Heredity and Environment for B.Sc. & M.Sc. students

1. Heredity and Environment
By Dr. Sadguru Prakash
Organisms can transmit some hereditary conditions to their offspring even if the parents
do not show the trait. Behaviour of an organism is influenced by two factors- Heredity and
Environment. The biological or psychological characteristics which are transmitted by the
parents to their offspring are known as Heredity. In other words, heredity is a biological process
of transmission of certain traits or behaviour of the parents to their offspring, by means of the
fertilized eggs. Heredity traits are innate and they are present at birth.
The term environment means the surrounding. It includes both physical or non-living or
abiotic and biotic (living) factors that act upon an organism or an ecological community and
ultimately determines the form and survival. Thus, the relationship existing between the
organism and the environment is of reciprocal nature.
An organism has hundreds of characters which are controlled by thousands of genes
acting singly or in combinations.
The characters of an organism are greatly influenced by the environment. A change in the
environmental condition often changes the phenotype of organisms. Phenotype is the expression
of genotype, but we must keep in mind that environment is absolutely critical about phenotypic
expression. Environmental variations may influence the individual cell of the body as well as
with the whole organism.
Some biologist claims that the difference in the traits of individuals or groups is due to
differences in their heredity. But other biologists explain that the variations of an organisms are
due to differences in environment.
Thus, the question arises here that, what is the relation between heredity and
environment? What is more important, Heredity or Environment? Seed or Feed? Nature
or Nurture?
The answer of this question can be explained by following example-
Example 1: Some pea plants are tall and some are dwarf. The tall plants have the genes
for tallness where as dwarf plants have the genes for dwarfness. Does the gene for tallness
always produce a tall plant?
If the two seeds of almost the same size and weight are selected from a plant which is
homozygous tall and are sown in different conditions. One is well watered and soil having rich
and adequate minerals where as other sown in soil that is poor in minerals and has an inadequate
water supply. The plants that grow / raised in these two different conditions will be different. The
one type grow in good soil will be tall like parents and other remain stunted and dwarf even they
have gene for tallness. It is well known fact in crop plants, yields and quality are controlled by a
number of genetic factors, yet the results obtained in actual practice are greatly influenced by
several environmental factors. But results are obtained only when the soil is well manured and
contain an adequate supply of water, temperature and light are optimum.
Example 2: In Cattle, production of high milk character is hereditary and is controlled by
certain combination of gene but the number of litters of milk produced are greately affected by
the amount and kinds of food given to a cow.

2. Example 3: ‘White leghorn’ variety of fowl gives large number of eggs per year but this
trait in influenced by the amount and kind of food given to an egg laying hen.
Example 4: A green plant develops chlorophyll due to presence of certain specific genes.
If this plant is cut off from light, it will remain pale yellow. The same condition arises in the
absence of iron, magnesium or manganese. Thus it is clear that a plant not inherited the green
colour but inherited the ability to produce green colour in the presence of light.
Example 5. In some types of corn the kernels will remain yellow until they are exposed
to sunlight. Once exposed, the kernels become various shades of red and purple.
Thus, from the above example and discussion it is clear that-
 A gene can express itself only in favourable environmental conditions.
 All characters are the result of interaction between genes and environment.
 The same gene in one set of environmental condition produces one character and in
another set of environmental conditions another character.
What is the genotype determining the mode of reaction of the organism to the
environment?
This is explained by following examples_
Example 1. In fowls, yellow leg breed truly and white leg also breed truly. The
difference is due to two alleles of a single gene. Yellow legs beings dominant to white legs. But
this results obtained only when fowl feed on normal diet. If we feed the white maize, the yellow
leg fowl breed white leg. To show yellow colour, their genes have to interact with a chemical
substance found in yellow maize, but not in white maize.
Example 2. Himalayan rabbit has white coat which is recessive to brown agouti colour of
wild type. This difference is due to a single factor pair. Himalayan rabbit produces white hair at
optimum temperature, but black hair at a lower temperature. At birth, all the Himalayan rabbit
are uniformly white, for the whole body has been maintained at high and constant temperature
before birth. But subsequently the extremities, the ears, muzzle and feet, which are subject to
chilling, turn black for these parts are cooler than the rest of the body and this difference is
sufficient to totally alter the effect of Himalayan factors. This is experimentally proved.
If the patch of hair be shaved off the black of two Himalayan rabbits and the one be kept
in a worm and the other in a very cold place, the hair which grow again will be white in the
former environment and black in the latter.
Example 3. In barley plant, the white (Albino) variety is due to a single recessive factor,
but it can only produce when the plant grows below 2.50
C. Above 180
C they develop the full
amount of green colour and cannot be distinguished from the genotypically green plants.
Example 4. The effect of environment in aquatic plant, Ranunculus aquatalis in unique.
The emerged leaves are often different from the leaves that are above the surface of water
although all the leaves have the same genotype.
Example 5. In man, a Negro develops black colour due to presence of dominant gene and
white race do not produce pigment due to lack of this gene. But when a white races living in
outdoor life in tropic where sunlight intensity and duration are more, they develop colour
pigment. Only the rate of pigment development in a white race under the influence of sunlight is
much slower than in the case of a negro whose genotype causes the pigment to develop rapidly

3. within a few days after birth and skin becomes dark independently of sun exposure. Here, the
genotypic differences are related to the different rates of pigment formation.
Example 6. In Drosophila, there is a hereditary trait in which the wings curl up sharply if
the files are raised at a temperature of 25 0
C. If, however, the files are raised at a lower
temperature, such as 16 0
C then the trait rarely appears. The wings seem to be straight, and the
flies look normal. The genetic trait is there, however, and will reappear in the next generation if
the temperature returns to 25 0
C.
Example 7. Human intelligence and social behaviour are also controlled by genes. Every
one inherits from its parents a certain quota of intelligence but it mainly depends upon what type
of education, training, diet and social environmental condition given to it. Only capacity to
develop the intelligence is inherited not the degree of intelligence. As an impact of
environmental factors decided the IQ (Intelligence Quotient) of a person. There is a famous
Punjabi saying “AK (Calotropis) will produce an Ak and the mango a mango”. This emphasises
the rate of heredity. But you cannot get good mangoes without good environment.
Thus, heredity and environment both work hand in hand. Only a set of gene transfer or
passed from parents to offspring. Only genotype is relatively constant but the phenotype
constantly changes during the life of an organisms. For example, the photographs of a person
taken in children, early boyhood, mature youth and old age will be differ due to effect of
environment.
A person whose growth was stunted on account of poor diet and unhygienic
environment or due to disease will pass onto his children who was well endowed mentally at his
birth but remained backward due to lack of opportunity can still pass on to his children the
capacity to develop high intelligence. Thus, the environment may modify the phenotypic effect
of the genes that an individual possesses but it does not affect the heredity potentiality of the
individual.
Warburton and Fraser have emphasized that the development of a fetus depends on a
precise and extremely intricate system of interactions between two sets of hereditary factors and
two environments, all acting at the same time on the growing baby. The mother and the fetus
each have their own environment and their own genotype.
On the other hand we see that some traits such as polydactyly, color blindness, baldness,
blood type, skin color, the ability to taste certain substances, the presence or absence of hairs on
the middle of the fingers, and free or attached ear lobes do not seem to be influenced by the
environment.
Though the arguments have been advanced by the supporters of both heredity and
environment, no exact conclusions with regard to the relative importance of these two factors
have been drawn, nor is it possible either to determine the relative values of both.