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 chi-squared 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

Genetic variation can be inherited

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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 Discontinuous VariationGives a full range of intermediate phenotypes between two extremes. The majority of individuals are close to the mean value with low numbers at the extremesThere are no distinct categories

Gives a few discrete categories (phenotypes) with no intermediates

Quantitative differences between phenotypes– examples give measurements

Qualitative differences between phenotypes– examples do not 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

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

Polygenic phenotypes are affected by the environment more than monogenic traits eg mass of an

No or very little environmental effects

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animal is affected by the availability of foodExamples:

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?

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 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

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 Phenotypes

Population sample size = 2000

One person has cystic fibrosis

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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