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VARIATION


Genetics is concerned with explaining how some characteristics are passed from generation to generation, i.e. heredity, or inheritance. However, it is important to firstly deal with some background information about the characteristics themselves. This may then be tied in with evolution and the development of different species of living organisms.
See also notes on classification, chromosome structure, cell division and the structure of DNA.

Like most living organisms, humans show variation. If you consider almost any characteristic, you will find differences between various people (or other animals or plants) in a population. There are two forms of variation: continuous and discontinuous variation.

Characteristics showing continuous variation vary in a general way, with a broad range, and many intermediate values between the extremes. As a matter of fact, if you consider a large enough sample from a population, perhaps plotting frequency as a histogram or as a frequency polygon, you will find that most of the values are close to the average (mean), and extreme values are actually rather rare.
diagram of normal distribution

Height is an example of a continuously variable characteristic, as long as you consider a consistent sample, for example a large number of people of a particular age and sex.

As you will find out later, it is usually difficult to give a straightforward explanation of the genetic basis for these continuously variable characteristics because they result from a combination of genetic factors as well as environmental influences.

List below some other examples of human characteristics which show continuous variation.

> weight
> foot length or any measurable dimension


Characteristics showing discontinuous variation fall into a few very distinct classes. The ability to roll the tongue, and blood groups, are examples of discontinuous variation. These characteristics can be explained much more easily by simple rules of genetics and are less likely to be affected by other factors.
For the tongue-rolling ability, how many classes are there? i.e.?
> 2 (rollers and non-rollers)

How many classes of blood group are there? i.e.?
> 4 (A, B, O, AB)

Causes of variation


Some of the characteristics possessed by an individual in a population can be said to be inherited - i.e. derived from the previous generation. These characteristics are passed on, in a fairly predictable way, as a result of sexual reproduction. Sexual reproduction also introduces an element of randomness, so that variation is brought about in a population. These two almost contradictory factors - dependable inheritance of characteristics from parents, and variation within the population - are essential to an understanding of the process of evolution.
Give some examples of characteristics which may be inherited by a child from his or her parents.
> hair colour
> eye colour
> skin colour etc etc.
We shall return later to a methodical consideration of the way in which these characteristics are passed on; it is certainly too simplistic to imply that characteristics like facial features of children can merely be attributed to parents by looking at them, without knowing the background of previous generations.

The examples often chosen give the impression that inheritance covers only trivial features, such as the shape of the human chin, or ear-lobes. However, an extremely wide range of characteristics are known to be passed on in this way. In fact, practically every aspect of normal human body functioning is under hereditary (genetic) control, because there are many examples of fairly rare "conditions" (diseases which cannot be transmitted from one person to the next, but which are caused by defective functioning of certain cells) which can be inherited in exactly the same way as hair or eye colour. These inherited forms of "disease" may also be called inborn errors of metabolism.

See if you can find some examples of these inherited human conditions which may be passed from one generation to the next.
> sickle cell anaemia
> Huntington's chorea
> cystic fibrosis
> haemophilia
Other characteristics are said to be acquired during life (non-inherited). These may also be said to be caused by environmental effects.
Give some examples of acquired human characteristics.
> scars
> fillings
> ability to speak other languages
> ability to ride bicycle etc.
It must also be said that some characteristics probably have both an inherited and an environmental basis, such as (possibly) I.Q. - intelligence quotient. The balance between them is the answer to the "nature versus nurture" question.

Similar considerations apply in all living organisms; for example different plants grown in different conditions of light or temperature may show differences in growth rate and vigour, and understanding the causes of this variation is quite fundamental in controlling or increasing agricultural and horticultural productivity.
From an experimental point of view, how may the effects of these environmental factors be shown, as distinct from effects due to different genetic make-up of the plant in question?
> use a clone of plants, from cuttings etc?

A Gene Mutation is a very rare event indeed. A mutation in a single inheritable characteristic (gene) is usually less likely than one in a million, but once it has happened, it may be passed on to the next generation, along the same lines as other inherited characteristics.

However, not all individuals carrying mutations survive; most have been found to be harmful, so that the organisms carrying them are at a disadvantage. In the wild, such organisms are unlikely to survive.

Nevertheless, some (beneficial) mutations confer an advantage, and others (neutral?) cause neither advantage nor disadvantage - at least until there is some reason for selection of adapted types to occur. This may be another reason for variation within a population. In fact, the few different forms resulting from mutation which are beneficial can spread through a population by natural selection, and this may have the eventual effect of changing a population so much that it differs from its original form - resulting in the evolution of a new species.

Chromosome Mutations may also result in change in the number of chromosomes incorporated into sex cells. A child produced as a result may have, for instance, an extra chromosome, or an extra part of a chromosome attached to the normal set.
Down's syndrome is caused by having 47 chromosomes instead of the normal 46 per cell.
What is the frequency of babies being born with Down's syndrome?
> 1 in 700

Mutations as described above may occur "naturally", but in the laboratory it has been shown that they may be caused (more efficiently) by other means. Similarly, various factors in the environment may increase the chance of mutations occurring. The risks associated with some lifestyle activities are nowadays known, and exposure to many of these are avoidable. For instance, chemicals in tobacco include mutagens, and several types of ionising radiation have mutagenic effects.
Give some examples of treatments which may cause mutations.
> X rays > gamma rays > ultra-violet rays


These are in fact similar to the causes of cancer, so the idea of "natural" causes for mutations (and cancer?) is probably rather dubious.

Examination of seedlings resulting from irradiated seeds

It is possible to obtain barley seeds which have been irradiated, i.e. exposed to various amounts of radiation. After these seeds are planted and allowed to grow, some of the effects of the radiation can be seen on the adult plants.
Examine the specimen plants provided, growing in seed trays. You should make measurements such as percentage germination, mean height, and estimates of degree of "greenness" and "healthiness".

Radiation
Dose
%
Germination
Mean
Height
Greenness Healthiness Other factors?
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What is the general trend?
>

INFORMATION GATHERING EXERCISE


Below is a list of characteristics which can be easily noted or measured. This includes a range of continuously variable and discontinuously variable characteristics, as well as features which are likely to be acquired rather than inherited. Carry out a survey of the class to gauge the extent of variation in these characteristics. Use the space below to record your results.
As this is recorded in a grid format, you should be able to detect a pattern of correlation between some characteristics. Use the bottom row to summarise the information.
Excel spreadsheet
Other characteristics which are interesting to test: the ability to taste PTC

Class survey

CONTINUOUS VARIATION DISCONTINUOUS ACQUIRED
Name/
initials
Height
/cm
Arm-span
/cm
Weight
/kg
Tongue
roller? (Y/N)
Ear lobe?
(free/joined)
Scars?
(Y/N)
Fillings?
(Y/N)
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Why is it possible to calculate mean values for the first 3 characteristics, but not the others?
>
these are continuously variable - others are either one category or the other
Now extend this survey to your family. For the purposes of the next topic, it may also be interesting to find out your own blood group, and those of your parents and other family members.




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