OCR A2 UNIT 5 VARIATION PART 1
Specification:
a) Explain the terms allele, locus, phenotype, genotype, dominant,
codominant and recessive
b) Explain the terms linkage and crossing-over
c) Explain how meiosis and fertilisation can lead to variation through the
independent assortment of alleles
d) Use genetic diagrams to solve problems involving sex linkage and
codominance
e) Describe the interactions between loci (epistasis). (Production of
genetic diagrams is not required)
f) Predict phenotypic ratios in problems involving epistasis
g) Use the chi-squared (χ2) test to test the significance of the difference
between observed and expected results. (The formula for the chisquared test will be provided)
h) Describe the differences between continuous and discontinuous
variation
i) Explain the basis of continuous and discontinuous variation by
reference to the number of genes which influence the variation
j) Explain that both genotype and environment contribute to phenotypic
variation. (no calculations of heritability will be expected)
k) Explain why variation is essential in selection
l) Use the Hardy-Weinberg principle to calculate allele frequencies in
populations
m) Explain, with examples, how environmental factors can act as
stabilising or evolutionary forces of natural selection
n) Explain how genetic drift can cause large changes in small populations
o) Explain the role of isolating mechanisms in the evolution of new
species, with reference to ecological (geographic), seasonal (temporal)
and reproductive mechanisms
p) Explain the significance of the various concepts of the species, with
reference to the biological species concept and the phylogenetic
(cladistic/evolutionary)species concept
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q) Compare and contrast natural selection and artificial selection
r) Describe how artificial selection has been used to produce the modern
dairy cow and to produce bread wheat (Triticum aestivum)
a) Describe the differences between continuous and discontinuous
variation
b) Explain the basis of continuous and discontinuous variation by
reference to the number of genes which influence the variation
c) Explain that both genotype and environment contribute to phenotypic
variation (No calculations of heritability will be expected)
d) Explain why variation is essential in selection
e) Use the Hardy-Weinberg principle to calculate allele frequencies in
populations
Definition of Variation
Variation in biology refers to

The differences between members of different species (interspecific
variation) – this variation is used to classify different species
and

The differences between members of the same species (intraspecific
variation) – this variation is the basis of natural selection
Causes of Variation
There are two causes of variation within a species:

Genetic causes

Environmental causes
Genetic Causes

Due to differences in the alleles that organisms from the same species
inherit from their parents

In sexually reproducing organisms, this genetic variation derives from
meiosis and random fertilisation

Mutations involving errors in DNA replication, also cause genetic
variation, and this could occur in both sexually and asexually
reproducing individuals
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
Genetic variation can be inherited
Environmental Causes

This variation occurs during an individuals lifetime and depends upon
the environmental effects on the expression of the genes

The availability of food will affect the size of animals

The availability of light, water and mineral ions in the soil will affect the
sizes of plants and possibly the colour of their leaves

Human intelligence is only realised if there is a stimulating learning
environment both at home and at school. Good nutrition is also
important for the growth and development of the brain and nervous
system

Environmental variation cannot be inherited
Continuous and Discontinuous Variation
These are the two types of variation within the same species. The table
below gives the characteristics and examples of these two types of variation
Continuous Variation
Gives a full range of intermediate
phenotypes between two extremes.
The majority of individuals are close
to the mean value with low numbers
at the extremes
There are no distinct categories
Quantitative differences between
phenotypes– examples give
measurements

Due to many two or more
genes
 Each gene provides an
additive component to the
phenotype
 Different alleles at each gene
locus has a small effect on the
phenotype
 Some characteristics are
controlled by many genes,
called polygenes. The
characteristic is described as
polygenic. The genes are
unlinked
Polygenic phenotypes are affected by
the environment more than
Discontinuous Variation
Gives a few discrete categories (phenotypes)
with no intermediates
Qualitative differences between phenotypes–
examples do not give measurements






In many examples, only one gene is
involved (monogenic inheritance)
Different alleles at a single gene locus
have large effects on the phenotype
In other examples, a few genes are
involved
If more than one gene are involved, the
genes interact in an epistatic way
Different gene loci have quite different
effects on the phenotype
Examples include codominance,
dominance and recessive patterns of
inheritance
No or very little environmental effects
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monogenic traits eg mass of an
animal is affected by the availability of
food
Examples:
 Mass and linear
measurements of organisms
 Such as height in humans
 Length of leaves on an oak
tree
 Skin or eye colour in humans
is also classified as showing
continuous variation –
controversial?
Data can be plotted as a histogram
after dividing the data into a number
of groups or classes.
With a large sample size, the data
gives a normal distribution curve
Examples:
 gender in animals (male or female)
 human blood groups
 resistance or not, to insecticides in an
insect species
 resistance or not, to an antibiotic, in a
bacterial species
 red, pink and white coloured flowers in
the snapdragon
Data is plotted as a bar chart with each
category represented by a bar, separated from
other bars by equidistant spaces on the graph
paper
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Some Examples of Continuous Variation
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Examples of Discontinuous Variation
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The graph below shows another example of discontinuous variation.
Within the same wheat species, some plants are resistant to mildew
infection and some are not
Data obtained from Gel Electrophoresis of Liver Enzymes from Eight
Different House Mice (Mus musculus)
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Observations and conclusions from this gel electrophoresis data:





The eight mice contained three common liver enzymes but there was
variation amongst them for four other enzymes
This variation in the liver enzymes, reflects genetic differences in the
mice
Each enzyme is coded for by a different gene. If the enzyme is
missing, the mouse must possess two alleles that do not code for the
enzyme
Among the eight mice, there is genetic variation relating to liver
enzymes giving rise to variation at the biochemical level
This genetic variation may affect the survival of the mice in their
environment, it is the interspecific variation that is the raw material for
natural selection
Variation and Selection

Natural selection, as proposed by Charles Darwin, involves
environmental selection pressures selecting those individuals of a
population that are best adapted to survival. These individuals will
survive to breed and pass on their desirable adaptative features.
Natural selection depends upon genetic variation within the
population

Artificial selection has been carried out by humans since agriculture
began. Humans have selected animals and plants with desired
features to breed from. Artificial selection depends upon genetic
variation within each population
Population Genetics
Definition of Population Genetics
In population genetics, geneticists focus on the alleles and genotype
frequencies in populations, from generation to generation
A population refers to members of the same species that can interbreed to
produce fertile offspring
Factors Affecting Population Size/Density

Birth and death rates

Emigration and immigration
Gene Pool refers to all the alleles within a population
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Measurements of Allele and Genotype Frequencies in Populations
1) For Codominant Traits
Since both alleles contribute to the phenotype, the genotype frequencies are
the same as the phenotype frequencies
Example 1
The frequencies of genotypes in the human population for the
inheritance of MN blood groups

A single gene controls inheritance of the MN blood group

There are two codominant alleles of the gene. LM and LN

Each allele controls the synthesis of a specific antigen in the red blood
cell plasma membrane
Genotypes in the Population
Corresponding Phenotypes

Population size = 100

MM – 36 individuals

MN – 48 individuals

NN – 16 individuals

Total number of alleles in the population = 200 (each individual has two
alleles)

Total number of M alleles: 72 + 48 = 120

Frequency of M alleles: 120/200 = 0.6

Since the frequency of M + N = 1

Frequency of N: (1.0 – 0.6) = 0.4
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2) For Traits with Dominant and Recessive Alleles
Hardy and Weinberg developed a mathematical model to calculate allele
frequencies for traits controlled by dominant and recessive alleles, in
populations.
The Hardy-Weinberg principle is a fundamental concept in population
genetics. This principle can only be applied to populations that fulfil the
following criteria:
Assumptions

The organisms are diploid

The population is very large (this eliminates sampling error)

Members of the population are sexually reproducing

There is random mating within the population (this wouldn’t be the case
in a zoo where animals are selected for mating)

No genotype has a selective advantage

There is no mutation, migration or change in allele frequency (genetic
drift) during the investigation
Changes in Allele Frequencies in a Population result in Evolutionary
Changes
Causes of these changes in allele frequency:

Changes in selection pressures resulting in natural selection

Genetic drift

Mutation

Migration
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Example: Cystic Fibrosis
Let F represent the normal allele for mucus production and f, the recessive
allele
Genotypes in the Population

Population sample size = 2000

One person has cystic fibrosis
Phenotypes
Question: How many in this population are carriers?
p = frequency of dominant allele F
q = frequency of recessive allele f
p2 = frequency of genotype FF
q2 = frequency of genotype ff
2pq = frequency of genotype Ff ( from the mating of two heterozygotes, as
seen on page 10)
Frequency of Genotypes from a Mating of Two Carrier Parents
Parental Phenotypes
Parental Genotypes
Gamete Genotypes
Offspring Genotypes from a Punnett Square
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The resulting genotypes of the offspring (FF, 2Ff and ff) can be expressed as
p2 + 2pq + q2
The frequency of the alleles (p + q) = 1
The frequency of the genotypes (p2 + 2pq + q2) = 1
We know that q2 = 1 in 2000 = 0.0005
Therefore q = 0.022
If (p + q) = 1, then p = (1-0.05) = 0.978
The frequency of carriers is given by 2pq = 2 x 0.978 x 0.022 = 0.043
A frequency of 0.043, means that 4.3 individuals out of 100 in the population
are carriers.
In a population of 2000, this means that 4.3/100 x 2000 = 86 are carriers
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