These reproductive cells continue from one generation to another.
Weisman has given two generalisations, by which he explains heredity.
(i) The parent are only the trustee of the germ plasma rather than the producer of the child.
(ii) As the germ plasma of the parents are carried to the child, and the same continues to the next generation, so the heredity of a child is ancestral, nothing special from his parents. In other words, Weisman believed in the Non- Transmission of Acquired Traits.
If a person has acquired skill in music, his son will not inherit the acquired musical talent. If his son actually develops into a musician, it is because of the musical environment at home. Weisman conducted experiments on rats. He cut their tails, but the successive generations did possess tails.
2. Galton’s Theory:
According to Sir Francis Galton heredity does not go to immediate parents but to remote ancestors. Only 50% of the heredity is due to the parents (25% to father, 25% to mother). One fourth (or 25%) is due to grandparents (parental grand-father and mother and maternal grand-father and mother), one-eighth is due to great grandparents, one-sixteenth to their parents and so on. Total inheritance is which is equal to 1. This is represented by the following diagram.
Illustration 1. Heredity of an Individual according to Galton.
F= father, M=Mother, I= individual
3. Mendal’s Law:
Mendal explained the cause of variation in the off-spring of the same parents. He discovered two degrees of the same trait – dominant and recessive. The recessive trait recedes in the presence of the dominant. Tallness is a dominant trait, and shortness recessive. Mendal performed experiments on cross-fertilisation of tall peas and small peas (TT and SS).
The first crop was all tall peas with shortness as a latent recessive trait (symbolised as TS). Second time these were self- fertilised, and the second crop was pure tails (TT) 25%, impure tails (TS) 50%, and pure shorts (SS) 25%. Third time these were again self-fertilised. Pure tails got only pure tails (TT), pure shorts got only pure shorts. The impure tails when self-fertilised gave again the same ratio i.e., 25% pure tails, 50% impure tails and 25% pure shorts.
This showed that the recessive trait (shortness) lay latent with 25% chances to come to the forefront. Mendal performed similar experiments on rats. Black (B) rats were crossed with white (b) rats – male white and female black. Black is dominant over white. In the first generation, all were black, with whiteness as a recessive latent trait. In the second generation, there were only 25% pure white. The 75% black contained 25% pure black and 50% impure black.1 it might be symbolically represented by the binomial.
Another example is the cross-breeding of human beings with two divergent skin colour – black dominant and white recessive. At the second generation we will have 25% pure black, 50% mixed and 25% pure white. At the fourth generation we will have pure black one in sixteen (B4), less black four in sixteen (4B3 W), mixed six in sixteen (6B2W2), less white 4 in sixteen (4B W3) and pure white one in sixteen (W4)2.
Thus two pairs of genes of opposite trait would produce five colour gradations. Sex of the offspring is determined by genes when the sperm unites with the egg. One pair of chromosomes are called sex chromosomes. The two are like in females and are called X chromosomes. Males have one X chromosome and one tiny Y chromosome.
When sperms and eggs are produced, the chromosomes separate. So half the sperms are X sperms and half are Y sperms, but all eggs have X chromosomes. There is equal chance of an X sperm or a Y sperm fertilizing the egg. XY produces a male and XX produces a female.
Illustration 2. Mendal’s Law, Cross-breeding of black (B) and white (W) skin colour. B is dominant and W is recessive.
Mendal’s law is clearly understood, if the MITOSIS and chance variation in chromosome is understood. If the male has B dominant and W recessive genes and the female also similarly, when the fertilisation takes place, there are 4 chances. First chance is male B with female B, and that will give pure black. Second chance is male W with female W, and that will give pure white. The third chance is male W and female B, and the fourth is male B and female W. In these two cases, we have the mixed type.
4. Darwin’s Theory:
During the late 19th century an English scientist Darwin studied the vast arena of the species of the world and arrived at some generalizations.
(i) Wide variation exists within and among animal species.
(ii) Animals tend to over-produce their kind, leading to competition for food, space and mates.
(iii) This endless competition creates a struggle for existence,
(iv) The survival of the fittest is a process of natural selection in which the less fit tend to die out and the fittest live to reproduce their kind.
(v) Because of variation the difference in animals – some are adapted to survive in this struggle for existence than others.
(vi) The species of animals change slowly in response to changes in their environment. New variations are produced. (Darwin could not give exactly the cause and manner of variations).
The theory of natural selection is further explained as under:
(i) Under new conditions harmful traits are eliminated by selection.
(ii) Useful traits are strengthened and modified.
(iii) The great number of traits which are neither harmful nor useful pass on as such through heredity.
Darwin goes one step further to Weissman, Galton and Mendal, when he explains how changes in the genes produce new characteristics in the offspring. Such changes are called MUTATIONS. The variation in species is due to mutations. Over millions of years enough advantageous mutations would occur to produce the gradual evolution of animals.
The earliest horse was of cat-size. There are four stages of the evolution of horse from that size to the present size. From fossil remains of bones and teeth, comparable stages in the evolution of elephants, camels and monkey have been reconstructed.
Thus evolution of life took place by Natural selection of variations produced by mutations.
5. Lamarcks’ Theory:
Variations have taken place in species. Darwin explains in terms of struggle for existence and survival of the fittest. But Lamarck explained in a different way.
(i) A living creature has an inner urge to realize its needs and to adapt itself to its environment.
(ii) The inner urge causes modifications in the organs. Some organs are used and some are not,
(iii) Modifications acquired by the parents during their life-time are transmitted to the off-spring.
(iv) Through the use and disuse of organs, when modifications are acquired and transmitted to the next generation, this process from generation to generation and after some ages produces new species.
We may compare Darwin’s Theory with Lamarck’s theory by means of an example.
What made Giraffe to be very tall?
Darwin’s explanation is as follows:
Giraffe at one stage of its development was four feet long. As it multiplied, all the foliage within reach of animals of his height would be quickly consumed. Hence only the taller animals survived. In successive generations, only the taller and still taller animal survived, and the result is Giraffe of today. Lamarck gave the following explanation:
Giraffe had an inner urge to adapt to environment. The leaves of trees were generally high. In order to reach these, giraffe habitually stretched its neck. Thus it modified through successive generation as its bodily structure. The slight gains in each generation were transmitted to the next generation. After ages, we have the present giraffe.
Both Darwin and Lamarck agree that slight variations do take place in each generation. The favourable variations are passed on to the next generation who in turn vary about a new mean. Darwin gives here one new idea, i.e., there are sometimes sudden leaps away from the normal i.e., saltatory variations or mutations. The reason for these sudden leaps is not given.
As regards the cause of variations in each generation, both disagree. Lamarck explained through use and disuse of organs for the sake of adaptation to environment. Darwin explained through prolific nature of species, their struggle for existence, their shortage of food, their survival of the fittest, and elimination of the unfit, natural selection of better specie with acquired characteristics.