Variation within a population is a critical and required component for natural selection. If you have understood all the work on DNA, chromosomes and Mendelian genetics, you should now have a good understanding of where the genetic causes of this variation comes from. But remember that variation can also be caused by the environment. Indeed all variations in reality come from an interplay between genetic and environmental factors. Genes by themselves cannot cause variation as without the environment of a cell to produce the protein, genes alone cannot affect the phenotype.
Genetic causes of variation
Sexual reproduction is the key to genetic variation in a population. Meiosis produces gametes that are haploid and genetically different from each other. The gametes may have a different combination of randomly “shuffled” chromosomes. Crossing over in meiosis also allows alleles that would not otherwise be combined in a gamete. So there are massive genetic differences between one gamete and the next. And then there is random fertilisation so that any one male gamete is equally likely to fuse with any female gamete. Random fertilisation (of gametes that meiosis has made genetically different to each other) is the key to genetic variation in a population.
A final cause of genetic variation has nothing to do with sexual reproduction and is mutation. DNA replication is not 100% accurate – the enzymes in eukaryotes make one error every billion base-pairs. These mutations are random and can lead to new alleles appearing in a population. Chromosomal mutations can also occur where chromosomes do not separate properly in meiosis or parts of a chromosome break off and re-join somewhere else….
Environmental causes of variation
Some of the differences seen in populations are due not to differences in genes but due to the differing environments in which an organism lives. Peas which have inherited two copies of the T allele (for tallness) will never grow tall unless they are planted in well-watered soil and given access to sunlight. You would never be a school teacher as a career without understanding that the environment a brain develops in can affect a person’s outcomes. Environment is as important as genes in many variations in the human population, in particular to do with health and disease. This is why so much emphasis for health for example is placed on promoting balanced diets, altering smoking habits, and moderating alcohol consumption.
Genes and Environment always interact to determine Phenotype
Don’t allow yourself to fall into the lazy thinking of the “nature-nurture” debate. It is lazy thinking because the debate is a nonsense. Nothing is either determined by your genes or your environment – it is always both. So when you read of a ‘gene for obesity’ or a ‘gene for domestic violence’ treat with extreme caution (and switch newspapers……) If you read that playing violent computer games causes violent behaviour in humans, treat with caution. None of these variations in a population will be just due to genes, none will be just do to the environment. It will always be some complex interaction between the two.
Mutations are changes in the DNA content of a cell. There are various ways the DNA of a cell could change and so mutations tend to be grouped into two main categories: chromosomal mutations and gene mutations.
This is a change in the number or length/arrangement of the chromosomes in the nucleus. For example, people with Down’s syndrome have an extra copy of chromosome 21 giving them three chromosome 21s as opposed to the normal two.
(How many chromosomes in total will a person with Down’s syndrome have in each cell?)
Chromosomal mutations are often found in tumour cells and so play a critical role in the development of various cancers.
Sometimes the number of chromosomes in a cell stays the same, but sections are deleted, duplicated or break off from one chromosome to attach elsewhere. If this happens, this too would be classed as a chromosomal mutation.
Gene mutations happen to change the sequence of base pairs that make up a single gene. As you all know, the sequence of base pairs in a gene is a code that tells the cell the sequence of amino acids to be joined together to make a protein. A gene then is the sequence of DNA that codes for a single protein. If you alter the sequence of base pairs in the DNA by adding extra ones in, or deleting some or inverting them, this will alter the protein produced.
A point mutation is a change to just one base within the gene – it occurs at a single point on the DNA molecule.
Mutations can happen at any time and occur randomly whenever the DNA is replicated. But there are certain things that can increase the rate of mutation and so make harmful mutations more likely. A mutagen is an agent that increases the chance of a mutation occurring.
a) Radiation can act as a mutagen
Some parts of the electromagnetic spectrum can cause mutations when they hit DNA molecules or chromosomes. This is called ionising radiation and includes gamma rays, X rays and ultraviolet. You probably know that the dentist goes out of the room whenever they take an X ray to protect themselves from repeated exposure to X rays and you all certainly know of the link between UV exposure and incidence of skin cancer.
b) There are chemical mutagens as well
Some chemicals can make the rate of mutation increase. These are called chemical mutagens and a good example is the tar in tobacco smoke. Tar can cause cancers to form wherever the cigarette smoke comes into contact with cells and this is because tar is a mutagen. It makes mutations in the DNA much more likely and mutations are needed to turn a healthy cell into a cancer cell.