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POPULATION GENETICS

PREPARED BY: ARUSHI ARORAA-2016-30-050

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CONTENTSTOPICS

• Population genetics• Variation in population• Hardy Weinberg law• Gene pool concept• Allele frequency• Rare and common alleles• References

SLIDE NUMBER• 3-8• 9-19• 20-26• 27-37• 38-42• 43• 44

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POPULATION

• A population is a summation of all the organisms of the same group or species, which live in a particular geographical area, and have the capability of interbreeding.

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POPULATION GENETICS• Defined as study of the distribution and change

in frequency of alleles within populations and is related to evolution.

• Population genetics is the study of the frequency and interaction of alleles and genes in populations.

• In population genetics population is a set of organisms in which any pair of members can breed together.

• This means that they can regularly exchange gametes to produce normally-fertile offspring, and such a breeding group is also known therefore as a gamodeme.

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PROCESSES CAUSING EVOLUTION IN A POPULATION

NATURAL SELECTION

GENE FLOW

MUTATION

GENETIC DRIFT

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Population genetics is a vital ingredient in the emergence of the modern evolutionary synthesis.

Its primary founders were • Sewall Wright(1945)• J. B. S. Haldane(1924)• Ronald Fisher,(1930) who also laid the foundations

for the related discipline of quantitative genetics.

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• Population genetics began as a reconciliation of Mendelian inheritance and biostatistics models

• In a series of papers starting in 1918 and culminating in 1930 the book The Genetical Theory of Natural Selection by Fisher.

• He showed that the continuous variation measured by the biometricians could be produced by the combined action of many discrete genes, and that natural selection could change allele frequencies in a population, resulting in evolution.

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• In a series of papers beginning in 1924, another British geneticist, J.B.S. Haldane worked out the mathematics of allele frequency change at a single gene locus under a broad range of conditions.

• Haldane also applied statistical analysis to real-world examples of natural selection, such as the Peppered moth evolution and industrial melanism

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VARIATION IN POPULATION• If all members of a species have the same set of

genes, how can there be genetic variation?NO VARIATION = NO NEW VARIETY= NO PLANT

BREEDING• In a population, for any given gene there can be one

to many different alleles;• however, because most organisms carry only one or

two chromosome sets per cell, any individual organism can carry only one or two alleles per gene.

• The alleles of one gene will always be found in one chromosomal position. Allelic variation is the basis for hereditary variation.

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 TYPES OF VARIATION IN POPULATION

CONTINOUS VARIATION

DISCONTINOUS VARIATION

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CONTINOUS VARIATION DISCONTINOUS VARIATION

1. They cannot be classified into distinct classes.

They can be put into various classes and categories.

2.Caused by number of genes Caused by a single genes or few genes

3. Tends to be quantative Tends to be qualitative

4. They refer to small, indistinct differences from the normal condition.

large, conspicuous differences from the parents.

5. . They are also known as fluctuations. They are also known as mutations or spurts

6. due to chance segregation of genes during gamete formation, crossing over or chance combination during fertilization.

They are produced by change in genes or genome.

Eg.height, weight,leaf length Eg.finger prints, blood groups

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Continous variation

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Discontinous variation

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HOW VARIATION ARISE?

MIGRATION

RECOMBINATION

MUTATION

GENE FLOW

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MIGRATION

• It is the movement of organisms from one location to another.

• If the migrating individuals mate or cross with the destination individuals, they can provide a sudden influx of alleles

• This way migrating individuals contribute gametes carrying alleles that can alter the existing proportion of alleles in the destination population.

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MUTATION

• They are the changes in DNA or nucleotide sequences which happen a sudden.

• A single mutation has a large efeect in creation of variation in a population

• It creates an entirely new character in a population which may be capable of selection

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RECOMBINATION• It is the production of offspring with the

combinations of traits that differ from those found in parents.

• Genetic recombination occurs during meiosis

• It is a major contributor in variation in a population

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GENE FLOW• Gene flow is the transfer of alleles or genes from one

population to another.

• These alleles bring about variation in population and this is related to migration as brings new alleles to the population.

• Generally horizonal gene transfers that is through hybridization which is used by breeder.

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GENE POOL CENTRES

• Gene pool centres refers to :areas on the earth where important crop plants and

domestic animals originated.

They have an extraordinary range of the wild counterparts of cultivated plant species and useful tropical plants.

Gene pool centres also contain different sub tropical and temperate region species

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HARDY- WEINBERG LAW

HARDY (1908) IN ENGLAND

WEINBERG (1909) IN GERMANY

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Hardy- Weinberg law states that gene and genotype frequencies in a Mendelian population remain constant generation after generation if there is no selection, mutation, migration or random drift.

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ACCORDING TO HARDY WEINBERG LAW

• The frequencies of 3 genotypes for a locus with two alleles,

• Say A and a, will remain constant at:• p² (AA), 2pq (Aa) and q² (aa). Where p and q

are frequencies of alleles A and a, respectively• Since, p+q=1. So,

p² + 2pq +q²=1

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ASSUMPTIONS OF HARDY – WEINBERG LAW

Large population

Random mating

No mutation

No migration

Natural selection

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ASSUMPTIONS LARGE POPULATION: large size of population limits

errors and also Allele frequencies in a small population are more likely to get affected by random drift as compared to allele frequencies in a large population.

RANDOM MATING. It refers to matings in a population that occur in proportion to their genotypic frequencies. If deviation from the expected value occurs, then no random mating. So, the assumption will be eliminated.

NO MUTATIONS. Mutations create and maintain variations within a population and these introduce new genes and alleles in a gene pool which violate the hardy weinberg law.

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NO MIGRATION. Immigration results in addition of new alleles into the existing gene pool which results in large changes in allele frequencies. Migration in population may cause unbalance in the law.

NO NATURAL SELECTION: Due to natural selection, certain phenotypes are favored over other. So, the allele frequencies will change and the population will evolve which will disturb the law.

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Hardy–Weinberg genotype frequencies for two alleles: the horizontal axis shows the two allele frequencies p and q and the vertical axis shows the genotype frequencies. Each curve shows one of the three possible genotypes.

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GENE POOL CONCEPT IN CROP BREEDING

• Harlan and de Wet (1971) proposed classifying each crop and its related species by gene pools rather than by formal taxonomy.

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GENE POOL

Set of all genes, or genetic information, in a single population, usually of a particular species

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GENE POOL

Sum of a population’s genetic material at a given time.

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TYPES OF GENE POOL

TERTIARY GENE POOL

PRIMARY GENE POOL

SECONDARY GENE POOL

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PRIMARY GENE POOL (GP-1):

• Members of this gene pool are probably in the same "species" and can intermate freely.

• Among forms of this gene pool, crossing is easy; hybrids are generally fertile with good chromosome pairing;

• gene segregation is approximately normal and gene transfer is generally easy.

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Example of primary gene pool

• The primary gene pool of both cultivated and wild varieties of sunflower(Helianthus annus) .

• A winter’s sunflower (H.winterii) a perennial grass in south Siberia whose genes are easy to be bought in cultivated ones.

• So both constitute members of gene pool 1.

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SECONDARY GENE POOL (GP-2):• Members of this pool are normally classified as

different species than the crop species under consideration (the primary gene pool).

• These species are closely related and can cross and produce at least some fertile hybrids. BUT…

hybrids may be weak hybrids may be partially sterile chromosomes may pair poorly or not at all recovery of desired phenotypes may be difficult in subsequent

generations The gene pool is available to be utilized, if the plant breeder

or geneticist is willing to put out the effort required.

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Example of secondary gene pool

• Aegilops tauschii AND Aegilopes speltoides, two wild relatives in secondary gene pool of bread wheat (Triticum aestivum)are diploid. That means that they have paired chromosomes whereas bread wheat is hexaploid(six copies).

• So, some crosses if result from this would be partially sterile or weak.

• THESE CONSTITUTE SECONDARY GENE POOL

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TERTIARY GENE POOL (GP-3):

• Members of this gene pool are more distantly related to the members of the primary gene pool. The primary and tertiary gene pools can be intermated, but gene transfer between them is impossible without the use of "rather extreme or radical measures" such as:

• embryo rescue (or embryo culture, a form of plant organ culture)

• induced polyploidy (chromosome doubling)• bridging crosses (e.g., with members of the

secondary gene pool).

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Example of tertiary gene pool• Triticum turgidum (AABB, 2n=28) and • Aegilops speltoides(BB,2n=14) produce amphdipoid hybrid (INTERGENERIC CROSS). This can be made fertile using various techniques like

colchicine treatment. EXAMPLE OF A SUCESSFUL CROSS BEING Triticale

DEVELOPED BY RIMPAU.

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ALLELE FREQUENCY• The allele frequency for an allele is the fraction of the

genes in the pool that is composed of that allele (for example, what fraction of moth coloration genes are the black allele).

• Evolution occurs when there are changes in the frequencies of alleles within a population; for example, the allele for black color in a population of moths becoming more common

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ALLELE FREQUENCY• Allele frequency, or gene frequency, is the relative

frequency of an allele (variant of a gene) at a particular locus in a population, expressed as a fraction or percentage.

• It is the fraction of all chromosomes in the population that carry that allele.

• Microevolution is the change in allele frequencies that occurs over time within a population.

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CALCULATION OF ALLELE FREQUENCY

Allele = number of specific type of allele frequency total number of allele in population

MONOPLOIDS• The frequency (p) of an allele A is the fraction of the

number of copies (i) of the A allele and the population or sample size (N), so

• p = i/N.

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DIPLOIDS• If f(AA),f(AB) AND F(BB) are the frequencies of the

three genotypes at a locus with two alleles,

• Then,

o p=f(AA)+1/2f(AB)=frequency of Ao q=f(BB)+1/2f(AB)=frequency of Bo Their sum must be equal to 1 as p+q=1

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EXAMPLE• Consider a locus that carries two alleles, A and B. In a diploid

population there are three possible genotypes, two homozygous genotypes (AA and BB), and one heterozygous genotype (AB). If we sample 10 individuals from the population, and we observe the genotype frequencies.

1.freq(AA) = 62.freq(AB) = 33.freq(BB) = 1

• Then there are 6 × 2 + 3 = 15 observed copies of the A allele and 1 × 2 + 3 = 5 of the B allele, out of 20 total chromosome copies. The frequency p of the A allele is p = 15/20 = 0.75, and the frequency q of the B allele is q = 5/20 = 0.25.

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RARE AND COMMON ALLELES

COMMON ALLELES• The alleles in a population

with frequencies >1%

• These alleles are frequent enough in a population

• They can be observed by genotyping in standard marker panels

RARE ALLELES• They are polymorphic

alleles with <1% frequency

• They are seen in only handful of individuals or are private to a single individual

• They can be observed by

sequencing technologies.

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REFRENCEShttps://en.m.wikipedia.org/wiki/Gene_poolhttp://www.fao.org/docrep/006/y4011e/y4011e0e.htmhttp://www.yourarticlelibrary.com/biology/5-factors-affecting-genetic-equilibrium-hardy-weinberg-equilibrium-theory/27259/https://en.m.wikipedia.org/wiki/Allele_frequencyhttp://www.ncbi.nlm.nih.gov/pmc/articles/PMC2914559/https://en.m.wikipedia.org/wiki/Hardy%E2%80%93Weinberg_principlehttp://www.cwrdiversity.org/about/what-is-a-genepool/http://study.com/academy/lesson/what-is-a-gene-pool-definition-example-quiz.htmlhttps://en.m.wikipedia.org/wiki/Population_geneticshttp://evolution.berkeley.edu/evolibrary/article/evo_17

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