Investigation III: Exploring the Behavior of Alleles

Investigation III: Exploring the Behavior of Alleles

Hypothesis: If a bottleneck effect were to occur in a population, decreasing the dominant allele frequency and increasing the recessive allele frequency, then the amount recessive alleles in the new population will increase.

Materials:

• 4 Spreadsheets
• Data Table

Procedure:

1. Create or copy a spreadsheet with the dominant and recessive alleles and genotypes.
2. Label your first spreadsheet parentals and create 250 rows with different genotypes.
3. Choose two frequency for both the dominant and recessive alleles.
4. Record all the totals of the alleles and genotypes in a data table.
5. Create a bar graph displaying this data.
6. Duplicate this spreadsheet and label it F1.
7. Decrease the number of rows containing the genotypes to 50 to mirror a bottleneck effect.
8. Change the allele frequencies.
9. Record all the totals in a data table.
10. Create a bar graph displaying this data.  
11. Duplicate the spreadsheet labeled F1 and label it F2.
12. Increase the number of rows containing the genotypes until it reaches 125.
13. Keep the allele frequencies the same and record the totals in a data table.
14. Create a bar graph displaying this data.  
15. Duplicate the spreadsheet labeled F2 and label it F3.
16. Increase the number of rows containing the genotypes until it reaches 250.
17. Keep the allele frequencies the same and record the totals in a data table.
18. Create a bar graph displaying this data.
19. Compare the differences in the allele frequencies between the parentals, F1, F2 and F3 by using the graphs.

Data Table 1:

	Parental	F1	F2	F3
Population	250	50	125	250
Allele Frequency	p = 0.7 q = 0.3	p = 0.2 q = 0.8	p = 0.2 q = 0.8	p = 0.2 q = 0.8
Number of Dominant (A) Alleles	233	42	124	259
Number of Recessive (B) Alleles	267	58	126	241
Homozygous Dominant (AA)	47	7	17	41
Heterozygous (AB)	139	28	90	177
Homozygous Recessive (BB)	64	15	18	32

Parental Graph:

This is a graph of a population of 250 where the pink bars represent the number of alleles in a population and the green bars represent the genotypes in a population.

Graph Justification: In this graph, there is a higher number of recessive alleles, B, then there are of dominant alleles, A, even if its allele frequency is lower. For this reason, there are more homozygous recessive genotypes, BB, and less homozygous dominant genotypes, AA.  Since the heterozygous genotype contains both alleles, there is a larger amount of heterozygous organisms.

Graph F1:

This is a graph of a population of 50 where the pink bars represent the number of alleles in a population and the green bars represent the genotypes in a population.

Graph Justification: After the change in population, there were more recessive alleles present compared to the dominant alleles. Since there are more recessive alleles, there is a larger amount of homozygous recessive genotypes than homozygous dominant genotypes. Even though there were less dominant alleles, most of the organisms in the population would display the dominant trait because of the large amount of heterozygous organisms. The increase in the recessive allele’s frequency was caused by a bottleneck effect that wiped out many of the dominant alleles.

Graph F2:

This is a graph of a population of 125 where the pink bars represent the number of alleles in a population and the green bars represent the genotypes in a population.

Graph Justification:  Over a period of time, the population increased and the number of dominant alleles increased. The dominant alleles are almost even with the recessive alleles and because of this, the amount of both homozygous genotypes are almost even. Even so, there are still more recessive alleles than dominant alleles.

Graph F3:

This is a graph of a population of 250 where the pink bars represent the number of alleles in a population and the green bars represent the genotypes in a population.

Graph Justification: Unlike the other graphs, there are more dominant alleles than there are of the recessive alleles. Because of this, there is a larger amount of organisms with homozygous dominant genotypes than homozygous recessive genotypes. Even though the frequency for the recessive allele is higher than the frequency for the dominant alleles, there are more dominant alleles present. Throughout each population, it seems that the heterozygous genotype was always larger than the rest.

Conclusion:

After the parental population decreased, there was a big change in the allele frequencies. In the original population,  the dominant allele frequency was higher than the recessive allele frequency but in the new population that switched. The result of this was that in the population of 50, there were more recessive genotypes than there were dominant. Now although there was a  change in the allele frequencies, the fact that there were more recessive alleles than dominant alleles did not change. As the population began to grow, more dominant alleles were passed on and this allowed it to reach a number closer to the recessive allele’s. When the population reached its original number, 250, there were more organisms that had inherited the dominant genotype because there were more dominant alleles present. This proved that my hypothesis was incorrect. Although there were more recessive alleles when the population was smaller, the number of dominant alleles grew quicker. Each time the alleles and genotypes in a population were recorded, the dominant alleles had a greater increase and that resulted in there being more dominant genotypes than recessive. A possible error that could have occurred would have been creating the wrong number of rows on the spreadsheet or inputting the wrong information in the graphs. If this experiment were to be repeated, the allele frequencies or the number of organisms could be increased or decreased. Since the parental graph and graph F3 in the experiment had a greater amount of alleles for the smaller allele frequency, then the hypothesis for the next experiment could be “ If a bottleneck effect were to occur in a population, then the smaller allele frequency will have a greater increase in alleles.”

Questions:

1. Why do recessive lethal alleles like cystic fibrosis stay in the human population?  Why don’t they gradually disappear?

Recessive lethal alleles like cystic fibrosis stay in the human population because of heterozygous individuals. Since heterozygous genotypes have both the dominant and recessive alleles, the recessive lethal allele will never die out. For some recessive alleles,  it is actually an advantage for it to stay in the population. An article written by Medha Hegde mentioned that "lethal alleles which are recessive result in death of the individual only in the homozygous recessive state". Although it is extremely harmful for a human to be homozygous recessive for these traits, it is beneficial for them to be heterozygous. Individuals with both the dominant and recessive allele, can become resistant to other diseases; for example, "it was hypothesized that carriers of mutant CFTR genes benefited from resistance to cholera". Those who are heterozygous for cystic fibrosis might be resistant to cholera just like people who are heterozygous for sickle cell anemia are resistant to malaria. In populations where these alleles give a person an advantage, like resistance to a disease, the allele is common and therefore cannot disappear. Even if it seems like the recessive allele frequency is decreasing, mutations in a population can create these alleles. On a website created by UC Berkeley, there was an article that stated, "the mutation producing the deleterious allele may keep arising in the population, even as selection weeds it out". Since mutations are always occurring in populations, lethal alleles will always be present.

2. Polydactyly, more than 5 digits per hand, is a dominant trait.  Why is polydactyly not a common trait in human populations?

When humans are born, they are born with two copies of each gene, one from their mom and one from their dad. Most humans are born with 5 fingers; the reason being that both their parents had 5 fingers. Even though having Polydactyly is a dominant trait, there are more humans that have the recessive trait for 5 fingers. Alisa Lehman from Stanford University wrote, "In some regions Polydactyly is more common though. Usually those are places where a founding member carried the gene for Polydactyly and this person passed that trait on to his or her children". It is uncommon for someone to inherit Polydactyly because it is only possible to be born with 6 or more fingers if one of their ancestors had the dominant trait. If the majority of the people in a population had ancestors with the recessive allele, which is the case in most human populations, then it is really uncommon for someone to be born with Polydactyly. In an article written by Natalie Wolchover, she mentioned that, "A duplicate finger contributes nothing new and so doesn't confer any worthwhile evolutionary advantage". Since there are no advantages in having Polydactyly, natural selection didn’t choose it to be passed onto future generations. Therefore, the trait for Polydactyly is rarely passed onto offspring.

3. How does the inheritance of traits or allele frequencies change in a population? How can this be beneficial or harmful to the organisms?

There are different ways a trait or an allele frequency can change. One way would be through natural selection where organisms with the traits and alleles better suited for survival are chosen to survive and reproduce more offspring. An an article about natural selection said, that those organisms "have a higher chance to survive, reproduce, and increase their population more than the ones that are less adapted to their environment". For example, if gray and green tree frogs were to live in an area full of green vegetation, the green frogs would be able to camouflage themselves while the gray frogs would be eaten. Natural selection would then choose the green frogs to survive and reproduce. This would then change the allele frequency of the population of frogs because the allele for green frogs would increase while the allele for gray frogs would decrease.  Mutations can also change the allele frequencies. For example, if you were to continually spray insects with a chemical that would kill them, the insects would develop a mutation over time that would help them resist the chemical. Another article stated that, "If fitness is improved by a mutation, then frequencies of that alleles will increase from generation to generation". Since the insects with the mutation survived, they would reproduce and increase the allele frequency for that mutation. The migration of organisms can also change the allele frequencies; "Migration will change gene frequencies by bringing in more copies of an allele already in the population or by bringing in a new allele that has arisen by mutation". If the insects with the mutation were to move to another population without the chemical resistance, they would pass on their traits to that population and the allele frequency for the mutation would increase in the new population. Although the change in traits and the allele frequencies are harmful to organisms, like the gray frogs who are eaten or organisms who developed a harmful mutation, it is beneficial to most organisms because it allows them to evolve and survive in changing environments.

Citations:

Question 1:

"The "bad" Gene." The "bad" Gene. Berkeley, n.d. Web. 22 Sept. 2016.

Hegde, Medha. "Lethal Alleles - Its Instances in Humans, Plants and Animals."Lethal Alleles
- Its Instances in Humans, Plants and Animals. Biotech Articles, Winter 2012. Web. 22 Sept. 2016.

"Cystic Fibrosis." New World Encyclopedia. New World Encyclopedia, 24 July 2013. Web. 25 Sept. 2016.

Question 2:

Lehman, Alisa. "Understanding Genetics." Understanding Genetics. The Tech Museum of Innovation, 31 Oct. 2012. Web. 22 Sept. 2016.

Wolchover, Natalie. "What If Our Hands Had 6 Fingers?" What If Our Hands Had 6 Fingers? LiveScience, 11 May 2012. Web. 22 Sept. 2016.

Question 3:

"Population and Evolutionary Genetics." Population and Evolutionary Genetics. N.p., n.d. Web. 22 Sept. 2016.

Whittaker, Danielle. "Evolution 101: Natural Selection." Evolution 101: Natural Selection. Beacon, 01 Oct. 2012. Web. 22 Sept. 2016.

Investigation II: Alleles in a Population

Investigation II: Alleles in a Population

Hypothesis: If the frequencies for the recessive allele and the dominant allele are both the same, then the number of organisms with homozygous dominant genes and homozygous recessive genes will be the same. 

Materials: Spreadsheet

Procedure:

1. Make a copy of the given spreadsheet.
2. Using the data from the spreadsheet, create a graph.
3. Label the independent variable (horizontal x-axis)
4. Label the dependent variable (vertical y-axis)
5. Title the graph

Graph 1:

This bar graph shows the alleles present in a population of 25 organisms.
A represents the dominant alleles while B represents the recessive alleles.

Graph Justification: 
In this first graph, you could see that the number of  dominant alleles, A, and the recessive allele, B, are the same. There are about 25 of each allele, A and B, because the frequencies of both alleles are 0.50.

Graph 2:

This bar graph shows the genotypes that are present in a population of 25 organisms.

Graph Justification: When the alleles were combined to form genotypes, the heterozygous genotype was found in a larger amount of organisms than the number of homozygous dominant and recessive genotypes. There were 11 heterozygous organisms compared to the 7 homozygous dominant and 7 homozygous recessive organisms. Both the homozygous recessive and dominant genotypes were equal because the alleles in these genotype had the same allele frequency.

Conclusion:
If all the homozygous genotypes, AA and BB, had an equal amount in a population, then my hypothesis would have been proven correct, but because the genotypes were randomly generated, my hypothesis won't always be correct.  There are times where amount of people with the homozygous dominant gene will appear greater than the amount of organisms with the homozygous recessive gene and vice versa.