The term ‘variation’ refers to observable differences within a species. All domestic cats belong to the same species i.e. they can all interbreed, but there are many variations of size, coat colour, eye colour, fur length, etc. Those variations that can be inherited are determined by genes. They are genetic variations. Phenotypic variations may be brought about by genes, but can also be caused by the environment, or a combination of both genes and the environment.
So, there are variations that are not heritable, but determined by factors in the environment. A kitten that gets insufficient food will not grow to the same size as its litter mates. A cat with a skin disease may have bald patches in its coat. These conditions are not heritable. They are cause by environmental effects. Similarly, a fair-skinned person may be able to change the colour of his or her skin by exposing it to the Sun, so getting a tan. The tan is an acquired characteristic. You cannot inherit a suntan. Black skin on the other hand, is an inherited characteristic.
Many features in plants and animals are a mixture of acquired and inherited characteristics see Figure.
For example, some fair-skinned people never go brown in the Sun, they only become sunburned. They have not inherited the genes for producing the extra brown pigment in their skin. A fair-skinned person with the genes for producing pigment will only go brown if he or she exposes themselves to sunlight. So the tan is a result of both inherited and acquired characteristics.
Variation continuous and discontinuous
There are two kinds of variation: continuous and discontinuous
Continuous variation is influenced by a combination of both genetic and environmental factors. Continuously variable characteristics are usually controlled by several pairs of alleles. There might be five pairs of alleles for height – (Hh), (Tt), (Ll), (Ee) and (Gg) – each dominant allele adding 4 cm to your height. If you inherited all ten dominant genes (HH, TT, etc.) you could be 40 cm taller than a person who inherited all ten recessive genes (hh, tt, etc.). There are no distinct categories of height; people are not either tall or short. There are all possible intermediates between very short and very tall see Figure.
There are many characteristics that are difficult to classify as either wholly continuous or discontinuous variations. Human eye colour has already been mentioned. People can be classified roughly as having blue eyes or brown eyes, but there are also categories described as grey, hazel or green. It is likely that there are smaller number of genes for eye colour and a dominant gene for brown eyes, which overrides all the others when it is present. Similarly, red hair is a discontinuous variation but it is masked by genes for other colours and there is continuous range of hair colour from bond to black.
The actual number of genes that control height, intelligence, and even the colour of hair and skin, is not known.
Continuously variable characteristics are greatly influenced by the environment. A person may inherit genes for tallness and yet not get enough food to grow tall. A plant may have the genes for large fruits but not get enough water, minerals or sunlight to produce large fruits. Continuous variations in human populations, such as height, physique and intelligence, are always the result of interaction between the genotype and the environment.
In discontinuous variation, the variations take the form of distinct, alternative phenotypes with no intermediates. The mice are either black or brown; there are no intermediates. You are either male or female. Apart from a small number of abnormalities, sex is inherited in a discontinuous way. Some people can roll their tongue into a tube. Others are unable to do it. They are known as non-tongue rollers. Again, there are no intermediates see Figure.
Discontinuous variation cannot usually be altered by the environment. You cannot change your eye colour by altering your diet. A genetic dwarf cannot grow taller by eating more food. You cannot learn how to roll your tongue.
|Continuous variation||Discontinuous variation|
|Properties||– No distinct categories|
– No limit on the value
– Tends to be quantitative
|– Distinct categories|
– No in-between categories
– Tends to be qualitative
– heart rate
– finger length
– leaf length
|– tongue rolling|
– finger prints
– eye colour
– blood groups
|Representation||Line graph||Bar graph|
|Controlled by||A lot of Gene and environment |
⟶ range of phenotypes between 2 extremes
e.g. height in humans
|A few genes|
⟶ limited number of phenotypes with no intermediates
e.g. A, B, AB and O blood groups in humans
New combinations of genes
If a grey cat with long fur is mated with a black cat with short fur, the kittens will all be black with short fur. If these offspring are mated together, in due course the litters may include for varieties: black-short, black-long, grey-short and grey-long. Two of these are different from either of the parents.
A mutation may occur in a gene or a chromosome. In a gene mutation it may be that one or more genes are not replicated correctly. A chromosome mutation may result from damage to or loss of part of a chromosome during mitosis or meiosis, or even the gain of an extra chromosome, as in Down’s syndrome.
An abrupt change in a gene or chromosome is likely to result in a defective enzyme and will usually disrupt the complex reactions in the cells. Most mutations, therefore, are harmful to the organism.
Surprisingly, only about 3% of human DNA consists of genes. The rest consists of repeated sequences of nucleotides that do not code for proteins. This is sometimes called ‘junk DNA’, but that term only means that we do not know its function. If mutations occur in these non-coding sequences they are unlikely to have any effect on the organism and are, therefore, described as ‘neutral’.
Rarely, a gene or chromosome mutation produces a beneficial effect and this may contribute to the success of the organism.
If a mutation occurs in a gamete, it will affect all the cells of the individual that develops from the zygote. Thus the whole organism will be affected. If the mutation occurs in a somatic cell (body cell); it will affect only those cells produced, by mitosis, from the affected cell.
Thus a mutation in a gamete may result in a genetic disorder e.g. haemophilia or cystic fibrosis. Mutation in somatic cells may give rise to cancers by promoting uncontrolled cell division in the affected tissue. For example, skin cancer results from uncontrolled cell division in the basal layer of the skin.
A mutation may be as small as the substitution of one organic base for another in the DNA molecule, or as large as the breakage, loss or gain of a chromosome.
Effects of ionising radiation and chemicals on the rate of mutation
- Mutations are normally very rare. However, exposure to radiation and some chemicals, such as tar in tobacco smoke, increases the rate of mutation.
- Exposure can cause uncontrolled cell division, leading to the formation of tumours (cancer).
- Exposure of gonads (testes and ovaries) to radiation can lead to sterility or to damage to genes in sex cells that can be passed on to children.
- Some scientists argue that there is a higher incidence of leukaemia (a form of white blood cells cancer) in the children of workers at nuclear power stations.
Sickle cell anaemia is caused by a mutation in the blood pigment haemoglobin. When the faulty haemoglobin is present in red cell, it causes the cell to deform and become sickle-shaped, especially when oxygen levels in the blood become low.
A person with sickle-cell disease has inherited both recessive alleles (HbSHbS) for defective haemoglobin. The distortion and destruction of the red cells, which occurs in low oxygen concentrations, leads to bouts of severe anaemia. In many African countries, sufferers have a reduced chance of reaching reproductive age and having a family.
There is thus a selection pressure, which tends to remove the homozygous recessives from the population. In such a case, you might expect the harmful HbS allele to be selected out of the population altogether. However, the heterozygotes (HbA HbS) have virtually no symptoms of anaemia but do have the advantage that they are more resistant to malaria than the homozygotes (HbAHbA). It appears that the malaria parasite is unable to invade and reproduce in the sickle cells.
The selection pressure of malaria, therefore, favours the heterozygotes over the homozygotes and the potentially harmful HbS allele is kept in the population see Figure. When Africans migrate to countries where malaria does not occur, the selective advantage of the HbS allele is lost and the frequency of this allele in the population diminishes.
With sickle-cell anaemia, the defective haemoglobin molecule differs from normal haemoglobin by only one amino acid (represented by a sequence of three bases), i.e. valine replaces glutamic acid.
This could be the result of faulty replication of meiosis. When the relevant parental chromosome replicated at gamete formation, the DNA could have produced the triplet – CAT-(which specifies valine) instead of -CTT- (which specifies glutamic acid). In this case, a change of just one base (from A to T) makes a significant difference to the characteristic of the protein (haemoglobin).
Down’s syndrome is a form of mental and physical disability, which results form a chromosome mutation. During the process of meisosis which produces an ovum, one of the chromosomes (chromosome 21) fails to separate from its homologous partner, a process known as non-disjunction. As a result, the ovum carries 24 chromosomes instead of 23, and the resulting zygote has 47 instead of the normal 46 chromosomes.
The presence of the extra chromosome causes unusual characteristics in the baby. These usually include lowered life expectancy, mental retardation (although some Down’s children are very intelligent), early puberty, and a distinctive round face and short neck. The risk of having a baby with Down’s syndrome increases as the mother gets older.
Mutations in bacteria
Mutation in bacteria often produce resistance to drugs. Bacterial cells reproduce very rapidly, perhaps as often as once every 20 minutes. Thus a mutation, even if it occurs only rarely, is likely to appear in a large population of bacteria. If a population of bacteria containing one or two drug-resistant mutants is subjected to that particular drug, the non-resistant bacteria will be killed but the drug-resistant mutants survive. Mutant genes are inherited in the same way as normal genes, so when the surviving mutant bacteria reproduce, all their offspring will be resistant to the drug.
Mutations are comparatively rare events; perhaps only one in every 100000 replications results in a mutation. Nevertheless they do occur naturally all the time.