Read this article to get study notes on the Effects of Heredity and Environment on a Person.
The behaviour genetics studies relations between heredity and environment, the two extremes in the level of biological organization. Heredity refers to a biological mechanism as a result of which a child obtains something in terms of specific species or ancestral characteristics by which he can trace his individuality through his parents.
Heredity involves the transmission of genes from generation to generation, their behaviour and functions. Heredity is defined as the transmission of characters or factors from parents to the offsprings.
Many aspects of human behaviour and development, ranging from physical characteristics such as height, weight, eye and skin colours, and the complex patterns of social and intellectual behaviour are influenced by a person’s genetic endowment. They also include physical deficiencies and the nature of glandular functioning.
Functional units of heredity are the genes. Heredity is a source of both similarities and differences among individuals. The same heredity mechanism that leads to resemblance of parent and offspring and also to produce differences among members of a family.
a. Sex-Linked Inheritance:
The nuclei of human cells contain 22 autosomes and two sex chromosomes. In females, the sex chromosomes are the 2x chromosomes; males have one X chromosome and one Y chromosome.
Thomas Morgan first discovered that some traits are sex-linked, that is passed on only to one of the sexes, i.e. either males or females.
Each egg produced by the female contains one X chromosome, but the sperm produced by the male can contain either a X or Y chromosome. The union of an egg, which always bears a X chromosome, with a sperm also bearing a X chromosome produces a female offspring carrying XX chromosomes. The union of an egg with a sperm that bears a Y chromosome produces a male offspring carrying XY chromosomes.
The human Y chromosome is approximately one-third of the X in its length and apart from its role in determining maleness, it appears to be genetically inactive. Most genes on the X have no counterpart on the Y. Recessive traits which are masked in females in the heterozygous state but will be exposed in males, who cannot be heterozygous at this loci on the X chromosome.
Sex-linked recessive traits appear in males at the same frequency as the alleles frequency. But most sex-linked recessives occur in the heterozygous state in females. Thus the phenotypic frequency of sex-linked traits is very much lower in females. Males transmit their X chromosome to their daughters but their Y chromosome to their sons.
Therefore, all alleles on the father’s X chromosome will be present in his daughter’s, but none in his son. X-linked recessives cannot be transmitted from father to son, but all daughters of an affected father will be at least carriers.
The genes occurring only in the X chromosome rare represented twice in female and once in male. If the recessive type of genes occur in X chromosomes of males they express themselves phenotypically.
This is because in such case Y chromosome contains no dominant allele to overcome the recessive gene of X chromosome or on the analogous Z chromosome called X- or Z-linked genes. The genes which occur in Y chromosome are called holandric genes. The inheritance of X- or Z-linked and holandric genes is called sex-linked inheritance.
In XX-XY type organisms, sex- linked genes can be classified into following three types:
The X-linked type sex-linked inheritance is performed by those genes which are localized on the non-homologous sections of X chromosome and that have no corresponding allele in Y chromosome. The X-linked genes are commonly known as sex-linked genes.
The Y-linked type sex-linked inheritance is performed by those genes which are localized in the non-homologous section of Y chromosome and that have no alleles in X chromosome. The Y-linked genes are commonly known as holandric genes.
The XY-linked type sex-linked inheritance is performed by those genes which are localized in homologous sections of X and Y chromosomes. Heredity, thus contributes something in the form of inheritance just as we inherit land, money and other assets or liabilities from our parents and forefathers.
The science of genetics is concerned with the way certain characteristics are transmitted through the species and through a family to an individual organisms. This transmission definitely occurs at the time of the conception of the child in the womb of the mother. It is the beginning of the new life.
b. The Role of the Genes:
Genetics takes its name from the ‘genes’ which represent the unique determinants of hereditary traits. Chromosomes act as the housing elements for the fundamental carriers of hereditary traits, the genes.
The gene is the biologists’ concept of the structure which is responsible for transmitting such traits as eye colour, hair colour, height, colour of skin, texture of hair, shape of the nose, etc.
There is sufficient evidence to assure us there is existence in numbers of genes from 20 to several hundred in a single chromosome. It is estimated that any individual carries a minimum of 12,000 pairs of genes.
c. How Two Sexes Produce Variation?
With the union of an egg cell and a sperm, there takes place the pairing of chromosomes from two parents, both of whom have diverse hereditary backgrounds of their own. From this reshuffling of chromosome pairs, there arises innumerable gene combinations. These show themselves in the endless chain of variables seen in the faces and bodies of men and women.
Beginning with a number of sperms and number of eggs manufactured by the average human and multiplying this by the number of genes involved, “Dr Bentley Glass”, computed the number of possible combinations resulting from the union of a human egg and sperm to be almost 300 billion. Thus no two individuals produced from separate fertilizations ever have chance to be identical.
Gregor Mendel, an Austrian, experimented for 8 years with many varieties of garden peas, he cross-bred and in-bred his varieties, always concentrating on pairs of contrasting traits by manipulating the pollens of these plants he crossed tall peas with short peas. Wrinkled peas with smooth peas, red flowers with white flowers, green with yellow seeds and many other combinations.
d. Mechanism of Conception:
When the male and female reproductive organs produce germ cells, that is, in the male their testes produce male germ cells, the spermatozoa while in the female the ovaries produce the female germ cells, the ova. Normally, the ovum or egg is produced in each menstrual cycle (28 days) by the ovaries of a normal woman.
The production of the sperm by the testis in the male is not confined and limited. Normally, they produce 10 million sperms per day per gram of testicular tissue from the onset of the puberty till death. Conception is the result of the union of these male and female cells in the natural way. This union occurs at the time of copulating between a man and a woman.
Here as a result of coitus, the male germ cells (millions in number) usually come in contact with the female germ cells. The male germ cells are deposited at the mouth of the interest and try to make contact with the single ovum.
Out of so many spermatozoa, in a normal case, only one sperm (single male cell) is able to establish contact with the ovum (single female cell) situated in the ovarian duct of the mother and makes it fertile. The fertilized ovum is technically known as zygote which is the starting single cell structure of a new life.
Human life thus starts from a single cell, produced by the union of two germ cells, one from each parent and gradually develops into a complicated composition of trillions of poly-cells and yet containing the same genetic material as was inherited at the time of conception.
The zygote, i.e. the fertilized ovum consists of a semifluid mass called cytoplasm and within the cytoplasm there is a nucleus which contains the chromosomes. Chromosomes always exist in pairs.
In human zygote there are 23 pairs of chromosomes (46 individual chromosomes), 23 of which are contributed by the father and 23 by the mother and this is why for the transmission of hereditary characteristics both mother and father are said to be equal partners.
Chromosomes possess a thread-like structure and are made up of very small units called genes. It is estimated that there are more than 1000 genes in each human chromosome cell, consisting the possibility regarding the combination of 30,000 characteristics each from father and mother which may help us to understand the uniqueness of each individual.
Genes are composed with DNA (Deoxyribonucleic acid), said to be the basic chemical substance primarily responsible for genetic inheritance and RNA (ribonucleic acid) acts as an active assistant to DNA for carrying out the genetic code message from parent to offspring.
Thus, what we get from ancestral stock through our parents at the time of fertilization of the ovum of the mother by the sperm of the father is in the form of chromosomes, genes and their respective classical constituents. This inheritance at the time of conception makes up the native capital and endorsement of an individual, i.e. present in the fertilized ovum and it is this, that is known as the heredity of an individual.
e. Dominants versus Recessive Traits:
Gregor Mendel observed that when he cross-bred his plants, certain traits where passed on without alteration from generation to generation. He also observed that some traits appeared more pronounced and more frequently than others. There was thus introduced the idea of dominance and recessiveness in traits.
For example, when Mendel crossed the tall strain of pea with the dwarf type of some species, all of the products of this first cross-breeding showed up as tall. Then when this first generation of tall peas where self-fertilized the resulting offspring showed a ratio of three tall peas to every one of the dwarf types. Tallness was thus designated as the dominant trait while the dwarf strain were considered to be a recessive character.
Blending of Traits:
Aside from exhibiting dominance and recessiveness, certain traits appeared to be inherited as a blend of two qualities. For example, the skin colour of human beings is thought to give either imperfect dominance or the fact that two or more pairs of genes are involved in producing the characteristic trait.
In the mating of negroes and whites, a true blend appears to operate in determining skin colour. From the resulting ratios, it seems that four genes in two pairs are involved with none of them dominant or recessive. Thus the first generation offspring of a negro and white will all be of an intermediate skin colour.
If a male and female offspring of two such mating were to get together and produce enough progeny, the following ratio would result, i.e. one in 16 would be pure negroid, one pure white, 4 dark- skinned intermediate skin colour, 4 light skin colour and 6 in between the dark and light-skinned colour.
Thus the offspring of intermediate parents can produce either dark-skinned children or white-skinned children, depending upon the number of offsprings and loss of chance.
f. Twins Mechanism:
There are two distinct types of twins, namely:
i. The identical twins
ii. The fraternal twins.
i. Identical twins:
In the process of the fertilization of the ovum by the sperm the ovum is made to split into two parts. In a normal process the parts are again united. Sometimes, however it so happens that these two split parts fail to unite together.
The result is that each part is developed into a complete individual in the form of pair of identical twins. The twins produced are thus, termed identical on account of the identical nature of the genetic material, i.e. exact number of chromosomes and genes, etc. They are of same sex and somatic nature.
ii. Fraternal twins:
Normally, in each menstrual cycle the female ovaries produces a single ovum that can be fertilized by a sperm cell. In an exceptional case, two ova may be produced simultaneously and be fertilized at the same time by two different sperms.
It may then result in the conception of two individuals who may grow simultaneously in the womb of the mother. They have different combinations of chromosome and genes as both ova are fertilized by different sperms. Thus they differ in many traits.
Principles of Heredity:
The five important principles of heredity are:
1. Like begets like:
Each species breeds true to type. In other words, offsprings possess characteristics and racial differences, especially physical, similar to their parents, e.g. tall parents generally have tall children.
2. Principle of variance:
Even if like tends to beget like, parents and offsprings are not exactly alike. Resemblances of parents and their offsprings are never perfect. All tall parents do not have tall children.
3. Principle of convergence of two life streams:
Offsprings inherit physical and mental characteristics from parents and their ancestors. A portion of inheritance comes from the maternal side and remaining from paternal side that is the child’s maternal and paternal lines contribute about 50% each of his inheritance. It is generally assumed that ½ comes from parents, 1/4 from his grandparents, 1/8 from his great grandparents and so on from all the other more remote ancestors.
4. Principle of chance possibility:
As chance plays important role on inheritance, making absolute predication is impossible because the pairing of the chromosomes is in the state of flux, a cell to which the set of maternal or paternal chromosomes goes during the reduction and division, the particular cell which unites with another in the maternal and paternal lines, the pattern of genes in any chromosome; genes carried in any particular chromosome; crossing over of genes from one paired chromosome to another.
5. Principle of dominant and recessive:
One of the reasons for individual differences is some trials are dominant while others are recessive causing apparent differences. The union of the best traits of the father with the best traits of the mother produces talented children. But such gifted may carry on the determines of genes, which are average. There may be chances that when they produce a child, their average traits combine and a child of average caliber may be born.
Principles of heredity observed help us to understand that:
(i) Many aspects of human behaviour and development ranging from physical characteristics such as height, weight, eye colour, skin colour, etc., complex patterns of social and intellectual behaviour are influenced by a person’s genetic endowment.
(ii) They also determine nature of physiological functioning cause physical deformities.
(iii) The heredity mechanisms lead to resemblance between siblings as well as the differences among the members of the family.
(iv) The heredity basis of individual differences lies in the unlimited variety of possible gene combinations that can occur. These differences become more pronounced with the age and maternity.
The disorder known as phenylketonuria (PKU) is due to the lack of a specific gene. It arises due to inability to metabolize phenylalanine. This on complete oxidation, results in the accumulation of phenyl pyruvic acid in the brain, causing severe mental retardation.
This inherited disorder can be identified by early medical testing and can be controlled by the diet. Another form of mental retardation is due to the presence of an extra chromosome, instead of two in the twenty first chromosomes, resulting in mongolism or Down syndrome.
Environment refers to any and every influence with which an individual comes into contact after the hereditary pattern has been received to the germ plasma. It includes the effects of training, trial and error, learning influences of the home, school, neighbourhood, hospital, church, playground, climate, geographical location and anything else that stimulates the senses in any way.
Thus the heredity plays its game only at the time of conception. What happens afterwards, i.e. after conception is the game of environment. It affects the individual, his bodily structure, and all of his personality make up and behaviour.
Environmental forces can be categorized into:
b. External environments.
The environment received by the individual from his conception till his birth in the womb of the mother, i.e. a period of about 9 months is called internal environment. In this environment, the embryo receives the nutrition through the bloodstream of his mother.
The physical and mental health of the mother including her habits, attitudes and interests, etc. all constitutes the inner surrounding or internal environment that affects the growth and development of the foetus along with the emerging behaviour in future.
After the birth what the child gets in terms of environmental influences is purely external in nature. These can be physical and social or cultural influences. The physical environments like earth, river, mountains, weather and climatic conditions, food, water, etc. constitute the external environment.
The parents, family members, friends, classmates, teachers, members of community and society, the means of mass communication and recreation, religious places, etc. include social and cultural environments helping to shape the personality and behaviour.
The roles of heredity and environment in the development of personality and behaviour have been extensively searched in the form of twins and family studies and on experimenting selective breeding. The results have failed to establish a clear- cut role of either the presence of particular behaviour or trait in an individual.
What behaviours are learned or what are inherited is a controversial question that can only be answered through a reasonable understanding that one’s behaviour or development of a specific personality trait is always a result of the interaction of the environmental process on the genetically inherited characteristics.
Conclusion: The Nature/literature Controversy:
The nature (heredity) – nurture (environment) debate really concerns the relative importance of heredity and environment. Today no one believes that nature alone completely determines the course of our development. Psychologists agree that development is shaped by the interaction of heredity and the environment.
Within this interaction our genetic endowment for many characteristics provides us with a reaction range of possible levels that we may ultimately reach depending on the availability of our experience in the environment.