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
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
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 bleeder’s 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.
0 Comments