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Meiosis and variation
Lesson 9
Continuous and discontinuous variation
Learning objectives
You should be able to:
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Describe the differences between continuous and
discontinuous variation
Explain the basis of continuous and discontinuous
variation by reference to the number of genes
which influence the variation
Explain that both genotype and environment
contribute to phenotypic variation. (No calculations
of heritability will be expected)
Explain why variation is essential in selection
Variation
All organisms display variation
This can be either continuous or
discontinuous
What’s the difference?
Discontinuous variation describes qualitative differences between
phenotypes. This means phenotypes fall in to clearly
distinguishable categories
Human blood group is a good example. The four possible
categories are O, A, B and AB
Continuous variation describes quantitative differences between
phenotypes. This means that there are no distinct categories
of phenotype.
Height and mass in humans are good examples
The genetic basis of
discontinuous and
continuous variation
Discontinuous variation
Discontinuous variation is usually the result of just one
gene controlling the phenotype. This is described as
being monogenic and cystic fibrosis is a good
example
The only way we can get discontinuous variation of a
phenotype that is controlled by two or more genes is
if the genes interact with each other through a
mechanism such as epistasis
In discontinuous variation, each allele will have a large
effect on the phenotype
Different gene loci may have very different effects on
the phenotype
Dominant, codominant and recessive alleles may be
involved
Continuous variation
Continuous variation is the result of two or
more genes contributing to the phenotype.
It’s genetic basis is usually polygenic, in other
words the characteristic is controlled by
many unlinked genes (called polygenes)
Each gene provides an additive component to
the phenotype
Each allele has only a small effect on the
phenotype
Made up example of how
continuous variation might
be inherited
Imagine banana plants in which the length of the banana is
controlled by 3 genes, A/a, B/b and C/c
The presence of a dominant allele at each locus will add 8cm to
the length of the bananas on the plant
Two recessive alleles as a locus means that only 4cm is added to
the length of the bananas on the plant
What length will the bananas growing on plants with the
following genotypes be?
AABBCC
AabbCc
aabbCc
aabbcc
Genotype and environment
interact to contribute to
phenotype
In the example we have just looked at a plant with alleles
AABBCC should produce bananas that are 24cm long
But suppose the plant didn’t get enough light, water or nitrate?
The plant and the bananas on it would be smaller
In other words, environmental factors may limit the expression
of some genes
Polygenic traits tend to be more affected by environmental
factors than those under monogenic control
Determination of human
intelligence
This is an immensely controversial topic, not least
because intelligence is very hard to define and
therefore measure
Some scientists propose that both genes and
environmental factors, such as good nutrition,
contribute to human intelligence
It’s probably best to avoid making any definitive
statements about this subject though
I
Variation and selection
Selection occurs where individuals with
particular characteristics are more likely to
breed and so pass on those characteristics
Over several generations this can lead to
significant genotypic and phenotypic
changes within a population
Selection always requires genetic variation to
exist within a population if it is to be
heritable
There are two kinds of
selection
Natural selection
This occurs where individuals within a population are at a
selective advantage and so more likely to breed and
pass on their advantageous alleles
Artificial selection (there’s no such thing as unnatural
selection)
This is where humans select which plants or animals to
breed from so that desired characteristics are passed on
The domestication of animals and plants for agricultural
purposes is the most common reason for this
How continuous and
discontinuous variation
relate to Mendel’s work
By showing that pea plants inherited discontinuous
traits, such as green or yellow pods, Mendel showed
that inheritance was particulate (it came in particles).
The trouble was that Mendel had no idea what these
particles of inheritance might be as noone knew
about DNA or chromosomes
Worse still for Mendel, his ideas clashed with prevailing
thought at the time which was that the
characteristics of the two parents somehow blended
to produce the phenotype of the offspring. Even
Charles Darwin believed this
This meant that Mendel’s work was ignored and then
forgotten
What changed this?
The discovery of chromosomes in the late nineteenth
century and the observation of their behaviour
during meiosis during the early twentieth century
supported Mendel’s idea of inheritance being
particulate
This coincided with scientists rereading Mendel’s work
after doing similar experiments themselves and
realising that he had been right all along
The only question that now remains about Mendel’s
work is his recording of the results he obtained.
There are those that say they are a little too
perfect…………………..