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Genetic Control - On and Off Genes
- Control of Growth & Development

This regulation of growth tutorial looks at the various factors involved that affect growth. When it comes to the crunch, it is the genetic coding of our bodies that determine the way we are and how we work, with the external environment either emphasising or inhibiting the effectiveness of some of these genes.

The Role of Genes

Genes are the blueprint of our bodies, a blueprint that creates the variety of proteins essential to any organisms survival. These proteins, which are used in countless ways by our bodies are produced by genetic sequences, i.e. our genes, as described in the cell biology section, protein synthesis pages.

Utilization of Genetic Information

All cells have originated from the single zygote cell that formed it, and therefore possess all the genetic information that was held in that zygote. This means that an organism could be cloned from the genetic information in the nucleus of one cell, regardless of the volume of cells that make the organism (be it one or billions).

However, this brings about the following question, how can cells become differentiated and specialised to perform a particular function if they are all the same? The answer to this is each cell performing its unique role has some of its genes 'switched on' and some 'switched off'.

In light of this, the cells in our body still contain the same genetic information, though only a partial amount of this information is being used in any one cell.

Switched On and Switched Off

Some genes are permanently switched on, because they contain the blueprint for vital metabolites (enzymes required for respiration etc). However, since cells become specialised in multi-cellular organisms such as ourselves, some genes become switched off because they are no longer required to be functional in that particular cell or tissue.

For instance, insulin is produced in pancreas cells, which must have the gene that codes for insulin switched on, and perhaps other genes that are un-related to the role of the pancreas can be switched off.

Some other genes that will be functional during specialisation determine the physical characteristics of the cell, i.e. long and smooth for a muscle cell or indented like a goblet cell

Skin Colour

Skin colour is an excellent example of genetic control at work. Skin colour depends on the degree of melanin found in skin cells. The amount of melanin is pre-determined by the genetic blueprint of some genes in each cell. To be exact, there are two genes that control the production of melanin, each of which has a dominant and recessive expression. This leads to a possible 16 combinations of genotype when coding for skin colour, as seen below.

Possible combinations of genotype coding for skin colour via melanin

# Genotype
1 M1M1M2M2
2 M1M1M2m2
3 M1M1m2m2
4 M1m1m2m2
5 m1m1m2m2

Black Skin
Dark Brown Skin
Brown Skin
Light Brown Skin
White Skin

Although there are 16 possible combinations in expressing the skin phenotype, there are 5 different possible genotypes that the genes of melanin can express for, as indicated above. Each expression of melanin has an accumulating effect on skin tone, until maximum expression of melanin through 4 dominant alleles leads to a black skin phenotype.

Therefore, when any person is born, they will be one of five colours. After this, external factors such as UV sunlight from the sun will change the skin colour away from the genetic expression of its initial colour.

Eye Colour

Melanin is also present in the iris of the eye, therefore its accumulating effect on colour determines the colour of the eye depending on how many dominant and recessive alleles are expressed. The coding for brown eyes is dominant to the coding of blue eyes.


Albinism is an occurrence caused by a deficiency of a particular enzyme in a biochemical pathway. The resultant effect is that no melanin is present in the organism, which show pale eyes and white hair/skin. This is not a lethal occurrence in organisms, provided they are not over exposed to ultraviolet radiation from the sun, which can be carcinogenic.

More information about the way genes control and determine the make up of our body is investigated upon on the next page.

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