Genetics and behavior are closely linked fields of study in biology and psychology. Genetics refers to biological inheritance of traits from parents to offsprings through genes, which are segments of DNA. Behaviour, on the other hand, encompasses the action and reaction of individuals, often influenced by their environment, experiences and biological factors
Behavioural genetics is branch of psychology that attempts to allocate and explain genetic and environmental contributions to human behaviour.
-Behaviour Genetics Association
A. LAWS OF BEHAVIORAL GENETICS
Three laws of behaviour genetics was formulated by Turkheimer(2000). The fourth law of behavior genetics was formulated by Christopher F Chabris (2015).
1. All human behavioural traits are heritable,i.e., they are affected to some degree of genetic variation.
2. The effect of being raised in the same family is smaller than affect of genes.
3. A substantial portion of the variation in complex human behavioural traits is not accounted for by the effects of gene or families.
4. A typical human behavioural traits is associated with many genetic variants , each of which accounts for a very small percentage of the behavioural variability.
HEREDITY
The concept of heredity is fundamental to understanding genetics and its influence on behavior.
Heritability is a statistical estimate of proportion of variation in a trait that can be attributed to genetic factors . Heritability typically expressed as percentage with higher values indicating a stronger genetic influence. There in a popular notion that life begins at birth. But biologically, life begins at the moment of fertilization (conception) i.e. when a matured sperm of a man fuses with matured ovum of woman. It is more crucial event when sperm and ovum determine the sex, biological and psychological characteristics of the emerging individual. The individual is a psychobiological organisms emerged out of the interaction of heredity and environment.
Moreover, which chromosome from ovum will pair with which chromosome from sperm depends upon chance .A million of permutations and combination is possible for the union of chromosome which contain genes.That explains why not to individuals are perfectly identical. Genes are the true carriers of hereditary traits. Which determine various physical and psychological traits of an individual. For example, colour of the skin and eyes, the height, skin texture, intelligence, temperament etc.
Often the genes determining characteristic are not identical. They are heterogenetic where one gene is dominant over the other which is said to be recessive.Thus, the genetic constitution of an individual contains dominant and mrecessive genes. The characteristics usually produced are the result of expression of dominant genes.
Genotype versus Phenotype
While discussing genes and chromosomes, it is also appropriate to discuss the concept of genotype and phenotype. A person may have appropriate genotype at birth but the phenotype expression of particular rate maybe suppressed due to many environmental factors. For example, a person may have genotype to be intelligent but malnutrition early in life can supressed expression of this trait and lead to retardation.
The genotype
It refers to the genetic make-up of an individual inherited from the parents. It refers to the genetic potentiality that the individual has.
The phenotype
It refers to the observed characteristics of the individual.
Identical twins, maternal twins and siblings
After the first mitotic division of a fertilized egg, the two cells may remain separated and develop independently giving rise to twins. The twins developed out of a single zygote are known as ‘monozygous’ or ‘identical twins’. As the share the same chromosomes, the sex of both the twins will be the same. So, they closely resemble each other in physical and mental characteristics .As they share the same heredity, any differences which appear at later age may be due to environmental factors.
Sometimes, two ova are released by the ovary. If both are fertilized simultaneously by two sperms, to individuals emerge, develop and grow to be the twins. Such twins born out of the two separate ova simultaneously but together are known as ‘fraternal twins” or ‘heterozygous’. As the share different hereditary traits, they maybe of different sex or the same sex but with but will have different characteristics.
Sibling are brother and sister born to the same parents but with a gap of months or years. Though they are born to the same parents, each is different from the other in sex as well as in many other characteristics.
ENVIRONMENT
The term environment refers to such factors in an around the individuals which stimulates the individuals and bring about changes in the individual physically and psychologically.
The environment is classified into two categories:
Internal environment(prenatal environment) The nucleus with its chromosomes and their genes is surrounded by jelly like substance known as cytoplasm. Cytoplasm is considered as an intracellular environment, this environment influences the genes and thereby the traits they carry. In fact what the organism becomes is determined by its cytoplasm and its heredity.
Later on when the endocrine glands release hormones into bloodstream another intracellular influences is produced, these hormones greatly affect the subsequent development of the individual.
In addition to intracellular environment, intrauterine environment like malnutrition of the mother, vitamin deficiency, excessive alcohol intake, excessive smoking, use of drugs and chronic disease like diabetes will have their own effects. Consequent to this intrauterine environment the individual will be born with physical or mental handicap or both.
For example – mental deficiency, born blindness, deafness etc. These will have long range effects on the individual.
External environment (after birth)
The environment after birth is more complex and powerful. It involves immense variety of physical and social contacts.
Social environment: It includes language, customs, traditions and many cultural aspects like value, moral, schools, community, family, peer group, neighbourhood also influences the individual.
Physical environment: It involves physical stimuli like the food we eat, the drinks we take, the clothes we wear, the shelter we live in, light, sound, smell, taste etc. All these will have their own impact on individual.
Individual from the time of conception to death is subjected to environmental influences. So human is a product of the interplay of heredity and environment.