INTRODUCTION
Everything you are and everything
you do depend on both your genes and your environment. Without your genes or
without an adequate environment you would not exist. So far, no problem. The
controversies arise when we discuss why people differ from one another in their
intelligence weight gain, sexual orientation mood tendency toward alcoholism
and so faith.
MENDELIAN GENETICS
Prior to the work of Gregory Mendel, a late 19th
century monk, scientists thought that inheritance was a blending process in
which the properties of the sperm and the egg simply mixed, much as one might
mix red and yellow paint.
Mendel demonstrated that in heritance
occurs through genes, units of heredity that maintain their structural identify
from one generation to another.
GENES RULE
As a ruler, genes come in pairs, because they are
aligned along chromosomes (which is a strand of gene) which also come in pairs
(as an exception to this rule, a males x and y chromosomes are unpaired having
different genes).
Gene is a portion of a chromosome which is composed of
double-strand chemical deoxyribonucleic acid DNA a stand of DNA serves as a
template for the synthesis of ribonucleic acid (RWA) molecules. And one for
belle eyes (recessive) will have brown eyes or he or she is a “carrier” for the
blue-eye gene and can transmit it to a child for a behavioral example. The gene
for ability to taste moderate concentration of phenylthoic carbonize (PTC) is
dominant the gene for low sensitivity is recessive only someone with two
recessive genes has trouble tasting it.
FOUR POSSIBLE OUT COMES OF A MATING
BETWEEN PARENT WHO ARE HETEROZYGOUS FOR A GIVEN GENE
1. A
child in this family has a twenty five percent chance of being homozygous for
the dominant gene.
2. Twenty
five percent chance of being homozygous for the recessive gene and fifty percent
chance of being heterozygous.
3. Fifty
percent chance of being a heterozygous taster
4. A
chance of being a homozygous nonstarter.
RNA
is a single stand chemical one type of RNA molecules serves as a template for
the synthesis of protein molecules.
HOMOZYGOUS AND HETEROZYGOUS
Homozygous:- An
individual who has an identical pair of genes on the two chromosomes is
homozygous for that gene.
Heterozygous:- An individual with an unmatched pair of genes is
heterozygous for that gene. For example: gene for blue eye might be on one
chromosome and a gene for brown eyes on the other.
DOMINANT AND RECESSIVE GENE
Dominant Gene: A dominant gene shows a strong effect in either a homozygous
or heterozygous condition
Recessive Gene A recessive gene shows its effect only in the
homozygous condition.
For
example some one with a gene for brown eyes (dominant)
SEX- LINKED AND SEX LIMITED GENES
The genes located on the sex chromosomes are known as
sex-linked gene. All other chromosomes are autonomic genes.
In mammals the two sex chromosomes are designated X
and Y: a female mammal has two X chromosomes, a male has an Y and a y. during
reproduction the female necessarily contributes an X chromosome and the male
contributes either an X a y. if he contributes an X, the offspring is female.
If he contributes a y the offspring is male the Y chromosomes is small and
carries few genes other than gene that causes the individual to develop as a
male the X chromosomes however, carries many gene thus when biologists speak of
a sex-linked genes they usually mean X linked genes.
An example of human sex-lined gene is the recessive
gene for red-green color vision deficiency. Any man with this gene on his X
chromosome has red-green color deficiency because he has no other X chromosomes.
A woman however is color deficient only if she has that recessive gene on both
of her X chromosomes. So for example if eight percent of human X chromosomes
contain the gene for colour vision deficiency then eight percent of all men
will be color deficient, but fewer than one percent of women will be.
SEX LIMITED GENE
Distinct from sex-linked genes. A sex limited gene is
present in both sexes but has an effect limited or almost limited to one sex.
For instance genes control the amount of chest hair in men. Breast size in
woman, the amount of crowing in roosters, and the rate of egg production in
hens. Both sexes have these gene but the genes become active only under the
influence of sex hormones.
SOURCES OF VARIATION
If reproduction always produced off spring that were
exact copies of the parents, evolution would not occur one source of variation
is recombination a new combination of genes, some from one parent and some from
the other that yields characteristics not found in either parent. For example a
mother with curly blonde hair and a father with straight black hair could have
a child with curly black hair or straight blonde hair.
A more powerful source of variation
is mutation or change in a single gene. For instance, a gene for brown eyes
might mutation of a given gene is a rare, random event: tat is the need of the
organism do not guide it. A mutation is analogous to having an untrained person
add, remove or distort something on the blue prints for your new house. Random
changes are only rarely helpful but those rare events are critical for
evolution.
Most mutations produce recessive
genes thus if you or one of your recent ancestors had a harmful mutation on one
gene. Your children will not show its effects unless your mate has the same
harmful gene for this reason. It is unwise to marry a close relative.
HERITABILITY
Most variations in behavior depend on the combined
influence of many genes and environmental influences. You may occasionally hear
someone ask about a behavior. Which is more important, heredity or environment.
That question as stated is meaningless. No behavior can develop without both
heredity and environment.
However we can rephrase it meaningfully do the
observed differences among individuals depend more on differences in heredity
or on if you sing better than I do the reason could be that you have different
genes that you had better training or of course both.
In meaning the relative contribution of heredity
researchers use the concept of hereditability, an estimate of how much of the
variances in some characteristic within some population is due to different in
heredity. Heritability ranges from 0 to 1. a heritability of 0 means that
genetic differences account for none of the observed variation in some
characteristic within the tested population. A heritability of 1 indicates that
genetic differences account for all of the observed differences. Heritability
is specific to the tested population it could be high in one population and low
in another. For example in a community in which everyone is closely related any
variation in their behaviors would be largely due to differences in their
environments and therefore heritability would be low conversely if all the
people in some other community have nearly identical environments, the heritability
of their behaviors will be high.
HOW GENES AFFECT BEHAVIOR
When biologist speaks of a gene for brown eyes he does
not mean that the gene directly produces brown eyes rather the gene produces a
protein that alters body chemistry to make the eyes brown instead of an other
color. If we speak of a gene for alcoholism we should not imagine that the gene
itself produces alcoholism rather the gene produces a protein that under
certain circumstances increases the probability of alcoholism. How a gene
increases the probability of a given behavior is a complex issue.
However gene can affect behavior indirectly by
changing other body characteristics. For example, a gene that increases a
person’s height increases the probability of playing basketball “if he or she
lives where people play basketball court is time not spent doing something
else. The increased height gene probably decreases the time that the person
spends playing the violin. The point is that genes that affect the body in any
way also affect behavior and a gene that affects one behavior also influences
other behaviors.
OVERESTIMATING
HERITABILITY
Investigators cannot control people’s mate choices and
have only limited control over their environments some of the difficulties
studying human lead to over estimated heritability Murray and Eaves 2001.
For example the closer we come to providing every one
with the same environment the higher the heritability becomes “Tiny
environmental differences will not produce much different in outcome. So the
remaining differences have to reflect heredity” such is the case for adopted
children. Adoption agencies try to place them all in high quality homes and
therefore differences in their environments are generally small.
Further-more studies on humans seldom distinguished between genetic and prenatal influences most
monozygotic twins share a single chorion and therefore a single blood supply
during prenatal life. Therefore monozygotic twins resemble one another more
closely in prenatal environment as well as heredity the low 1Q criminal or
mentally ill parents gave the children their genes but they also gave them
their prenatal environment. Therefore what looks like a genetic effect could be
wholly or partly an effect of prenatal environment.
Still another problem is that your genes not only
affect you but also influence how other people treat you. For example good
looking children get treated different from less attractive children. If your
genes cause you to be even-tempered others react to you in a friendly way
thereby improving your already calm disposition. If your genes lead you to
frequent temper tantrums other people including your parent will react harshly
giving you still further reason to feel hostile. Dickens and Flynn “2001”call
this tendency a multiplier effect if genetic or prenatal influences produce
even a small increase in some activity the early tendency will change the
environment in a way that magnifies that tendency.
ENVIRONMENTAL
MODIFICATION OF HERITABLE BEHAVIORS
A trait with high heritability can be modified by
environmental interventions. For example, different genetic strains of mice
behave difference in the elevated plus mare. Some stay almost entirely in the
walled arms. Others venture onto the open arms. But even when different
laboratories use the same genetic strains and nearly the same procedures
strains that are adventure some in one laboratory are less active in another.
Evidently, the effects of the genes depend on subtle differences in procedure,
such as how the investigators handle the mice or may be even the investigators
odors.
For example phenylketonuria is a form of mental
retardation caused by a genetic inability to metabolize the amino acid
phenylalanine. Because of the pku gene phenylalanine accumulates to toxic
levels, impairing brain development and leaving children mentally retarded
restless and irritable. Approximately one percent of Europeans carry a
recessive gene for pku fewer Asians and still fewer Africans have the gen.
Ordinarily the heritability of pku would be virtually
one. However physicians in many countries routinely measure the level of
phenylalanine or its metabolites in babies blood or urine.
THE
EVOLUTION OF BEHAVIOR
Every gene is subject to evolution by natural
selection. Evolution is a change over generations in the frequencies of various
genes in a population by this definition evolution includes any change in gene
frequencies, regardless of whether it is helpful or harmful to the species in
the long run.
GENES AND
BEHAVIOR
Genes are neither all important nor irrelevant certain
behaviors have a very high heritability such as ability to taste PTC many other
behaviors are influenced by genes but also subject to strong influence by
experience. Our genes and our evolution make it possible for us the flexibility
to change our behavior as circumstances warrant. Understanding the genetics of
human behavior is particularly important but also particularly difficult. Separating
the roles of heredity and environment is always difficult. Especially so with
humans because researchers have such limited control over environmental
influences inferring human evolution is also difficult partly because we do not
know enough about the lives of our ancient Caveman ancestors. Finally we should
remember that the way things are is not necessarily the same as the way they
should be for example. Even if our genes predispose people to be have in one
way or another we still have great flexibility in acting on our
predispositions.
CONCLUSION
Genes
are chemical that maintain their integrity from one generation to the next and
influence the development of the individual. A dominant gene affects
development regardless of whether a person has pairs of the gene or only a
single copy parcel. A recessive gene affects development only in the absence of
the dominant gene.
Some
behavioral differences demonstrate simple effects of dominant and recessive
genes. More often however, behavioral variations reflect the combined
influences of many genes and many environmental factors. Heritability is an
estimate of the amount of variation that is due to genetic variation as opposed
to environmental variation.
Researchers
estimate heritability of a human condition by comparing monozygotic and
dizygotic twins and by comparing adopted children to their biological and
adoptive parents.
The
results often overestimate human heritability for several reasons. First the
environmental quality varies little for most adopted children. Second most of
our results do not distinguish between the effects of genes and those of
prenatal environment. Third, after genes produce an early increase in some
behavioral tendency, that behavior may lead to a change in the environment that
magnifies the tendency thus leading to what appears to be a huge effect of
heredity.
The
fact that some behavior shows high heritability for a given population does not
necessarily indicate that it will show an equal heritability for a different population.
It also does not deny the possibility that a change in the environment might
significantly alter the behavioral out come.
Genes
influence behavior directly by altering chemicals in the brain and also
indirectly by affecting virtually any aspect of the body.
The
process of evolution through natural selection is occur in genes and
individuals with certain sets of genes reproduce more successfully than other
do.
Evolution
spreads the genes that are associated with the individuals who have reproduced
the most. Therefore, if some characteristic is widespread within a population
is reasonable to look for ways to which that characteristic is or has been
adoptive. However we cannot take it for granted that all common behaviors are
adoptive.
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