Download 4- Random change student

RANDOM CHANGE
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The Hardy-Weinberg principle shows that if a certain set of conditions are met, the gene pool
remains unchanged generation after generation.
By showing what needs to happen to keep the gene pool unchanged, the principle also outlines what
has to happen to change the gene pool.
When the gene pool changes____________________ will occur (any change in gene frequencies
within a population of species).
The key points that lead to evolution are:
o ___________: new alleles can be created or one allele can change into another thereby
changing the allele frequencies and the gene pool
o Migration:_____________________________________________________________
___________________________________________
o Non-random mating: individuals that are ___________________ will pass on their alleles
in greater numbers than less preferred mates
o ________________________: individuals with certain alleles have greater reproductive
success and increase the relative frequency of their alleles in the next generation
o Genetic drift:
____________________________________________________________________
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More about Genetic Drift
Consider the following example:
Assume only 1 in 50 frogs carries a certain allele, C. In a large population of 10 000 frogs you would
expect 200 to have the allele. If severe weather conditions led to the death of half the frogs,
then about 100 of the surviving 5000 frogs would have the allele. Therefore, the allele frequency
of the C allele would not change (200 / 10 000 = 100 / 5000).
If the frog population were endangered with only 100 individuals then we would expect only 2 frogs
to have the allele. If half died, then there would be a good chance that both frogs with the allele
would be eliminated which would remove the allele from the population entirely. If both survived
then the allele frequency would be doubled (2 / 100 < 2 / 50).
When a severe event results in a drastic reduction in numbers a population may experience
a___________________________________(a dramatic yet often temporary reduction in
population size usually resulting in significant genetic drift).
When this form of genetic drift occurs, a very small sample of alleles survives to establish a new
population. Their relative frequencies of alleles may differ from the original population resulting in
a drastically different gene pool.
A MODEL OF GENETIC DRIFT
INTRODUCTION
Scientific models are used when it is difficult to observe a principle in action. Change within a
population of real organisms due to genetic drift is impossible to observe in a short amount of
time. Therefore, a model can be used to simulate this change over many generations. In this
model, beads represent individuals within a population. A particular trait in this population has
two alleles represented by two colours, red and blue. An opaque bag is used to allow for
random sampling of the population.
PURPOSE: To observe a simulation of genetic change in a sample population.
MATERIALS: - 25 red beads
- 25 blue beads
- 1 opaque bag large enough to hold all the beads
- 2 beakers
PROCEDURE:
1. Set up a model of a population of a diploid organism using 25 red beads and 25 blue
beads. The 2 colours represent the 2 alleles of a particular trait.
2. Place the entire population into the opaque bag.
3. Assume that 10 individuals from this population migrate to a new location. Randomly
withdraw their genes from the bag and place them into a beaker.
4. Record the genes that are present in the new population.
5. Observe and compare the gene frequencies of the new population with those of the
original population.
6. Repeat steps 3 to 5 two additional times.
7. Place the genes of the original population aside in a beaker. Into the opaque bag, place
the genes of the 10 individuals from your first migrant population. Assume that only 6
of these individuals succeed in reproducing successfully (3 males and 3 females) to form
the next generation and that each pair produces 2 offspring.
8. Randomly draw the genes of the parents from the bag and record them.
9. Determine and record the possible gene frequencies of the offspring and compare them
with the genes of their parents.
OBSERVATIONS:
Record all information from the procedure in an appropriate chart.
DISCUSSION:
1. How did the three trials (steps 3-5 in the procedure) differ?
2. How did the gene frequencies of the offspring in step 9 compare with the gene
frequencies of their parents? How did the gene frequencies of the offspring
compare to the original population?
3. Define the term genetic drift.
4. What effect have you demonstrated?
EXTENSION:
1. The world’s population of cheetahs is almost identical genetically. Male cheetahs
are known to have low sperm counts and the species in general has a low resistance
to many infectious diseases. All cheetahs are thought to be homozygous in over
99.9 % of their genes. Explain how a severe bottleneck effect in the past could
account for these observations.