Friday, 25 March 2011

tMoL: Genotype vs. Phenotype

It's the end of another term, time for me to move on again. I'll be going home soon to write about what's been going on, and there's a lot to say. The talks I did in schools went really well, although there's still stuff to improve on. This lab placement has been really great and I've been enjoying all the other projects I'm involved in. Next up is an unconference, SciBarCamb, which I strongly recommend to anyone reading this blog.

This is a peice I wrote for a friend of a friend, she's doing a course in genetics and was looking for clarification of the difference between genotype and phenotype, it was a bit of a rush job but hopefully it clarifies matters.

In order to live, grow and make more of themselves, cells need to have instructions. These are stored as DNA, which is mostly found in the cell nucleus. Each set of instructions is called a gene, each gene produces a particular protein. The full set of human DNA, the human genome, can make thousands of proteins and each protein performs a specific function within a cell.

Genes are specific DNA sequences (source)
 Different people have different versions of the same gene and hence of the same protein, this is what makes us genetically different. These different gene alleles have different effects within our body. As a simplified example; there is a protein which is responsible for hair colour. If it is functioning properly then you have brown hair. Some versions of the protein do not function properly so there is no colour made, this results in blonde hair. There are various other versions with intermediate effects resulting in all different hair colours. This is a slight simplification, as there are actually multiple proteins involved in hair colour, but it illustrates the principle.

There are two terms that geneticists (scientists who study genes) like to use a lot when discussing the effects genes have on a living creature. The first is genotype, this is a description of which version of a gene a particular individual has. It is often written as an abbreviated code which describes the gene and how its activity differs from other versions of the same gene, whilst it is very useful in the field it can be quite inaccessible to others. The second term is phenotype, meaning the observed effect of having a specific genotype i.e. of having certain versions of a protein. This is generally easy to understand and was the basis of a lot of scientific research prior to our discover of DNA. Hair colour, eye colour and gender are all obvious examples of phenotypes they are the observed effects of specific genes.

A schematic of the structure of a gene. The exact sequence of each gene is your genotype. (Source)

Many of the common, well studied examples of genotypes and phenotypes come from research into illnesses. A genetic illness (as opposed to an infection/environmental disease) is a phenotype, it is a description of a set of easily observed symptoms e.g. haemophilia, Down’s syndrome, Muscular dystrophy, Type I diabetes. Genetic diseases are something you are born with – although in some cases, like Huntingdon’s disease, there are no effects until later life. You cannot catch them from other people or transfer them to other people, they are as much a part of who you are as your hair colour. Illnesses provide an interesting demonstration of the differences between genotypes and phenotypes. Primarily, a specific phenotype can be the result of any one of many different genotypes.

Diabetes occurs when you are unable to correctly control storage of sugars in your body. Sugar in the blood is the pancreas which then produces insulin. Insulin encourages storage of sugar and reduces the amount circulating in the blood. Most people develop type II diabetes later in life as a result of a high sugar diet. Type I diabetes is much more dependent on your genes but there are many ways that your genes can cause it. There are many genotypes for the same phenotype. There can be changes in detection of blood glucose, the production of insulin or the response of other cells to insulin and sugar storage. Each of these processes includes many proteins with certain functions and if any of these does not work correctly the result is the same.

Eye colour is a phenotype, it is the observable result of the activity of specific gene variants (Source)

The reverse can also be true in that one genotype has many phenotypes, firstly it depends on how you look for a phenotype. When you look at the whole body you can have one phenotype but if you look at a specific part of the body, or when you look at the cells the phenotype can be different. Huntingdon’s disease is characterised by development of unusual behaviour patterns and severely reduced mental capacity which usually only develops around middle age. However it can also be described, in the same patient, on a cellular level as brain cell death. This is one genotype giving different phenotypes. It also depends on where you look for your phenotype, the same gene can do different things in different parts of the body. Changing the gene will have different effects in each of these different body parts.

Your genotype, the forms of genes that you have, is responsible for which versions of different proteins you have and, on many different levels, this causes your phenotype, it makes you who you are. There are many arguments over the contributions of upbringing and environmental factors to who you are and it is very difficult to study this reliably, it is generally dependent on ‘twin studies’ studying those rare cases where twins (which have exactly the same genotype) are separated at a young age and brought up in different environments. But that’s probably a topic for another time.

1 comment:

Anonymous said...

Thank you for the explanation. Have you thought about teaching?!!