Populus? Population Genetics?

1

The Rh (Rhesus) polymorphism in humans is coded for by a

single locus with two alleles, D and d. D is dominant to d, so

that both the genotypes DD and Dd produce the Rh antigen. This

can cause problems during pregnancies of dd women, because the

fetus may produce the D antigen, which the mother’s immune

system may recognize as non-self and cause a spontaneous

abortion. A D_ fetus (D_ means there is at least one D allele.

Since D is dominant, the identity of the other allele does not

matter.) is not a problem in the first pregnancy because the

mother has not yet formed anti-D antibodies, but can be a

problem with the second pregnancy with a D_ fetus. You will use

Populus for this problem.

In the Populus program menus, choose “Selection”, then

“Frequency and density dependent selection”, and finally

“Frequency dependent selection: Diploid model”. Read the

instructions you find there about the basic model. When you

run the program, you will be asked to fill in an array of

fitnesses, and the initial allele frequency of allele “A.” Set

up a fitness model of the evolution of the Rh polymorphism,

making the following assumptions:

i. allele A in Populus is D, a is d.

ii. The fitness of couples that have a DD father and a dd

mother is 0.8, which means that the fitness of an average

couple where one is DD and the other is dd is 0.9. This is

because half the time, the mother is DD, and the father dd,

which is no problem for the offspring.

iii. The fitness of couples with one Dd individual, and one dd

individual is 0.975 (after averaging the fitness of mothers and

fathers as in ii).

iv. All other fitnesses are 1.0.

Once you have put these fitnesses in the right places in the

matrix, run the program with different starting allele

frequencies. If your matrix is correct, at some allele

frequency, you should find that the behavior of the model

switches from D going to fixation, to d going to fixation.

Find allele frequencies no more than 0.01 frequency units apart

that show this change in behavior.

Hand in:

A. The fitness matrix you used.

B. The two allele frequencies that come closest to bracketing

the point of switching from fixation of D to fixation of d.

C. Answer the following question: Is this a case of frequency

dependence?

3 Answers

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  • Vlado
    Lv 4
    1 decade ago
    Favorite Answer

    Population genetics is the study of the allele frequency distribution and change under the influence of the four evolutionary forces: natural selection, genetic drift, mutation, and gene flow. It also takes account of population subdivision and population structure in space. As such, it attempts to explain such phenomena as adaptation and speciation. Population genetics was a vital ingredient in the modern evolutionary synthesis, its primary founders were Sewall Wright, J. B. S. Haldane and R. A. Fisher, who also laid the foundations for the related discipline of quantitative genetics.

    Perhaps the most significant "formal" achievement of the modern evolutionary synthesis has been the framework of mathematical population genetics. Indeed some authors (Beatty 1986) would argue that it does define the core of the modern synthesis.

    Lewontin (1974) outlined the theoretical task for population genetics. He imagined two spaces: a "genotypic space" and a "phenotypic space". The challenge of a complete theory of population genetics is to provide a set of laws that predictably map a population of genotypes (G1) to a phenotype space (P1), where selection takes place, and another set of laws that map the resulting population (P2) back to genotype space (G2) where Mendelian genetics can predict the next generation of genotypes, thus completing the cycle. Even leaving aside for the moment the non-Mendelian aspects revealed by molecular genetics, this is clearly a gargantuan task.

    The three founders of population genetics were the Britons R.A. Fisher and J.B.S. Haldane and the American Sewall Wright. Fisher and Wright had some fundamental disagreements and a controversy about the relative roles of selection and drift continued for much of the century between the Americans and the British. The Frenchman Gustave Malécot was also important early in the development of the discipline. John Maynard Smith was Haldane's pupil, whilst W.D. Hamilton was heavily influenced by the writings of Fisher. The American George R. Price worked with both Hamilton and Maynard Smith. On the American side, Richard Lewontin and the Japanese Motoo Kimura were heavily influenced by Wright. Luigi Luca Cavalli-Sforza is a Stanford-based population geneticist particularly interested in human population genetics.

  • vaka
    Lv 4
    3 years ago

    i might elect D. organic determination isn't probably random. those with genes that enhance their odds of survival and duplicate are systematically chosen for and those with detrimental features are chosen against. Eg. A hairless polar bear would not accomplish that properly in the arctic.

  • 4 years ago

    The collection of all the alleles of all of the genes found within a freely interbreeding population is known as the gene pool of the population. Each member of the population receives its alleles from other members of the gene pool (its parents) and passes them on to other members of the gene pool (its offspring). Population genetics is the study of the variation in alleles and genotypes within the gene pool, and how this variation changes from one generation to the next.

    Factors influencing the genetic diversity within a gene pool include population size, mutation, genetic drift, natural selection, environmental diversity, migration and non-random mating patterns. The Hardy-Weinberg model describes and predicts a balanced equilibrium in the frequencies of alleles and genotypes within a freely interbreeding population, assuming a large population size, no mutation, no genetic drift, no natural selection, no gene flow between populations, and random mating patterns.

    In natural populations, however, the genetic composition of a population's gene pool may change over time. Mutation is the primary source of new alleles in a gene pool, but the other factors act to increase or decrease the occurrence of alleles. Genetic drift occurs as the result of random fluctuations in the transfer of alleles from one generation to the next, especially in small populations formed, say, as the result adverse environmental conditions (the bottleneck effect) or the geographical separation of a subset of the population (the founder effect). The result of genetic drift tends to be a reduction in the variation within the population, and an increase in the divergence between populations. If two populations of a given species become genetically distinct enough that they can no longer interbreed, they are regarded as new species (a process called speciation).

    In many cases, the effects of natural selection on a given allele are directional. The allele either confers a selective advantage, and spreads throughout the gene pool, or it confers a selective disadvantage, and disappears from it. In other cases, however, selection acts to preserve multiple alleles within the gene pool and a balanced equilibrium is observed. This situation, labelled balanced polymorphism, can arise because of a selective advantage for individuals heterozygous for a given allele. For example, the disease sickle cell anaemia is caused by a mutation in one of the genes responsible for the production of haemoglobin. Individuals with two copies of the mutant gene for sickle haemoglobin (HbS/HbS) develop the disease. Individuals that are heterozygous - one copy of the sickle gene and one copy of the normal gene (HbS/HbA) - are carriers of the condition. It is believed that these heterozygous individuals are more resistant to malaria than individuals homozygous for the normal gene (HbA/HbA), and that this selective advantage maintains the presence of the HbS gene in the population. As a result of balanced polymorphism, the gene pools of most populations contain a number of deleterious alleles that reduces the overall fitness of the population (known as the genetic load).

    Genetic variation within populations and species can now be analysed at the level of nucleotide sequences in DNA (genome analysis) and the amino acid sequences of proteins (proteome analysis). The genetic differences between species can be used to infer evolutionary history, on the basis that the closest relatives will have gene pools that are most similar. Recent advances in the sequencing of genomes, allied to computer-based techniques for storing and comparing this information, have led to the construction of detailed evolutionary trees. The use of molecular clocks - nucleotide sequences (or amino acid sequences) in which evolutionary change accumulates at a constant rate - allows dates to be attached to the points at which populations start to diverge to form new species. These approaches are also proving useful in other useful areas (for example, in tracing the transmission routes of infectious diseases).

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