In this article we will discuss about:- 1. Definitions of Heredity 2. Mechanism of Heredity 3. Theories of Heredity 4. Laws of Heredity 5. Studies and Experiments on Heredity.
Definitions of Heredity:
Under heredity we include all the factors that were present in the organism at birth. In other words, it is the sum-total of inborn individual tracts. Heredity has two aspects – biological and psychological. Biologically, it means the sum total of biological traits that are present in the fertilised ovum. Psychologically it means the traits, innate tendencies, and capacities that resemble between the parents and the child.
(i) Biological Heredity:
It is a general observation that cow begets cow, horse begets horse, dog begets dog, and human beings beget human beings. There is no exception to this rule. Then again, in the same, the brothers and sisters bear resemblance to their parents as regard the physical characteristics such as size, form, appearance, colour of the skin, colour of the eyes, strength of muscles, invertible disease etc. Identical twins resemble each other at birth in every respect. Fraternal twins resemble in majority of the traits. Siblings resemble in many characteristics.
(ii) Mental Heredity:
Besides the physical characteristics, many mental characteristics are inherited. Every child at birth instinctively sucks, breathes, laughs, cries and moves limbs. All the instincts are inherited. Intelligence also seems to be inherited, because in a number of cases the intellectual caliber of children resembles that of their parents. Some tendencies to commit crime also are reported to be inherited, as evidences by case-studies.
Definitions of Heredity:
1. James Drever:
“Heredity is the transmission from parents to offsprings of physical and mental characteristics.”
2. H.A. Paterson:
“Heredity may be defined as what one gets from his ancestral stock through his parents.”
3. R.S. Woodworth:
“Heredity covers all the factors that are present in the individual when he begins life not at birth, but at the time of conception about nine months before birth.”
4. O.B. Douglas and B.F. Holland:
“One’s heredity consists of all the structures, physical characteristics, functions or capacities derived from parents, other ancestry or species.”
5. Ruth Bandict:
“Heredity is transmission of traits from parents to offsprings.”
6. J.A. Thomson:
“Heredity is mainly a convenient term for the genetic relation between successive generations.”
7. P. Gisburt:
“Every act of generation in nature is the transmission by the parents to their off-spring of certain characteristics biological or psychological. The complex of the characteristics thus transmitted is known by the name of heredity”.
Mechanism of Heredity:
Mating is the first step for reproduction. It is essential for the union of the male germ cell spermatozoa with the female ovum. The union causes fertilisation of the ovum. Growth and development takes place by the multiplication of the fertilized cell through continuous division (or mitosis).
Growth involves repeated division of the fertilised cell or zygote.
The fertilised ovum consists of a semi-fluid mass called cytoplasm, and within that there is a small nucleus of denser material. The nucleus includes the elementary carriers of the traits of both the parents, called chromosomes. In a fertilised ovum, there are 24 pairs of chromosomes, 24 from father and 24 from mother.
Each chromosome consists of small particles numbering 40 to 100, which are called genes. In fact the genes are the real carriers and determinants of heredity. Each gene carries some structure which is responsible for the development of a particular trait. There is experimental evidence to prove that the physical traits, like the colour of the eyes, the skin, the size, the type of blood, and some organic diseases are represented by respective genes.
5. Chance factor:
Both the ova and the sperm before fertilisation contains 24 pairs of chromosomes each. On fertilisation, there will be not 24 + 24 or 48 pairs, but only 24 pairs. Others drop out equally from both. Which 24 of 48 chromosomes of the sperm or the ova will continue, is a matter of chance. A mathematician will tell us that there are millions of permutations and combinations possible for the chromosomes. That is why no two persons can be identical.
Theories of Heredity:
It will not be out of place to discuss here 5 major theories regarding the mechanism of heredity, which shed considerable light on the subject.
The theories bear the names of its protagonists viz.:
(iv) Darwin, and
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 parents are only the trustee of the germ plasm rather than the producer of the child.
(ii) As the germ plasm 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 killed their tails, but the successive generations did possess tails.
2. Gallon’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 grand-parents (parental grand-father and mother and maternal grand-father and mother), one-eighth is due to great grand-parents, one-sixteenth to their parents and so on.
Total inheritance is:
This is represented by the following diagram:
3. Mendel’s Law:
Mendel explained the cause of variation in the offspring 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 talls (TT) 25%, impure tails (TS) 50%, and pure shorts (SS) 25%. Third time these were again self-fertilised. Pure tails got only pure talls (TT), pure shorts got only pure shorts. The impure talls when self-fertilised gave again the same ratio i.e., 25% pure talls, 50% impure talls and 25% pure shorts.
This showed that the recessive trait (shortness) lay latent with 25% chances to come to the forefront. Mendel 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. 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 (4B3W), mixed six in sixteen (6B3W2), less white 4 in sixteen (4BW3) 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. Mendel’s Law, Cross-breeding of black (B) and white (W) skin colour. B is dominant and W is recessive.
Mendel’s law is clearly understood, if the MITOSIS and chance variations 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 generalisations.
(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 Mendel, 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 live 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.
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, elimination of the unfit, natural selection of better specie with an acquired characteristics.
Laws of Heredity:
The above discussion of the various theories leads us to the following generalisations:
1. Like Begets Like:
It is a common observation that children are alike their parents. ‘A youth is chip of the old block’. A child takes after his parents in size, colour, dullness, brightness etc. It has been explained by some biologists that children are bound to resemble their parents closely because of the continuity of the germ plasm. Thus a cat gives birth to kittens. A negro gives birth to a negro.
This theory has now been criticised in numerous ways. Firstly, we see that many exceptions take place. Children of bright intelligent persons are not always bright. Criminals do not always beget criminals. Secondly it ignores the effect of environment. Role of environment is equally important. In words of Bagley, “environment affords the stimulus; heredity determines largely the nature of the reacting substance; the reaction or behaviour is the product of the two.”
The effect of environment is discussed below:
2. Law of Variation:
The exceptions to the above rule are explained by this law. Variations take place at a number of steps during the process of fertilization and birth. There is double the number of chromosomes in the sperm or ovum. There is chance for variations when only 24 pairs are selected. There is chance for variations where these take place certain combinations of genes.
In any human this permits 16,777,216 different combinations of chromosomes. So, 16,777,216 different kinds of sperms and 16,777,216 different kinds of eggs are possible. Any sperm might fertilize any egg. So almost 300 trillion different kinds of fertilised eggs are possible. Thus each human is literally unique.
This process called MEIOSIS explains how different combinations of genes give birth to different types of children. The parents are the same, but the chromosomes that combine are different for each off-spring. These chromosomes also do not remain constant throughout the life. Much depends upon the physical and mental characteristics of the parents at the time of each conception.
Mendel has further explained the causes of variations when two dissimilar chromosomes combine.
To what extent do variations take place? There is a limit to variations. There is a greater tendency in the offspring to be like the parents or like each other, than to be dissimilar.
In this respect, Woodworth gives the following correlation coefficients between children of the same parents:
3. Law of Regression:
The law states that “In successive generations variations tend to move towards the average of the species of which they form a part.”
If the father is a tall man, his son may be tall, but not so tall as his father. He will regress towards the normal height of the human beings. This law is outcome of statistical calculations, and the statistical law called ‘Normal Probability’. There is always, a mean height, mean weight, mean I.Q. mean strength etc. 67% cluster round this mean.
Only 16 ½ % are abnormal on the positive side, and 16½% on the negative side. There is a limit beyond which no healthy or normal person goes. The limits are determined by three standard deviations on each side of the mean. Suppose 60 is the average height. The range of maximum and minimum may be 50-70.
Persons belonging to the height range 55-65 may be considered average or normal. Their children will have the same height. But suppose a parent’s height is 70. Will the off-spring have the same height ? The offspring will avoid the abnormality. It will try to regress to the normal height, and be of 65″ height, but in no case 70″. Similarly a dwarfish parent of 50″ height will have an offspring with 55″ height, but in no case 50″. The regression is always, towards the normal.
Galton has already given a reason for regression, when he states the heredity of a child is determined by his immediate parents only 50% (½), and the remaining is determined by the ancestoral parents.
4. Transmission of Acquired Traits:
This is the most controversial topic:
(i) Weisman refuted the possibility of the transmission of acquired traits from one generation to another, as he thought the parent is only the trustee of the germ plasm than the producer of the child. He proved it by experimenting on rats and cutting their tails.
(ii) Lamarck had a definite belief that favourable variations are passed on to the next generations, and so did Darwin.
(iii) McDougall conducted an experiment, by which he could prove that acquired habits are in some degree transmissible. He placed rats in a tank with two gangways, one of which was charged with electricity. The first generation of rats committed errors of getting shock by going through the charged gangway. But in succeeding generations, errors decreased. In the 23rd generation, only 25 errors were committed. Thus the lessons learnt by each generation were unconsciously transmitted to the next.
McDougall states that instincts are also modified and transmitted. Instincts are racial habits which have been transmitted slowly to each generation. These were originated for self-preservation and race preservation. Animals in danger acquired the habit of running away. Successive generations acquired this habit and transmitted to the next. Thus the engram complex determining the instinctive action developed in the individual independently of his experience. This constituted an instinct.
Studies and Experiments on Heredity:
A number of studies have been conducted regarding the influence of heredity. The studies are either case studies or genetic studies or experimental studies.
1. Everyday Experience:
We have experience of observing that like begets like, that the children of intelligent and cultured people tend to grow bright from the very beginning. Great men had great mother or great father. There are individual variations in the achievement of children brought up in the same environment. Such a difference must be largely the result of heredity or native difference in endowment.
2. Biographies of Great Men:
Great men were exceptionally bright from the very beginning:
(i) Goethe, at an early age, had done extensive reading in 4 languages viz. German, French, Latin and Hebrew. He also played piano and flute. He knew the history of chief European countries in detail.
(ii) Sir Francis Galton could read when two and half year old. Darwin, his cousin was a similar genius.
(iii) Voltaire began to read at three. At twelve he wrote a tragedy.
(iv) Macaulay could read at four. At eight, he wrote a treatise to convert the natives of Malabar to Christianity.
(v) Sir Issue Newton, in his childhood could devise many types of machines, especially water- clocks and kites.
(vi) Sir Walter Scot and John Stuart Mill were child prodigies.
(vii) Shankaracharya, the great Indian philosopher had a complete mastery over the whole of Sanskrit literature at the age of 16. At that age he wrote his famous treatise on Vedanta.
(viii) Swami Rama Tirtha, Lala Hardayal, Ramanujam (the mathematician), Vivekananda, and many other Indian genii vouchsafe the fact that they had some inner potentiality.
3. Studies of Family Histories:
Some psychologists have studied different generations of some families to find the connection between the traits of the individual with the family:
(i) Galton Francis, in 1869, prepared a list of977 famous individuals, their nearest blood relations out of whom 536 were famous. Hence, there was high correlation. In another list 977 common individuals he found only 4 near relatives who were famous.
(ii) Dr. A.E. Winship, studied some Edward family. The children of Edward born of a talented lady, and their successive generations came out to be famous. But the children of Edward born of another ordinary lady came out to be ordinary.
(iii) Karl Pearson studied the family of Galton, Darwin and Bezwood (all cousins). For five successive generations, the members of this family had a place of the ‘Royal Society’ of England.
(iv) Dugdall and Istabruke studied the Juke family. Juke was a corrupt man and he married a corrupt woman. Out of the 5 generations of this family, about 1000 persons were born. Out of these 300 died in infancy, 310 were in orphanages, 440 were chronic patients, 130 were criminals, and only 20 could learn some vocation.
(v) H. H. Goddard studied Kalikak family and its five generations. During the American revolution Martin Kalikak (the name is fictitious) had an illegitimate son by a girl known to be mentally defective. From his son have come 480 descendents. Of these 143 were feeble-minded, 46 normal, and the rest abnormal. Among the total lot, 36 were illegitimate, 33 were sexually immoral (mostly prostitutes), 24 were drunkards, 3 were epileptics, 82 died in infancy, 3 were criminals, and 8 were ill-famed.
After the American Revolution, Martin married a normal woman. Goddard traces 496 descendants of this union, all of whom were normal mentally and morally. Several were superior too. In this group, we find, lawyers, professors, doctors etc. The two lives of generations thus differed sharply.
4. Study of Siblings:
(i) Leathly in one of the best controlled study of adopted children found a considerable high relationship between the intelligence of children and their own parents, than between adopted children and their foster parents.
(ii) Pearson found that the coefficient of correlation between 2000 pairs of siblings as regards their personality traits was 50.
5. Study of Twins:
Twins are of two types-identical and fraternal. The twins that are developed from one single fertilized ovum are identical. Thus their genetic elements are the same. On numerous studies of identical twins, it has been found that identical twins, even though reared part had the same I.Q., with only a slight difference (maximum 5). Their correlation of traits was 9.
As regards fraternal twins, the correlation was .63 to .7, and the I.Q. differences were in no case more than 9.