Download LAB – Modeling a Gene Pool

Name___________________________________________________________ Period______ Date_ _____
LAB – Modeling a Gene Pool
Introduction
A population is a group of organisms of the same species that live together in a particular location. Each population is
normally isolated from other populations of the same species. Populations can be observed for many characteristics.
Population genetics is the study of genes in a population of organisms. The sum total of all the genes in a population is
called a gene pool. Biologists who study population genetics are interested in how frequently alleles of a gene appear in a
gene pool. The number of times an allele occurs in a gene pool compared to other alleles for the same gene represents its
relative frequency. Biologists are also interested in any changes in allele frequencies that may be occurring.
In this investigation beans will be used to represent individuals in a simulated population of organisms. Black beans will
represent homozygous dominant individuals (RR). Red beans will represent heterozygous individuals (Rr). White beans
will represent homozygous recessive individuals (rr). You will study various crosses using the beans and analyze
relative frequency and changes in relative frequencies as they appear in a simulated population in which a harmful
phenotype occurs. In addition to this, you will be able to see how a potentially lethal genetic disorder affects allele
frequency in the population over generations.
Objective

To observe and understand how relative frequency of an allele changes in a population.
Pre-lab
* COPY and ANSWER the following questions on a separate sheet of paper. Read the INTRODUCTION and PROCEDURE to
find the answers.
1. Define the following terms: gene pool, population, relative frequency.
2. Why are you asked to set aside and not use some of the white beans at the start of the lab?
3. If the initial breeding population (P generation) is composed of 16 RR, 32 Rr, and 8 rr individuals, how many R
alleles are present in the P generation? How many r alleles are present in the P generation?
4. In this lab, can an individual’s phenotype (physical appearance) be used to determine its genotype (combination
of alleles)? Explain your answer.
5. How would you expect the gene pool of this population to change over time?
Procedure
Part I: P generation parent pairs
1. Obtain a cup from the back. Place into it 16 black beans (RR), 32 red beans (Rr), and 16 white beans (rr). The
white beans represent individuals having the potentially lethal phenotype (rr genotype). To represent the early deaths in
the parent generation, remove half of the white beans and set them aside.
2. Mix the beans in your cup. Then, without looking, remove the beans in pairs. Each pair will be one of the six possible
combinations: (1) RR x RR, (2) RR x rr, (3) Rr x Rr, (4) RR x Rr, (5) Rr x rr, (6) rr x rr. Count the number of pairs of
each of the 6 combinations. These will represent the parents that will produce the first generation of offspring (F1
generation). To help organize and count the pairs, divide a sheet of paper into 6 columns with labels. Your teacher
will demonstrate for you.
3. Record the number of parent pairs for each combination in Data Table 1 on the back of this sheet.
4. Assume that each pair of parents will have 4 offspring. Calculate the expected number of offspring for each category
of parent pairs. Record these numbers in Data Table 1. For example, if you counted 6 Rr x Rr parents pairs, and each
pair has 4 offspring, there will be 24 offspring for that combination.
 Recall from your work with Punnett squares that the expected genotypes of 4 offspring produced from a
cross of 2 heterozygotes (Rr) is 1 RR, 2 Rr, and 1 rr (1:2:1 ratio). If 24 offspring are produced, the genotypes
would then be 6 RR, 12 Rr, and 6 rr. Record the numbers that reflect your data in Data Table 1.
5. Using these totals, count the total number of R’s and r’s in the offspring population. Remember that each RR
individual has 2 R’s, and each Rr individual has 1 R and 1 r. Record the data in the bottom row of Data Table 1.
Then calculate the relative frequency of each allele by using the formulas below and record in Data Table 1.
Frequency of R =
total R___
(total R + total r)
Frequency of r =
total r___
(total R + total r)
Part II: F1 Generation parent pairs
6. Based on the totals in Data Table 1, put enough beans in your cup to represent all the F1 generation offspring into
your container (you will need to get more beans). As before, remove ½ of the total number of white beans (rr) to
represent those that die before reproducing
7. Mix the beans. Then, without looking, remove the beans in pairs. Arrange these pairs into the 6 possible combinations
as you did in step 2. These 6 combinations will represent the F1 parents that will produce the second generation (F2)
of offspring. Record the number of counted parent pairs in Data Table 2 below.
8. Assume that each parent pair will have 4 offspring. Again, calculate the expected number of offspring for each
category of the parent pairs and record in Data Table 2.
9. Repeat the same calculations you did in step 6 for this F2 generation. Record your results in Data Table 2.
Part III: F2 Generation parent pairs
10. Repeat steps 6-9 to produce another generation of offspring (F3 generation). Record your results in Data Table 3.
Data Table 1 (P Generation parent pairs)
Parent
# of parent
# of offspring with
Pairs
pairs
genotypes
RR
Rr
rr
RR x RR
Data Table 2 (F1 Generation parent pairs)
Parent
# of parent
# of offspring with
Pairs
pairs
genotypes
RR
Rr
rr
RR x RR
RR x rr
RR x rr
Rr x Rr
Rr x Rr
RR x Rr
RR x Rr
Rr x rr
Rr x rr
rr x rr
rr x rr
Totals 
Totals 
# of R’s in offspring _____ Frequency of R=______
# of R’s in offspring _____ Frequency of R=______
# of r’s in offspring _____ Frequency of r =______
# of r’s in offspring _____ Frequency of r =______
Analysis: Answer in COMPLETE SENTENCES on the
same sheet of paper you used to answer your Pre-lab
questions. Staple it to this sheet for your final lab report.
1. Which of the six parent pairs of the P generation is
least likely to occur? Why is this so?
2. Would you predict that another group of students in
your class would have identical data to yours? Explain
your answer.
3. Can a single factor that affects the reproductive
success of only some individuals in a population result
in evolution? Explain your answer.
4. Explain why alleles that are lethal in a homozygous
individual can be maintained in a gene pool.
Data Table 3 (F2 Generation parent pairs)
Parent
# of parent
# of offspring with
Pairs
pairs
genotypes
RR
Rr
rr
RR x RR
RR x rr
Rr x Rr
RR x Rr
Rr x rr
rr x rr
Totals 
# of R’s in offspring _____ Frequency of R=______
# of r’s in offspring _____ Frequency of r =______
Parent Pairs
# of parent pairs
# of offspring with genotypes
RR
RR x RR
RR x rr
Rr x Rr
RR x Rr
4
4
7
8
Rr x rr
3
rr x rr
1
Rr
rr
Totals 
# of R’s in offspring _____
Frequency of R=______
# of r’s in offspring _____
Frequency of r =______
Frequency of R =
total R___
(total R + total r)
Frequency of r =
total r___
(total R + total r)