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Name: Benoit L.
Status: Student
Age: 16
Location: N/A
Country: N/A
Date: December 2002

In the Hardy-Weinberg theory, one of the important aspect is that the frequency of the gene are always constant. Is there situations or events where this frequency can be changed ?

Actually the Hardy-Weinberg theory models a population that is stable and not evolving. For the population to be stable, the conditions of the theory need to be met, ie. no immigration/emigration, a small population, etc. In a stable population the gene frequencies don't change. If they DO change, then the population is evolving. This is actually what the theory is designed to show.


The gene frequency can change in response to environmental, cultural (e.g., migration of populations) or catastrophic events (e.g., war). Say, for instance, every person with blue eyes was sent overseas on a special blue-eye scholarship, and the ship carrying the blue-eyed people was lost in a storm. The remaining population would consist of the brown-eyed homozygotes and brown-eyed heterozygotes only.

The point of Hardy-Weinberg is that, if the remnant of the population continue to mate randomly with regard to eye color, a new equilibrium will be established in just one generation.

In a war, let us say all the able-bodied men are sent to the front and many of them die, and those with medical exemptions for various reasons stay behind and get to marry the pretty girls. If there is a genetic basis for the difference between those who go and those who stay, then we would expect the frequencies of the genes causing physical strength to be decreased and the frequencies of certain disease genes prevalent in the non-soldiers to be increased in the next generation.

Let us say the people of Russia are oppressed by poverty and the Czar, and many of the young people decide to emigrate to America. Perhaps those with ADHD (attention deficit disorder) are more likely to emigrate, because they have a more adventurous, restless streak. If this is true, and if there are genes causing ADHD, then the gene frequency of those genes would tend to be higher in the new American population and lower in those left behind.

Let us say a small Jewish village in Europe is invaded by Crusaders on their way to "fight the Infidel" in the Holy Land. After a rampage of rape and destruction the Crusaders leave and are never seen in the village again. They leave a sizable percentage of women pregnant and so have altered the gene frequencies. For instance, if every Crusader had blue eyes, then every child born of these assaults will carry the gene for blue eyes. If some of the attackers had blue eyes, but still more than the local population, then the frequency of the blue-eyed gene will be increased in the population of children born. When all the children of the village grow up and get married and have children, IF the children of these attacks are equally accepted in matings compared with children not born of violence, then the new generation will attain new genotype frequencies in the very first generation of random mating.

Sarina Kopinsky, MSc, HED, CGC

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