Genotype and phenotype

We are all unique. Even monozygotic twins, who are genetically identical, always have some variation in the way they look and act. This uniqueness is a result of the interaction between our genetic make-up, inherited from our parents, and environmental influences from the moment we are conceived.

Understanding genotype and phenotype

Wilhelm Johannsen was a scientist working in Denmark in the late 19th and early 20th centuries. During a series of experiments, he observed variations in genetically identical beans. He concluded the variation must be due to environmental factors and coined the terms ‘genotype’ and ‘phenotype’ in 1911.

Genotype is the genetic make-up of an individual organism. Your genotype functions as a set of instructions for the growth and development of your body. The word ‘genotype’ is usually used when talking about the genetics of a particular trait (like eye colour).

Phenotype is the observable physical or biochemical characteristics of an individual organism, determined by both genetic make-up and environmental influences, for example, height, weight and skin colour.

How genotype affects phenotype

The term ‘genotype’ is usually used to refer to specific alleles. Alleles are alternative forms of the same gene that occupy the same location on a chromosome. At any given locus, there are 2 alleles (1 on each chromosome in the pair) – you get 1 allele from your mother and 1 from your father. The 2 alleles might be the same or they might be different. Different alleles of a gene generally serve the same function (for example, they code for a protein that affects eye colour) but may produce different phenotypes (for example, blue eyes or brown eyes) depending on which set of 2 alleles you have.

For example, the ability to taste PTC (a bitter tasting compound) is controlled by a single gene. This gene has at least 7 alleles but only 2 of these are commonly found.

An upper case ‘T’ represents the dominant allele that confers the ability to taste – ‘dominant’ means that anyone with 1 or 2 copies of this allele will be able to taste PTC. The non-tasting allele is recessive and is represented by a lower case ‘t’ – ‘recessive’ means that an individual will need 2 copies of the allele to be a non-taster.

Each pair of alleles represents the genotype of a specific individual, and in this case, there are 3 possible genotypes: TT (taster), Tt (taster) and tt (non-taster).

If the alleles are the same (TT or tt), the genotype is homozygous. If the alleles are different (Tt) the genotype is heterozygous.

It is actually rare that 1 gene determines 1 characteristic as in the case of PTC tasting (a monogenic trait). Most traits are complex and have genes that affect them at more than 1 locus (polygenic).

How environmental factors affect phenotype

Your genes carry the instructions for the growth and development of your body. However, your phenotype is influenced during embryonic development and throughout your life by environmental factors. Environmental factors are many and varied and include diet, climate, illness and stress.

With the PTC tasting example, scientists estimate that the gene controls about 85% of the ability to taste. Environmental factors that play a role include how dry your mouth is or how recently you have eaten.

The degree to which your phenotype is determined by your genotype is referred to as ‘phenotypic plasticity’. If environmental factors have a strong influence, the phenotypic plasticity is high. If genotype can be used to reliably predict phenotype, the phenotypic plasticity is low.

Overall, the amount of influence that environmental factors have on your ultimate phenotype is a hotly debated scientific issue. It is often referred to as the ‘nature (genes) versus nurture (environment)’ debate. Scientists commonly study monozygous (identical) twins to investigate the genotype/phenotype relationship.

In conclusion, your genotype or genetic make-up plays a critical role in your development. However, environmental factors influence our phenotypes throughout our lives, and it is this on-going interplay between genetics and environment that makes us all unique.

Useful links

Visit the Learn Genetics website to learn more about the gene for tasting PTC.
http://learn.genetics.utah.edu/content/begin/traits/ptc/ external link

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