1 CHAPTER 13 CHAPTER 13 Population Genetics Outcome 2 Pages 464 - 504.

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CHAPTER 13 CHAPTER 13 Population Genetics

Outcome 2

Pages 464 - 504

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VARIATION Variation: refers to a condition in which one or

more members of a population differ in one or more traits.

A population is: a group of the same species living in the same region at a given time.

KINDS OF VARIATION in populations

The variation that is present in a population may be classified in several ways:

Structural: example: polydactyly; tail vs. no tail

Behavioural: example: territorial behaviour; mating

Biochemical: example: ABO Blood type; cat pigment

Developmental: example: ripening colours;

Physiological: example: Red / green colour blindness

_______________________________________

Geographical: evident when comparing locations (cline: gradual changes/ graded sequence within a population across a range)

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EXTENT OF VARIATIONin relation to a particular characteristic e.g skin colour

NO VARIATION

One Gene – One Trait Monomorphic: A population that shows no variation in

traits is monomorphic.

example: white plumage of wild short billed corellas

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VARIATION

One Gene – Two or more traits A population that shows two or more variations in a trait

is said to be polymorphic.

Discontinuous Variation: a few diverse and discrete traits (caused by monogenic inheritance)

example: flower colour of snapdragons (3 traits!)

Continuous Variation: ‘continuum’ of variation

example: adult male human height

Questions 1-3 p471

Chapter 14 - VariationChapter 14 - Variation 44

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CAUSES OF VARIATION

Variations seen in a population can have different

causes.

Some variations are due to:

Environmental Factors

Inherited Factors

A mixture of both (remember:

phenotype = genotype + environment

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ENVIRONMENTAL VARIATIONS

Are variations caused by factors of the organisms

external environment.

Casual Factors Include:

Soil Type Diet Up-bringing Temperature Substance Abuse

THE INTERNAL ENVIRONMENT

Variations caused by the internal environment

include:

Levels of hormones Types of hormones

Questions 4-6 p474

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INHERITED VARIATIONS

A number of inherited factors influence variation

within a species including:

Number of inherited chromosomes:

sources of variation: polyploidy/ annueploidy

Combination of chromosomes:

sources of variation: meiosis, crossing over

Alterations to genes

sources of variation: mutations, chromosome abberations

Number and type of alleles sources of variation: mutations,

Combination of allelessources of variation: multiple alleles, independent assortment

Monogenic inheritance sources of variation: 2 or more alleles

Polygenic inheritance sources of variation: 2 or more genes in combination

MONOGENIC INHERITANCE

Monogenic Inheritance: inherited variations (ie blood type) are due to the action of a single gene (with two or more alleles!).

These are called monogenic traits.

Table 13.4 p475 – Monogenic Traits

Question: Question: How much variation in ABO Blood types?

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POLYGENIC INHERITANCE

Polygenic Traits are those that cause variation in traits due to a number of different genes.

Examples of polygenic traits include:

Skin Pigmentation Fat content in Milk Seed Mass in Bean Plants


Polygenic VariationsPolygenic Variations

No of + No of + AllelesAlleles

Possible GenotypesPossible Genotypes

Gene 1 Gene 2Gene 1 Gene 2ColourColour

44 ++++ ++++ Very Very DarkDark

33 ++++

+-+-

+-+-

++++

DarkDark

22 ++++

+-+-

----

----

+-+-

++++

InterInter

DarkDark

11 +-+-

----

----

+-+-

MedMed

DarkDark

00 ---- ---- LightLight

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VARIATIONS- MUTATIONS

Gene mutation is the process which produces new alleles of genes in various species and so generates new genetic variation.

Questions 7-10 p479

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GENES IN POPULATIONS

The genetic information present in a population of organisms is known as a gene pool.

For example fleece colour in sheep (W=white & w=black)

Gene Pool – WW, Ww, ww

Gene pools are analysed and expressed in terms of frequencies of allele combinations.

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The Gene PoolThe Gene Pool

Activity Sheet – The Gene Pool p311

Activity Sheet – Factors Affecting Gene Pools p312

ALLELE FREQUENCY

When all genotypes in a population are known – we can directly count the number of each kind of allele and calculate its frequency.

Allele Frequencies can have a value between 1 and 0.

If a particular allele has a frequency of 1 – then every organism will be homozygous with that particular allele.

See Figure Fig 13.24 p479

This population contains:

10 Sheep in total

2 black (homozygous - ww) 2 heterozygous white (Ww) 6 homozygous white (WW)

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The total number of alleles = 20

No of W alleles = 14

No of w alleles = 6

Freq of W alleles = (14/20) = 0.7

Freq of w alleles = (6/20) = 0.3

A convention commonly used is that the frequency value for the dominant trait is given the letter p, while the recessive trait is given the letter q.

Therefore p + q = 1 (0.7+0.3=1)

ALLELE FREQUENCY

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Hardy-Weinberg Equilibrium1908 G. H Hardy and W. Weinberg

recognised

a pattern in allele frequencies.

Hardy Weinberg Principle states:

• given the right conditions, allele frequencies remain constant, generation after generation

Conditions for a stable Allele Frequency: Must be a large population Members must mate at random (i.e. not selective

mating) All matings are equally fertile, producing equal numbers

of variable offspring The population is closed; no migration in or out

Hardy Weinberg Equilibrium: the allele frequencies

in a large closed population will remain constant from

Generation. (until an agent of change acts on the population)

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Hardy-Weinberg Equilibrium

This is said to be the Hardy-Weinberg Equilibrium and it is a formula used to calculate the proportions of genotypes in a population.

p2 + 2pq + q2 = 1

Where,

Proportion with WW= p2

Proportion with Ww

= 2pq

Proportion with ww

= q2

So, p2 + 2pq + q2 = 1

482

If we know the p and/or q values for a population – we can calculate the proportions of heterozygous and homozygous genotypes.

We can always work out the number of q2

Q2 = number of homozygous recessives

Therefore,

√ Q2 = q

and, number of ps = 1 – q

Example:

Plant flower color with B = blue b = white

36 members of the population have white flowers!

(see Figure 13.28 – p482)

Biozone: Changes in Gene Pool, pages

Quick Check Page 483: 11 - 15

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Hardy-Weinberg Equilibrium

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CHANGE AGENTS IN POPULATIONS

Most gene pools are not ‘closed’, unaffected by chance or by selection factors (predators, disease

etc). Consequently, allele frequencies often change within populations. Lets examine some of these

agents of change.

Agents of Change in Populations

Selection: Natural Selection/ Artificial SelectionMigration: in and out of populationsChance: genetic drift due to chance events

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SELECTION AS AN AGENT OF CHANGE

Under natural conditions, members of the same

population compete with each other for: Access to living space Food Mating partners

In a polymorphic population, different phenotypes

may be selected for (selective advantage) and

have different survival and reproductive chances and

consequently have a higher fitness value.

The ‘agent of change’ that causes these differences

to occur between phenotypes is the selecting agent.

Note: a selective advantage is not static but is

Subject to change (consider: mega fauna existed in

Australia before human settlement!).

Genetic fitness varies in different environments.

(consider: tree kangaroos climb trees. Would they be

successful in a desert environment?)

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ARTIFICIAL SELECTION

When farmers, horticulturalists, horse breeders or geneticists deliberately select particular organisms from a population to be the parents of the next generation.

Under conditions of artificial selection – the parents that contribute to the next generation may not be the fittest (in a genetic sense).

LEVELS OF SELECTION

Complete Selection: against a phenotype occurs when any organism with a given phenotype cannot reproduce because of death before a reproductive age is reached or because of sterility.

Partial Selection: against a phenotype occurs when matings involving that phenotype produce on average fewer viable and fertile offspring relative to othermatings .


Artificial Selection In DogsArtificial Selection In Dogs

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The Biggy!

NATURAL SELECTION

NATURAL SELECTION: The action of selecting agents on populations living in the wild is called natural selection.

Natural selection refers to any environmental agent that acts on a wild population and results in differential reproduction.

Differential reproduction occurs when one inherited variety (phenotypic trait!) in a population produced more viable offspring (live to reproductive age) than other varieites, and thus contribute disprooprtionally to the gene pool.

In wild populations more offspring are produced than will survive to maturity.

Phenotypes with the highest survival rates and fitness values will make the greatest contribution to the gene pool of the next generation.

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NATURAL SELECTION


Natural SelectionNatural Selection

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Natural Selection Tasks

Activity Sheet – Natural Selection p314

Activity Sheet – Darwin’s Finches p315

Activity Sheet – Artificial Selection p324

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MIGRATION AS AN AGENT OF CHANGE

Migration (in), also known as gene flow, can change the allele frequency of a population.

Unlike selection, which requires several generations to have an effect, changes due to migration can occur very quickly.

Emigration (out) can also change allele frequencies if the emigrant group is not a representative sample of the original population.

Human Migrations

Two great migrations occurred in human prehistory:

Homo Erectus from Africa to Europe and Asia (2 000 000 years ago).

Modern Homo Sapiens from Africa to Europe and Asia, and then onto Australia and the America’s. (130 000 years ago).

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CHANCE AS AN AGENT OF CHANGE

Chance effects can cause allele frequencies in a population to change over time.

When change effects operate, the direction of the change is unpredictable and can vary from one generation to the next.

The resulting pattern of change is known as random genetic drift.

The smaller the population, the greater the impact of chance events.

If the population is greatly reduced, the effect could include bottlenecking

Bottlenecking can also result in a population founded by a small under-represented sample of the original population. This is known as the Founder Effect.

Biozone: Genetic Drift p323

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Different examples of genetic drift can be seen on page 491.

These include:

The Bottleneck Effect The Founder Effect

Activity Sheet – The Founder Effect p321

Activity Sheet – Population Bottlenecks p322

CHANCE AS AN AGENT OF CHANGE

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EVOLUTION WITHIN A SPECIES

The allele frequencies in a gene pool of a population can change as a result of the action of change agents such as:

Migration Immigration Emigration Selection Genetic Drift

Members of one population may be separated into a number of populations spread over the range of the species.


Through exposure to certain selection agents, these isolated populations may give rise to new races or sub-species.

EVOLUTION WITHIN A SPECIES

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STUDYING POPULATIONS USING mtDNA

Mitochondrial DNA (mtDNA) is an important tool for tracing the evolutionary history of a species.

In addition mtDNA can also be used to outline migrations of species through history.

mtDNA is structurally identical to nuclear DNA of the chromosomes, however there are several difference between the two that makes mtDNA useful for studying populations.

These differences include:

Descent via the maternal line – mtDNA is inherited from the mother only, so that all offspring receive just one kind of mtDNA exclusively from the mother.

Lack of Recombination – mtDNA passes unchanged from a female parent to all of her offspring.

High Copy Number – Each mitochondrion contains 2 to 10 mtDNA molecules, and each cell has up to several hundred mitochondria. So many copies of mtDNA are present in each cell.

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The coding region of mtDNA mutates very slowly, but the two non-coding regions in the D-loop of mtDNA have a higher mutation rate.

These are hypervariable regions or segments and are designated as HVR1 and HVR2.


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Each of us carries in our mtDNA a story about our maternal ancestry.

The mtDNA in each of your cells was inherited from your mother – she received her mtDNA from her mother, and so on, and so on through thousands of generations.

By identifying similarities between individuals mtDNA can suggest an ancestral link.

The distinctive mtDNA sequences found in different populations are known as haplogroups and each is designated by a capital letter.


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Studying Populations Studying Populations Using mtDNAUsing mtDNA


Studying Populations Studying Populations Using mtDNAUsing mtDNA

Using mtDNA to Identify Using mtDNA to Identify Prehistoric Migration p497Prehistoric Migration p497

Quick Check Question 22 p498Quick Check Question 22 p498


Chapter ReviewChapter Review

Biochallenge 1-2 p499Biochallenge 1-2 p499

Chapter Review QuestionsChapter Review Questions

Understand Key WordsUnderstand Key Words

Questions 1-6 p500-504Questions 1-6 p500-504

1 CHAPTER 13 CHAPTER 13 Population Genetics Outcome 2 Pages 464 - 504.

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