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Transcript of Cellular Control Unit 1 Communication, Homeostasis and Energy.
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Cellular Control
Unit 1Communication, Homeostasis and Energy
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Meiosis
Module 1: Cellular Control
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Learning outcomes
describe, with the aid of diagrams and photographs, the behaviour of chromosomes during
meiosis, the associated behaviour of the nuclear
envelope, cell membrane and centrioles.
(Names of the main stages are expected, but not the subdivisions of prophase);
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Reproduction and variationAsexual reproduction
Single organism divides by mitosis New organism is genetically identical to
the parentSexual reproduction
Meiosis produces haploid gametes Which fuse at fertilisation to form a
diploid zygote This produces genetic variation amongst
offspring
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Human Life Cycle
Adult46
HaploidSperm
23
HaploidEgg23
Diploid Zygote
46
fertilisation
Meiosis
Mitosis
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Self assessment questions The fruit fly Drosophila melangaster has
eight chromosomes in its body cells. How many chromosomes will there be in a Drosophila sperm?
The symbol n is used to indicate the number of chromosomes in one set – the haploid number of chromosomes. For example in humans n = 23, in a horse n = 32. How many chromosomes are there in a gamete
of a horse? What is the diploid number of chromosomes
(2n) of a horse?
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Meiosis
Meiosis is a reduction division Resulting daughter cells have half the original
number of chromosomes Daughter cells are haploid Can be used for sexual reproduction Source of genetic variation
Meiosis has two divisions meiosis I and meiosis II
Each division has 4 stages Prophase, metaphase, anaphase, telophase
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Meiosis
You can view an animation of Meiosis at http://www.cellsalive.com/meiosis.htm
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Meiosis I
Prophase IChromatin condensesHomologous pairs form a bivalentNucleolus disappearsSpindle forms
Metaphase IBivalents line up on equator of cell
Anaphase IHomologous chromosome in each bivalent are pulled to opposite poles
Telophase ITwo new nuclear envelopes formCell divides by cytokinesis
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Early Prophase 1
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Late Prophase 1
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Metaphase 1
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Anaphase 1
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Telophase 1
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Cytokinesis 1
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Meiosis II
Prophase IINucleolus disappearsChromosomes condenseSpindle forms
Metaphase IIChromosomes arrange themselves on equatorAttach by centromere to spindle fibres
Anaphase IICentromeres divideChromatids pulled apart to opposite poles
Telophase IInuclear envelopes reform around haploid nucleiCell divides by cytokinesis
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Prophase II
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Metaphase II
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Anaphase II
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Telophase II
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Cytokinesis II
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Learning outcomes
explain how meiosis and fertilisation can lead to variation through the independent assortment of alleles
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Key words
AlleleLocusCrossing overMaternal chromosomePaternal chromosome
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Alleles, locus and homologous chromosomes
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Meiosis and variation
Meiosis enables sexual reproduction to occur by the production of haploid gametes.
Sexual reproduction increases genetic variation
Genetic variation increases the chances of evolution through natural selection
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Meiosis and Variation
Crossing over – prophase I Independent assortment of chromosomes
– metaphase I Random assortment of chromatids –
metaphase II Random fertilisation Chromosome mutations
Number of chromosomes▪ Non-disjunction - polysomy or polyploidy
Structure of chromosomes▪ Inversion, deletion, translocation
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Crossing over
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During metaphase I
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During metaphase I
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No crossing over
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Crossing over – new combinations of alleles
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Independent Assortment
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Learning Outcomes
explain the terms allele, locus, phenotype, genotype, dominant, codominant and recessive;
explain the terms linkage and crossing-over;
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Glossary
Gene Locus Allele Genotype Phenotype
Heterozygous Homozygous Monohybrid cross Dominant allele Recessive allele
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Genetics
Genetics is the study of inheritanceAllele
different varieties of the same geneLocus
position of a gene on a chromosome
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Genetics
Dominant An allele whose effect is expressed in
the phenotype if one copy presentRecessive
An allele which only expresses as a homozygote
Co-dominant Both alleles have an effect on the
phenotype
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Genotype genetic constitution of the organism
Phenotype appearance of character resulting from
inherited information
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Homozygous Individual is true breeding Possesses two alleles of a gene e.g. RR
or rrHeterozygous
Two different alleles for a gene e.g. Rr
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Monohybrid inheritance
Mendel’s First Law principle of segregation
“The alleles of a gene exist in pairs but when gametes are formed, the
members of each pair pass into different gametes, thus each gamete
contains only one of each allele.”
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Inheritance of height in pea plants
Follow out the following cross to the F2 generation Homozygous tall pea plant with a homozygous
dwarf pea plant Write out the genotypic and phenotypic ratios
from the F2 generation
gene Allelerelationshi
pSymbol
Height of pea
plants
Tall Dominant Tdwar
frecessive t
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Inheritance of height in pea plants Laying out the cross
P phenotype P genotype Gametes F1 genotype F1 phenotype F1 self-fertilised Gametes Random fertilisation F2 genotypic ratio F2 phenotypic ratio
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Pupil Activity
Answer the questions on monohybrid inheritance Remember to write out each cross in
full.
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Cystic Fibrosis
Cystic Fibrosis is caused by a mutation to a gene on one of the autosomes.
Mutation Changes the shape of the transmembrane
chloride ion channels (CFTR protein) The CFTR gene is found on Chromosome 7 The faulty gene is recessive
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Genetic Cross conventions Use symbols to represent two alleles Alleles of the same gene should be
given the same letter Capital letter represents the dominant
allele Small letter represents the recessive
allele Choose letters where the capital and
small letter look different The examiner needs to be in no doubt
about what you have written
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Inheritance of cystic fibrosis Three possible genotypes
FF unaffected Ff unaffected ff cystic fibrosis
Remember gametes can only contain one allele for the CFTR gene
At fertilisation, any gamete from the father can fertilise any gamete from the mother This can be shown in a genetic diagram
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Genetic diagram showing the chances of a heterozygous man and a heterozygous woman having a child with cystic fibrosis.
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Phenotype ratio of offspring Genotype ratio 1FF:2Ff:1ff
Phenotype ratio 3 unaffected:1cystic fibrosis
Can also be expressed as 25% chance of the child having cystic fibrosis Probability of 0.25 that a child will inherit the
disease Probability that 1 in 4 that a child from these
parents will have this disease.
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Learning Outcome
Use genetic diagrams to solve problems involving sex-linkage and codominance.
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Sex-Linkage
Sex-linked genes are genes whose loci are on the X or Y chromosomes
The sex chromosomes are not homologous, as many genes present on the X are not present on the Y.
Examples Haemophilia Fragile X syndrome Red green colour blindness
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Sex Chromosomes
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Factor VIII and Haemophilia
Haemophilia is caused by a recessive allele of a gene that codes for a faulty version of the protein factor VIII XH normal allele Xh haemophilia allele
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possible genotypes and phenotypes
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Inheritance of Haemophilia
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Pedigree for a sex linked recessive disease
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Codominance
Codominance describes a pair of alleles, neither of which is dominant over the other.
This means both have an effect on the phenotype when present together in the genotype
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Codominance example
Flower colour in plants CR red Cw white
Genotypes CRCR red flowers CRCW pink flowers CWCW white flowers
Write out a genetic cross between a pure breeding red plant and a pure breeding white plant.
Carry out the cross to the F2 generation. Write out the genotype
and phenotype ratio for the F2 generation
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Revision Question
Coat colour in Galloway cattle is controlled by a gene with two alleles. The CR allele produces red hairs and therefore a red coat colour. The Cw allele produces white hairs.
A farmer crossed a true-breeding, red-coated cow with a true-breeding white-coated bull. The calf produced had roan coat colouring (made up of an equal number of red and white hairs).
Explain the result and draw a genetic diagram to predict the outcome of crossing two roan coloured animals.
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Inheritance of A, B, AB and O blood groups
Human blood groups give an example of codominance and multiple alleles There are 3 alleles present▪ IA
▪ IB
▪ Io
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IA and IB are codominant Io is recessive
Remember each human will only have two alleles
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Blood Groups
Genotype Phenotype
IAIA Blood Group A
IA Io Blood Group A
IAIB Blood Group AB
IBIB Blood Group B
IB Io Blood Group B
Io Io Blood Group o
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Inheritance of blood groups
Carry out genetic crosses for the following examples Two parents have blood groups A and B,
the father is IAIo and the mother is IBIo
Father has blood group AB and the mother has blood group O
Mother is homozygous blood group A and the father is heterozygous B.
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Learning Outcome
Describe the interactions between loci (epistasis).
Predict phenotypic ratios in problems involving epistasis.
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Dihybrid Inheritance
Monohybrid cross Inheritance of one gene
Dihybrid cross Inheritance of two genes
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Example – dihybrid cross
Tomato plants Stem colour
A purple stem a green stem Leaf shape
D cut leaves d potato leaves
NOTE In the heterozygote AaDd due to independent
assortment in meiosis there are 4 possible gamete combinationsAD Ad aD ad
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Crosses
Cross a heterozygous plant with a plant with a green stem and potato leaves
Cross two heterozygous tomato plants
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Dihybrid Inheritance
A woman with cystic fibrosis has blood group A (genotype IAIo). Her partner does not have cystic fibrosis and is not a carrier for it. He has blood group O.
Write down the genotypes of these two people.
With the help of a full and correctly laid out genetic diagram, determine the possible genotypes and phenotypes of any children that they may have.
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Autosomal linkage
Each Chromosome carries a large number of linked genes
If two genes are on the same chromosome then independent assortment can not take place.
The genes are transmitted together and are said to be linked.
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Linked Genes
Where linked genes are involved the offspring of a dihybrid cross will result in a 3:1 ratio instead of the 9:3:3:1 ratio.
Example: In peas, the genes for plant height and
seed colour are on the same chromosome (i.e. linked)
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Learning Outcome
Describe the interactions between loci (epistasis).
Predict phenotypic ratios in problems involving epistasis.
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Flower colour in sweet pea Flower colour
Colourless precursor of a pigment C Gene that controls conversion of this pigment
to purple P Both dominant alleles need to be present for
the purple colour to develop Cross
Cross two white flowered plants with the genotypes CCpp and ccPP
Follow this cross through to the F2 generation
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Interactions of unlinked genes
A single character maybe influenced by two or more unlinked genes.
E.g. determination of comb shape in domestic poultry Dominant allele P pea comb Dominant allele R rose comb Two dominant alleles walnut comb No dominant alleles single comb
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Genetic Crosses
Carry out a genetic cross between a true-breeding pea comb and a true breeding rose comb
Follow this cross through to the F2 generation
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Inheritance of coat colour in mice
Wild mice have a coat colour that is referred to as “agouti”. Agouti (A) is dominant to black (a) C is a dominant gene required for coat
colour to develop A homozygous recessive cc means that
no pigment can be formed and the individual is albino
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Inheritance of coat colour in mice
Carry out a cross between a pure-breeding black mouse (aaCC) and an albino (AAcc)
Follow this cross through to the F2 generation.
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Epistasis
This is the interaction of different gene loci so that one gene locus masks or suppresses the expression of another gene locus.
Genes can Work antagonistically resulting in
masking Work complementary
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Epistasis ratios
9 : 3 : 4 ratio Suggests recessive epistasis
9 : 7 ratio Suggests epistasis by complementary
action12 : 3 : 1 ratio or 13 : 3 ratio
Suggests dominant epistasis
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Predicting phenotypic ratiosRead through pages 132 and 133 of
your textbook Answer questions 1 – 7
Complete the stretch and challenge question on “eye colour in humans”
Read through and complete the worksheet provided for you on epistasis
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Learning outcome
Use the chi-squared (χ2) test to test the significance of the difference between observed and expected results.
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χ2 (chi-squared) test
Allows us to compare observed and expected results and decide if there is a significant difference between them.
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χ2 (chi-squared) test
Where Σ = the sum of O = observed value E = expected value
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χ2 (chi-squared) test
Compare the χ2 value to a table of probabilities The probability that the differences between
our expected and observed values are due to chance.
If the χ2 value represents a probability of 0.05 or larger, the differences are not significant
If the χ2 value represents a probability of less than 0.05, it is likely that the results are not due to chance and there is a significant difference.
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Degrees of freedom
The degrees of freedom takes into account the number of comparisons made. Degrees of freedom
= number of classes of data - 1
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Table of χ2 values
Degrees of freedom
Probability greater than
0.1 0.05 0.01 0.001
1 2.71 3.84 6.64 10.83
2 4.60 5.99 9.21 13.82
3 6.25 7.82 11.34 16.27
4 7.78 9.49 13.28 18.46Critical value95% certain that the results are
not due to chance
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Table of χ2 values
Degrees of freedom
Probability greater than
0.1 0.05 0.01 0.001
1 2.71 3.84 6.64 10.83
2 4.60 5.99 9.21 13.82
3 6.25 7.82 11.34 16.27
4 7.78 9.49 13.28 18.46Accept null hypothesisThere is no significant difference, results
have occurred due to chance
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Table of χ2 values
Degrees of freedom
Probability greater than
0.1 0.05 0.01 0.001
1 2.71 3.84 6.64 10.83
2 4.60 5.99 9.21 13.82
3 6.25 7.82 11.34 16.27
4 7.78 9.49 13.28 18.46Reject null hypothesis: accept experimental hypothesis
Difference is significant, not due to chance
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Mammal question
χ2 value = 51.8Degrees of freedom = 3Critical value (p=0.05) = 7.82
Reject the null hypothesisThere is a significant difference
between observed and expected results
Suggestions? The two genes are linked
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Variation
What did you learn at AS level?
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Learning Outcomes
Define the term variation.Discuss the fact that variation occurs
within as well as between species.Describe the differences between
continuous and discontinuous variation, using examples of a range of characteristics found in plants, animals and microorganisms.
Explain both genetic and environmental causes of variation.
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Variation
Variation is the differences that exist between individual organisms. Interspecific variation (between species)▪ Differences that are used to assign
individuals to different species Intraspecific variation (within a species)▪ Individuals of the same species show variation
Variation can be inherited or influenced by the environment.
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Types of variation
There are two main types of variation Continuous variation Discontinuous variation
There are two main causes of variation Genetic variation Environmental variation
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Continuous variation
Existence of a range of types between two extremes
Most individuals are close to a mean value Low numbers of individuals at the extremes Both genes and the environment interact in
controlling the features Examples
Height in humans Length of leaves on a bay tree Length of stalk of a toad stool
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Continuous variation
Use a tally chart and plot results in a histogram
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Discontinuous variation
2 or more distinct categories with no intermediate values
Examples Earlobes attached or unattached Blood groups A, B, AB or o Bacteria flagella or no flagella Flowers colour of petals
Genetically determined The environment has little or no effect on
discontinuous variation
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Discontinuous variation
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Causes of variation
Genetic Variation Genes inherited from parents provide
information used to define our characteristics
Environmental Variation Gives differences in phenotype (appearance)
but not passed on by parents to offspring Examples▪ Skin colour tans with exposure to sunlight▪ Plant height determined by where the seed lands
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Variation
What you need to know for A2!!
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Learning outcomes
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.
Explain why variation is essential in selection.
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variation
Variation can be: Discontinuous▪ Each organism falls into one of a few clear-cut
categories, no intermediate values▪ Qualitative differences between phenotypes
Continuous▪ No definite categories▪ A continuous range of values between two
extremes▪ Quantitative differences between phenotypes
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Genes and variation
Discontinuous (qualitative) variation Monogenic inheritance Different alleles at same gene locus Different gene loci have different effects Epistasis, codominance, dominance and
recessive patterns of inheritance
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Genes and Variation
Continuous (quantitative) variation Polygenic inheritance Two or more genes Each gene has an additive effect Unlinked genes
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Polygenic Inheritance
Example –length of corn cobs Three genes – A/a, B/b and C/c Each dominant allele adds 2 cm length Each recessive allele adds 1 cm length
So AABBCC = 12 cm long aabbcc = 6 cm long
Hmmm!! How long would AaBBCc be? How long would aaBbCc be?
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Genotype, environment and phenotype
The environment can affect the expression of the genotype
examples AABBCC has the genetic potential to
produce cobs 12cm long▪ This could be affected by▪ Lack of water, light or minerals
Obesity in humans▪ Affected by diet and exercise
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Genotype, environment and phenotype
The environment influences the expression of polygenic traits more than monogenic traits.
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Learning Outcomes
Use the Hardy–Weinberg principle to calculate allele frequencies in populations.
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Population genetics
What is a population? Group of individuals of the same species
that can interbreed Populations are dynamic
The set of genetic information carried by a population is the gene pool.
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Allele Frequency
To measure the frequency of an allele you need to know Mechanism of inheritance of that trait How many different alleles of the gene
for that trait are in the population
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Hardy-Weinberg principle
The Hardy-Weinberg principle is a fundamental concept of population genetics
It makes the following assumptions Population is very large Random mating No selective advantage No mutation, migration or genetic drift
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The equations
p frequency of the dominant allele q frequency of the recessive allele
The frequency of the allele will be in the range 0 – 1. 0 – no one has the allele 0.5 – half the population has the allele 1 – only allele for that gene in the
population
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Ok – the equations
Equation 1p + q = 1
Equation 2p2 + 2pq + q2 = 1
Where▪ p2 frequency of genotype DD▪ 2pq frequency of genotype Dd▪ q2 frequency of genotype dd
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Calculating the frequency of cystic fibrosis in the population 1 in 3300 babies are born with cystic
fibrosis All babies with cystic fibrosis have
genotype nn Calculate q2
Calculate q Calculate p Calculate frequency of genotype Nn If we have 30,000 people in our population
how many will be carriers of the cystic fibrosis allele
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Question
Phenylketonuria, PKU, is a genetic disease caused by a recessive allele. About one in 15 000 people in a population are born with PKU.
Use the hardy-Weinberg equations to calculate the frequency of the PKU allele in the population.
State the meaning of the symbols that you use, and show all your working.
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The Answer
Calculate q2 = 1 / 15000 = 0.000067Calculate q = 0.0082
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Another question
Explain why the Hardy-Weinberg principle does not need to be used to calculate the frequency of codominant alleles.
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Pupil Activity
Answer the Hardy-Weinberg practice question.
You have 10 minutes Starting NOW!!
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The Answers
q2 = 0.52 / q = 0.72 p = 1 – 0.72 = 0.28 p + q = 1 p2 + 2pq + q2 =
1
Answer = 2pq / use of appropriate numbers;
Answer = 40%;
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The other answers
Any three from: Small founder population / common ancestor; Genetic isolation / small gene pool / no
immigration / no migration / in-breeding; High probability of mating with person having
H-allele; Reproduction occurs before symptoms of
disease are apparent; Genetic argument – Hh x hh = 50% / Hh x Hh =
75% affected offspring; No survival / selective disadvantage;
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Learning Outcomes
Explain, with examples, how environmental factors can act as stabilising or evolutionary forces of natural selection.
Explain how genetic drift can cause large changes in small populations.
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Variation and Natural Selection
The set of alleles in a population is it’s gene pool
Each individual can have any combination of alleles in the gene pool producing variation Some individuals more likely to survive They reproduce and pass genes on to offspring Advantageous alleles become more frequent in
the population
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Environmental Resistance
Environmental factors that limit the growth of a population offer environmental resistance
These factors can be biotic or abiotic
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Selection pressures
An environmental factor that “selects” for some members of a population over others
Confers an advantage onto certain individuals
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Stabilising Selection
If the environment stays stableThe same alleles will be selected for
in successive generationsNothing changes, this is called
stabilising selection
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Stabilising Selection
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Stabilising Selection
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Directional Selection
Change in the environment resulting in a change in the selection pressures on the population
Previously disadvantageous alleles maybe selected for
Change in the genetically determined characteristics of subsequent generations of the species
A.k.a. evolution
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Directional Selection
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Directional Selection
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Genetic Drift
A change in the gene pool and characteristics within the population.
This change has occurred by chance rather than as the result of natural selection.
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Genetic Drift and Islands
Genetic drift is thought to happen relatively frequently in populations on islands. Small populations Geographically separated from other
members of their speciesEvidence
Many isolated islands have their own endemic species of plants and animals
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Genetic Drift
Reduces genetic variationReduce the ability of the population
to survive in a new environmentMay contribute to the extinction of a
population or speciesCould lead to the production of a
new species
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Genetic Drift – Frog Hoppers The colours of the
common frog-hopper are determined by seven different alleles of a single gene.
The range of colours and their frequencies, on different islands in the Isles of Scilly, are very variable,
There are different selection pressures on the different islands
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Genetic Drift – Frog Hoppers
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The answers
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Learning Outcomes
Explain the role of isolating mechanisms in the evolution of new species, with reference to ecological (geographic), seasonal (temporal) and reproductive mechanisms.
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Speciation
Speciation is the formation of a new species.
Species Group of organisms, with similar
morphology and physiology, which can interbreed with one another to produce fertile offspring.
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Speciation
In the production of a new species, some individuals must Becomes morphologically or
physiologically different from members of the original species
No longer be able to breed with the members of the original species to produce fertile offspring.
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Isolation
Splitting apart of a “splinter group”Geographical isolation
Organisms are separated by a physical barrier
Reproductive isolation Two groups have become so different
that they are no longer able to interbreed
They are now a different species
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Isolating Mechanisms
Large populations may be split into sub-groups by Geographic barriers Ecological barriers Temporal barriers Reproductive barriers
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Geographical Barriers (AS recap)
Geographical barrier separates two populations of a species
Two groups evolve along different lines Different selection pressures Genetic drift
If barrier breaks down and two populations come together again, they may have changed so much that they can no longer interbreed
They are now two different species
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Isolating Mechanisms
Speciation occurs when organisms live in the same place
The barriers which can prevent two closely related species from interbreeding include Ecological Temporal Reproductive
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Ecological Barriers
Ecological barriers exist where two species live in the same area at the same time, but rarely meet.
Example Two different species of crayfish,
Orconectes virilis and orconectes immunis, both live in freshwater habitats in North America
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Meet the Crayfish
Orconectes virilis Not good at digging, can’t
survive summer drying Lives in streams and lake
margins Orconectes immunis
Lives in ponds and swamps,
Can easily burrow into the mud when the pond dries up
In streams and lake margins O. virillis is more aggressive and will drive O. immunis out of crevices where it tries to shelter
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Temporal Barriers
Two species live in the same place, and may even share the same habitat
Do not interbreed as they are active at different times of the day, or reproduce at different times of year
Example – flowering shrubs in Western Australia
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Meet the shrubs
Banksia attenuata flowers in the summer
Banksia menziesii flowers in the winter
They can not interbreed
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Reproductive barriers
Even if species share the same habitat and are reproductively active at the same time, they may not be able to interbreed Different courtship behaviours Mechanical problems with mating Gamete incompatibility Zygote inviability Hybrid sterility
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Meet the Mallards
Different courtship behaviours A male mallard duck will
only mate with a female who displays the correct courtship behaviour
Although the pintail female looks similar to the Mallard female, her courtship behaviour will only attract a pintail male.
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Learning Outcomes
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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The Species Concept
In AS biology you defined a species as “a group of organisms, with similar
morphological, physiological, biochemical and behavioural features, which can interbreed to produce fertile offspring, and are reproductively isolated from other species”
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The two species concept
Group of organisms Capable of
interbreeding Capable of producing
fertile offspring Reproductively
isolated from other groups
This is the Biological Species concept
Group of organisms showing similarities in characteristics Morphological Physiological biochemical Ecological Behavioural
This is the phylogenetic species concept
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Biological Species concept
Group of organisms that can interbreed and produce fertile offspring.
Clear cut definitionLimitation
Can only be used for organisms that reproduce sexually
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Phylogenetic species concept
Also known as the Evolutionary species concept Cladistic species concept
Different morphology between the two groups and certain that they evolved from a common ancestor
Not rigorous but allows decisions to be made
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Comparing the genetics
Closely related organisms have similar molecular structures for DNA, RNA and proteins.
Biologists can compare specific base sequences (haplotypes) The number of differences caused by
base substitutions can be expressed as the % divergence
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Cladistics
Clade Group of organisms with similar
haplotypes In cladistic classification systems is
assumes that the taxa are monophyletic, this means that it includes an ancestral organism and all it’s descendents.
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Cladistic classification
Focuses on evolutionPlaces importance on using
molecular analysisUses DNA and RNA sequencingUses computer programmesMakes no distinction between extinct
and still existing species
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Learning Outcomes
Compare and contrast natural selection and artificial selection.
Describe how artificial selection has been used to produce the modern dairy cow and to produce bread wheat (Triticum aestivum).
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Selection
Natural Selection Mechanism for evolution Organisms best adapted to their
environments more likely to survive to reproductive age
Favourable characteristics are passed on Produces organisms that are well
adapted to their environment
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Artificial Selection
Humans select the favourable characteristics
Humans allow those organisms to breed
Produces populations that show one characteristic to an extreme Other characteristics retained may be
disadvantageous
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Artificial Selection and the modern dairy cow
Breeds of cows with higher milk production have been artificially selected for Milk yield from each cow is measured and
recorded Test progeny of bulls Elite cows given hormones to produce many
eggs Eggs fertilised in vitro Embryos implanted into surrogate mothers
A few elite cows produce more offspring than they would naturally
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Disadvantage to high milk yields
Health costs for artificially selected cows is higher due to Mastitis Ketosis and milk fever Lameness Respiratory problems
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Artificial selection and bread wheat (Triticum aestivum)Polyploidy
Nuclei contain more than one diploid set of chromosomes
Wild species of wheat have a diploid number (2n) of 14
Modern bread wheat is hexaploid (6n), It has 42 chromosomes in the nucleus of every cell
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Getting from the ancestors to modern bread wheat
Wild einkorn
AUAU
2n = 14
Wild GrassBB
2n = 14
EinkornAUAU
2n = 14x
Domestication and artificial selection
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Wild GrassBB
2n = 14
EinkornAUAU
2n = 14x
Sterile hybrid P
AuB
Wild GrassDD
2n = 14
Emmer WheatAUAUBB4n = 28
x
Mutation that double chromosome number
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Wild GrassDD
2n = 14
Emmer WheatAUAUBB4n = 28
x
Sterile hybrid Q
AuBDMutation that double chromosome numberCommon Wheat
AUAUBBDD6n = 42
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Continuing selection in wheat
Breeders are continuing to try and improve wheat varieties Resistance to fungal infections High protein content Straw stiffness Resistance to lodging Increased yield