Biological Basis of Human Behaviour


Biological Basis of Human Behaviour

A major problem that bothered early scientists and psychologists was why one child in a family would have brown eyes while the other child has blue eyes. Psychologists wondered how individuals acquired their unique physical structures and traits. The answer to the question relates to the biological makeup of the human beings. A person’s heredity determines their unique characteristics. In this page, we shall consider genetic transmission. We will discuss genetic abnormalities. We will also discuss the contributions of the nervous system and the endocrine

system to human behaviour.

At the end of this page, you should be able to:

· outline how genetic materials are transmitted from parents to offspring

· explain how the nervous system contributes to differences in human behaviour

· discuss the role of the endocrine system in determining human behaviour.


Heredity will be discussed under the following sub-topics:

1. Gene Operations

The basic unit of human life is the cell. Groups of cells organise to form different structures such as: organs, muscles, tissues. Every cell has a nucleus. The nucleus contains 46 chromosomes arranged in 23 pairs.

However, the reproductive cells contain 23 units of chromosomes. Chromosomes are threadlike molecules of Deoxyribonucleic acid (DNA). The DNA carries the genetic instruction.

One of each pair of the 23 pairs of chromosomes in each human cell is from the father, and the other from the mother. These chromosomes carry coded instructions called genes. The gene is the basic unit of heredity, or genetic blueprint.

During reproduction, each parent contributes 23 units of chromosomes. When the sperm cell fertilizes the ovum, the chromosomes from both parents pair up.

2. Trait Transmission

Trait transmission is a process by which definite structures or genes are transmitted from parents to offspring. The gene for any specific trait is transmitted in pairs of alternate states called gene alleles. Genes operate either dominantly or recessively.

When opposing or alternate characteristics, such as brown and blue eye colour, are transmitted through genes, one overrides the other and becomes the trait evidenced. The trait evidenced is said to be operating dominantly. This means that its features are observed in the physical appearance of the individual.

In the specific example of brown and blue eye colour, when brown and blue gene alleles pair to determine eye colour, brown overrides blue. The eye colour of the individual is observed to be brown. The brown trait is dominant over blue. However, this same child with brown eyes carries the genes for the blue eye, hidden in the genetic makeup.

The blue eye trait cited here is swamped or hidden. It is less potent than the brown eye trait. The blue eye trait here is said to be operating recessively. In a nutshell, a dominant trait is a trait transmitted through genes that overrides an opposing trait. It is expressed in the physical features. A recessive trait is a trait transmitted through genes that is less potent than an opposing trait. It therefore remains hidden or unexpressed.

When an individual has a pair or identical gene alleles determining a trait, the individual is said to be homozygous for that trait. However, when a trait is determined by a pair of dissimilar gene alleles, the individual is said to be heterozygous for the trait.

Note that a dominant trait is only dominant when in a heterozygous condition. And a recessive trait is equally recessive only in a heterozygous condition. It holds therefore that in a homozygous condition, a recessive trait will express itself; there being no potent trait

swamping or overriding it.

10 The features of a recessive trait appear in the observed physical appearance of the individual when the recessive trait is in the homozygous condition. The sickler is an example where a recessive blood trait is in a homozygous condition. The carrier of the sickle cell anemia is an example of a pair of gene alleles determining a trait being in a heterozygous condition. The carrier has the trait hidden but does not manifest the sickness.

In the examples cited here, the brown eyed child, and the sickler are examples of outward appearance, or observable manifestation of inherited traits. The brownness or sicklerness indicate the way genes express themselves in the structure of the individual. Such outward expression of inherited traits is known as phenotype. However, behind the outward appearance is the actual genetic composition or genetic constitution. This is known as the genotype.

Note that the phenotype may not reflect the underlying genetic structure or the genotype as is the case with the brown eyed child and the carrier of the sickle cell anemia.

Note: Genes do not cause behaviour

It must be stressed that genes do not directly cause behaviour, thoughts, or emotions. Genes instruct the making of proteins and hormones. That is, genes instruct the making of chemicals that may make a child prone to behaving in certain ways, such as being anxious, impulsive, depressed. Take the emotion of anxiety as an example, the proteins and  hormones produced by DNA carry messages between brain cells. Some of these messages deal with the response to dangers.

The chemicals in the brain cells that make individuals respond to dangers may be coded to make one person highly responsive to danger. This person is then easily anxiety-provoked. The same chemicals in the brain of another person may be coded to cause a low-level reaction to dangers. This individual then expresses less anxiety. Therefore, even when the environment presents the same danger to these two persons, their responses will be quite different. The same explanation goes for observed individual differences in most human behaviours.

3. Genetic Abnormalities

The genetic code of every individual may be likened to a computer software program. The software program tells the computer what to do.

The genetic code is the child’s personal biological program. This personal biological program is constructed from the software of both the father’s and the mother’s sides of the family. Like a software program, the biological program sometimes gets hiccup or goes awry.

As a result, a substantial number of children are born with congenital defects or genetic abnormalities. It is estimated that about five percent of infants are born with genetic abnormalities, and approximately three percent of newborns have birth defects (Plomin, De Fries, and McClearn, 1990). Some of these conditions can be serious and debilitating. Families can be deeply affected by the birth of a child with a genetic abnormality.

Some of the well known genetic diseases are caused by dominant genes. Some of the diseases are caused by recessive genes. Some are caused by sex-linked genes. Still some other genetic diseases are caused by structural defects in chromosomes (too many or too low chromosomes).

Some of the more commonly genetic diseases are discussed here.

Sickle-cell Anemia

Sickle-cell anemia is transmitted through recessive genes from both parents. Both father and mother must have the sickle-cell disease trait in their genotype, either as carriers or sicklers, for their offspring to suffer sickle-cell anemia. The consequence of sickle-cell disease is a defect in the red blood cell structure. The red blood cells are therefore disabled and cannot effectively carry oxygen to body tissues and organs.

Individuals with sickle-cell anemia have regular severe pain in their limbs and joints. They are also easily fatigued. In extreme cases, death occurs from heart or kidney failure due to oxygen shortage.


Hemophilia is a blood diseases transmitted through a sex-linked recessive gene. The disease is carried on the X-chromosome. Hemophilia is more prevalent in male children. The sex genotype of females is XX; that of males is XY pair. Thus, males lack the second X chromosome than can counteract the genetic information that produces the disorder. The consequence of the disease, hemophilia, is inability of the blood to clot. This is why some call it the bleeders disease.

Down Syndrome

Normal human beings have 46 chromosomes, arranged in 23 pairs. In Down Syndrome individuals, there is an extra chromosome on the 21st pair of chromosomes. Thus, Down Syndrome is a disorder produced by the presence of one extra chromosome on the 21st pair, so there are three instead of two chromosomes.

The term chromosome trisomy has also been used to describe this situation. Down Syndrome has also been referred to as mongolism. It is an example of a genetic disease caused by a structural defect in chromosomes.

The consequence of Down Syndrome is mental retardation. Sometimes there is evidence of arrest in physical growth. Significantly greater number of Down Syndrome babies are born to mothers above 45 years of age. Very old fathers have also been cited as contributing significantly to the birth of Down Syndrome babies.

Turner’s Syndrome

This is a genetic disease caused by abnormal sex chromosomes. It is found among females. The female has only one X chromosome instead of two. The genotype if expressed as XO. The consequence of Turner’s Syndrome includes: lack of functioning ovaries; inability to develop

secondary sex traits; short physical stature, and poor spatial perception. Heart problems are also common complications.

Klinefester’s Syndrome

Klinefelter’s Syndrome is a genetic disease resulting from the presence of an extra X chromosome in the sex genotype. That is, the sex genotype is expressed as XXY. The disease is caused by abnormal sex chromosome. 35

The consequences of Klinefester’s Syndrome include: underdeveloped genitals – small testicles with no sperm; feminine appearance – enlarged breasts and high-pitched voice.

How the Nervous System Contributes to Differences in Human Behaviour

The Nervous System

The nervous system is made up of the brain, the nerve cells (the neurons), the synapses, and the specialised sensory modalities. The sensory modalities include the visual, the auditory, the olfactory, the tactile, and the taste organs. The feelings, the movements and the thoughts a child may experience are brought about by a complex network in the nervous system.

The infant is born with between 100 and 200 billion neurons or nerve cells. No new neurons are created after birth. The number the child is born with lasts them a lifetime. The amazing capabilities of the brain are achieved by increasingly more complex connections created between

the neurons and a pruning down process of unused neurons. That is, neurons that do not become interconnected with other neurons, in the course of the child experiencing of the world, become unnecessary. The unused or unnecessary neurons eventually die out.

Thus, according to Kolb (1995), the development of the neurons systemproceeds most effectively through the loss of cells, and not cell multiplication or division like other aspects of human growth.

The sensory modalities or sensory organs, the eye, the ear, the nose, the skin, and the tongue receive input information from the child’s environment. The sensory organs convert the stimulus from the environment into electrical activity or nerve impulses. The chemical

substances in the synapses and the neurons transmit nerve impulses to the brain; and from the brain to target organs.

Any child’s speed of reaction to environmental stimulation will depend on the nature of the chemical substances that transmit messages in the nervous system. Genes instruct the making of chemical substances in the nervous system. In other words, genes determine any child’s speed of reaction to environmental stimulation. Therefore, the efficiency of the functioning of the nervous system is genetically determined.

One can see how the malfunctioning of some of the sensory organs (for example, long sightedness and short sightedness of the visual organ) is attributed to genetic make up. Malfunctioning of sensory organs leads to perceptual impairment. This has implications for the child’s behaviour, school adjustment, and achievement.

The Role of the Endocrine System in Determining Human Behaviour

The endocrine system consists of the ductless glands. These are glands that secrete chemical substances (hormones or enzymes) that regulate body chemistry and activities. Among the import ductless glands are:

The Pituitary Gland: The pituitary gland is also known as the master gland. It secretes the hormone that controls all other glands. Primarily, the pituitary gland secretes the growth hormone which regulates the physical growth of the body parts.

The Thyroid Gland: The thyroid gland secretes the hormone, thyroxin. This hormone is responsible for the control of food metabolism and the sensitivity of the nerves.

The Adrenal Gland: This gland secretes the hormone commonly known as the emergency hormone. This hormone controls the body systems that regulate the body’s reactions to changes and danger signals.

The Pancreas: The pancreas secretes the hormone known as bile. This hormone regulates sugar metabolism and sugar levels in the tissues and the bloodstream.

The Gonads: For males, the gonad is the testes which produce the male gametes or male sex cells. For females, the gonad is the ovary. The ovary controls the maturation of the female sex cells or the ova. The gonads also regulate the development of secondary sexual characteristics. Hyper-activity or hypo-activity of any of these glands will hamper body systems activities. Logically, therefore, malfunctioning of the endocrine system will hamper normal growth and development of the child. This will in turn hamper normal behaviour and adjustment.

It is the genes that instruct the making of the enzymes or hormones of the endocrine system. Therefore, the level of functioning of the endocrine system has a biological origin. The endocrine system contributes significantly to human behaviour.

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