Environmental Influence on Genes(18)

7/30/2019 Environmental Influence on Genes(18)

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Differences in phenotype between

the members of a population arecaused by differences in:

Genetic make up or genotype.

The environment in which anindividual develops.


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The ability of animals to achieve theirfull genetic potential in terms of growth

depends heavily on environmentalfactors (amount of food available).

Genetically identical plants grow verydifferently when exposed to varyingamounts of light and soil nutrients.


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Examples of gene and environmentalinteractions

1. Human height

2. Skin color in human

3. Animal hair color

4. Cancers5. The strange case of monoamine

oxidase A (MAO A)


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In humans

Environments in the uterusaffects the development of thefoetus even before birth.

1. Malnutrition in mother

2. Smoking mother3. Heavy use of alcohol


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Human height

Human height is controlled by severaldifferent genes (polygene).

Human height is also influenced by

environment.(The amount and quantity of the

nutrients received at critical stage of thebody growth )


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Reasons for increasing human height overrecent generation

1.Better nutrition.

2.Greater movement of people resulted in lessinbreeding.

3.Improved health with reduction in infectious disease.

4.The end of child labour has allowed more energy to go

into growth.

5. Better heating of houses and better quality clothingreduces the amount of energy needed to heat thebody.


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n co our Skin colour depends on a pigment called melanin

produced bymelanocyte cells bymelanocyte-

stimulating hormone (MSH)

Melanocyte cells are present in the epidermis of theskin which contain MSH receptors.

Melanin is packed in organelles called melanosomes.

It is produced by the catalytic activity of the enzyme

tyrosinase upon tyrosine amino acid.

Two forms of melanin are produced-

red-yellow-common in lighter skin

brown-black- common in darker skin


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Environmental influence. Enhanced exposure to UV rays increases the enzyme

activity.

Melanocytes are stimulated to transfer melanosomesto skin cells.

They collect around the nucleus and dispersed in thecytoplasm.

Skin darkens.

Skin cells form more effective barrier against UV rays.

Low risk of skin cancer


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Low exposure to UV rays causes less MSH and MSH

receptors to form.

Collection of melanosomes around the nucleus is

less.

Decreased protection from UV rays.

Higher risk of skin cancer.


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Seasonal colour changeArctic foxes have brown fur in summer and

white fur in winterDuring summer season, more MSH and MSH

receptors are formed.

Leads to more melanosomes.Produce more melanin.

During winter season low intensity of UV

rays.Little or no production of MSH and MSH

receptors.

Melanin is not produced.

l l bb


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Hair colour in Himalayan rabbitsand Siamese cats

Have mutant allele for tyrosinase. Formed enzyme is unstable and inactive at normal

body temperature.

Active at low temperature- regions like tips of the

body

Converts tyrosine to melanin.


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cancerOccur when the rate of cell multiplication is faster

than the rate of cell death.This causes the growth of tumours.Caused by damage to DNA due to

1.physical factors such as UV rays2.carcinogens

Cancer cells do not respond to the controlmechanism of the cell.


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Two types of genes have a role in control of the cell

cycle and play a part in triggering cancer.1.Oncogenes2.Tumour suppressor geneOncogenesCode for the protein that stimulate the

transition from one stage in the cell cycle tothe next stage.

Mutation in this gene leads to excessive cell

division resulting in tumour.


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Tumour suppressor geneProduce suppressor protein that stop the

cycle.

There is no break on the cell cycle if the geneis mutated.

Example : p53

Stop the cell cycle by inhibiting the enzymes atthe G1/ S transition

Prevent the cell from copying its DNA

There is no p53 in cancer cells.

Cannot stop the entry into the S phase


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Environment and cancer Damage from the environment is either chemical or

physical.

Smoking

The greatest chemical risk

Increases the chance of lung cancer

Tar in the smoke is a carcinogen.

Tar lodges in the bronchi.

Cause damage to the DNA in the surrounding epithelial

cells.


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UV rays

Cause physical damage to the DNA in skin cells.

If a tumour is not removed, cancer cell sometimesspread to other parts of the body, carried in the bloodand lymphatic systems (metastasis)

Diet

Radicals cause cancer.

Radicals are chemicals from the diet, from environmental

factors such as smoke, UV or from cells metabolism.

A diet with plenty of fresh fruits and vegetables provideplenty of antioxidants which destroy radicals.


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Monoamine oxidase A (MAOA)An enzyme that catalyze the break down of

neurotransmitters serotonin, norepinephrin,and dopamine in the brain.

These are involved in the regulation of the

behaviour including the response to stress.Two alleles for the MAOA gene.

One for high level of the enzyme and the

other for low level.

Too much and too little of the enzyme leadsto problems like depression and Parkinson's

disease.

Th i di li k b MAOA l l


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There is no direct link between MAOA leveland antisocial behaviour.

Maltreated children with high levels ofMAOA were less likely to exhibit violentbehaviour.

Maltreated children with low levels of MAOAwere more likely to exhibit violent behaviour.

Protective children with low level of MAOA

were less likely to exhibit violent behaviour.


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Continuous variation There is a complete range of measurement from one

extreme to the other.

Controlled by the combined effect of many genes(polygene) and environment.

Example: Height, B.P, skin colour, eye colour etc


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Discontinuous variation Individuals fall into a number of distinct classes or

groups.

Controlled by alleles of a single gene or a small numberof genes.

The environment has little effect on this type of

variation. Example : blood group


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Polygenic inheritanceGroups of genes which together determine a

characteristic are called polygenes.

The inheritance of phenotypes that are

determined by the collective effect of severalgenes is called polygenic inheritance.

Polygenes are often located on differentchromosomes.

These genes interact with one another.


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Eye colourGenes at three loci are involved in the

inheritance of eye colour.

Alleles A, B, C and a, b, c.

A, B and C add pigment to the iris but the allelesa, b and c does not add any pigment.

All the three loci- dominant allele- dark browniris

All the three loci-recessive allele- pale blue.


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Gametes formed are

ABC

ABcAbC

aBC

abC

aBc

Abc

abc

N b f Ph


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Number ofdominantalleles

Phenotypes

6 Darkest brown

5 Dark brown

4 Brown

3 Light brown

2 Dark blue

1 Light blue

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Environmental Influence on Genes(18)