Pre-Lab #5: Inheritance
Name _______________________________
1. Define the following terms:
Monohybrid Cross (see Part I)
Allele Frequency (see Part II)
2. Describe how you will “mate” in Part 1 of this lab.
3. What is the allele frequency of “a” (little a) if your population has 40 “AA”, 50 “Aa”, and 10 “aa”?
(Show your work for full credit)
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Lab #5: Inheritance
This lab is demonstrates some of the principles of genetics - the study of how heritable traits are passed
from generation to generation. Genes are small segments of DNA that determine physical characteristics
called traits. Alleles are variations of a single gene that are present in the gene pool of a population. An
individual may carry two different alleles of the same gene or identical alleles of the same gene. This exercise
demonstrates how different alleles of the same gene segregate and re-assort (separate and come together) in a
population.
Part I: Human Genetic Traits
§
Some human traits exhibit the simple dominant and recessive behavior of a monohybrid cross similar to
Mendel's peas. However, the expression of most other human traits, including skin color and height, is much
more complex and the genetics cannot be easily studied. We will look at a few easily observed human genetic
traits to illustrate the simple dominant-recessive relationship between alleles in humans.
Phenotype frequencies for a population will not necessarily show the same dominant/recessive ratios that you
see for the offspring of a specific mating pair. When we calculate gene frequency, it is unlikely that we will
have a 3:1 ratio of dominant traits to recessive traits. In our case, it is probably due more to small sample size
than anything else. If you had a large immediate family to survey, these should segregate in a more classically
Mendelian fashion.
A. Mid-digital Hair The presence of hair on the middle joint of the finger is a dominant trait. Hair may not
be present on all of your fingers, but if you have hair on even one finger, you are dominant.
B. Tongue Rolling The ability to roll the tongue upward from the sides is a dominant trait. For some
reason, people who exhibit this trait seem to think it is a desirable thing to do. As far as anyone knows,
tongue rolling has no obvious anatomical or physiological advantage or disadvantage.
C. Widow's Peak A distinctive downward point of the frontal hairline is a dominant trait known as a
widow's peak. If you have a straight hairline, you are recessive for this trait.
D. Free Earlobes Free earlobes are dominant over attached earlobes. Read the introduction to Chapter 9 for
an explanation of how this trait is produced during fetal development.
E. Facial Dimples Dimples, or indentations, at the corner of the mouth are a dominant trait.
F. PTC Tasting The ability to taste the chemical phenylthiocarbamide, or PTC, is a dominant trait. Place a
piece of PTC paper on the back of your tongue. If you can detect this chemical, it will have a bitter taste.
If the paper does not taste nasty to you, then you are recessive for this trait.
1. Mark your appropriate phenotype for each trait in Table 1. on the data sheet at the end of this lab.
2. Record the data for the class and calculate the frequency (percentage) of each phenotype in the class
population.
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Part II: Monohybrid Cross
A monohybrid cross is the genetic transmission of a single trait. For this exercise, each person will receive
two colored beads that represent the two different alleles of a gene. Purple will represent the allele for the
dominant characteristic (designated in writing as “A”), while green will represent the allele for the recessive
characteristic (written as “a”). Since gametes contain only one copy of each gene, a single bead also
represents a gamete. The genotype of an organism is the combination of specific alleles. The phenotype of
an organism is the physical traits resulting from the genotype.You will “mate” (i.e. exchange gametes) with
your lab partner and determine the genotypes and phenotypes of the offspring that result from each mating, or
“cross”.
Work in groups of two
A. F1 Generation - Homozygous Parents
1. One lab partner holds two purple beads (the homozygous dominant genotype) while the other lab
partner holds two orange beads (the homozygous recessive genotype).
2. Mate with your lab partner by shaking your beads in your cupped hands and selecting one bead
WITHOUT LOOKING. This ensures that one of your alleles, or gametes, is chosen randomly. Pair it
with your partner's randomly chosen allele; the two beads represent the two alleles of your offspring,
i.e. the offspring’s genotype. Record the genotype of the offspring in Table 2.
3. Return to your original homozygous genotype and mate with your lab partner 9 more times (ie. repeat
steps 1-3) to produce a total of 10 offspring. Record your results.
4. Report your results to the class and determine the class total for each genotype.
B. F2 Generation - Heterozygous Parents
This mating involves self-fertilization of the offspring produced by the first cross.
1. Each person will start with the heterozygous genotype of the F1 offspring.
2. Mate with your lab partner ten times to produce ten offspring. Record the genotypes of your offspring
in Table 2. (Remember to return to your original heterzygous genotype after each mating).
3. Report your results to the class and determine the class total for each genotype.
C. Create Your Own Monohybrid Cross
1. Make up a single gene trait and assign letter symbols and bead colors to the different alleles.
2. Decide what parental genotypes you wish to have. Draw a Punnet square on your notes/observation
page, and predict what the expected genotype and phenotype ratios will be in the F1 generation.
3. Mate with your lab partner ten times to produce ten offspring. Record the genotypes of your offspring
in Table 3. (Remember to return to your original genotype after each mating).
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D. Pedigree Analysis:
After carefully studying generation after generation of cats, a monk living in a High Sierra retreat established
that the trait of cuteness R is dominant while the trait for being ugly r is recessive. To demonstrate this he
mated two cats. First, there was Fred a “cute” male cat whose father was quite handsome, but Fred’s brother
was really “ugly”. Before Fred met Susie, he had an affair that resulted in one “ugly” kitten. Second, there
was Susie a “cute” female cat whose father was “nice” but really “ugly” and whose mother was purebred for
cuteness. Use the following pedigree chart to help understand Fred’s pedigree.
If the monk is correct, what would be the answer to the following?
1. The genotype of Fred was ____________
1. The gametes from Fred could be __________ or __________ .
1. The gametes from Susie could be __________ or __________ .
1. The chances of them having a heterozygous kitten __________.
1. The chances of them having a cute kitten __________ .
1. The chances of them having a homozygous kitten __________.
1. The chances of the kitten being homozygous recessive __________.
1. The genotype of a kitten whose phenotype was ugly __________.
1. The phenotype of the homozygous dominant kittens is ____________________.
1. The genotype of the homozygous recessive kittens is __________.
1. The genotype of Fred’s father is __________.
12. The genotype of Fred’s brother is __________.
13. The genotype of Susie’s mother is __________. 14. The genotype of Fred’s mother __________.
15. The genotypes of Susie’s grandparents on her father’s side are _____________.
15. The phenotypes of Susie’s grandparents on her mother’s side are ___________.
15. The parent from which Susie’s mother inherited cuteness is ________________.
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Part III: Allele Frequency
Allele frequency is the relative amount (expressed as a percent) of a particular allele in a population. Evolution
can be defined as a change in the frequency of alleles in a population over time. The allele frequency for a
given trait changes over time in response to changes in the environment. This exercise illustrates how natural
selection influences genotype.
A. How to Calculate Allele Frequency
Think of a population as a pool of genes instead of individuals, as though each of the individuals pulled out
his or her alleles and threw them in a big pot.
Suppose we have a population of 100 individuals, 10 of which are "AA", 50 of which are "Aa", and 40 of
which are "aa". This population contains 200 total alleles, because 100 individuals x 2 alleles each = 200
alleles total.
Here is how to calculate the allele frequency of the “A” allele – first count up all the “A’s” in the
population, as follows
10 individuals are “AA”, they contribute 20 “A” alleles to the population (they have no “a” alleles)
50 individuals are “Aa”, they contribute 50 “A” alleles to the population (they also have 50 “a” alleles)
40 individuals are “aa”, so they contribute 0 “A” alleles to the population (they have 80 “a” alleles)
The total number of “A” alleles is: 20 + 50 + 0 = 70 . Now divide the number of “A” alleles by the
total number of alleles:
Allele frequency of "A" =
70 “A” alleles = 0.35 , or 35%
200 total alleles
Similarly, the allele frequency for "a" in this population is 0.65, or 65%.
B. What Happens with Selection Against the aa Genotype
We have seen how two alleles of a gene can be passed on if both alleles have an equal chance of surviving.
Now we will demonstrate what happens to allele frequency when there is a selective disadvantage to a
particular genotype. In particular, genotype “aa” causes a fatal disease that results in death during childhood.
1. Everyone will begin with a heterozygous genotype. (Everyone is healthy, but carries a recessive gene for
the disease.) Record your initial genotype on the first line of Table 3.
2. Instead of mating with your lab partner, find another person in the class to mate with AT RANDOM
(without respect to charm, appearance, or even gender). Mate twice. The two offspring you produce will
replace you and your partner in the next generation. If one offspring has the genotype "aa", it dies. This
means that one of you cannot reproduce again. If both offspring are “aa”, then neither of you can
reproduce again.
Stop at this point until everyone in the class has finished mating.
3. You will now assume the identity of one of your offspring and take on its genotype. Remember, if you are
“aa”, you cannot mate again. If you are "AA" or "Aa", replace your beads with the new alleles of the
appropriate colors. Record your new genotype in the 1st generation row of Table 3.
4. Find a new partner to mate with and produce two offspring. Again, if you produce an "aa" offspring,
one of you cannot mate in the next generation. Record your new genotype in the 2n d generation row of
Table 4.
5. We will repeat the matings until a total of five generations have been completed.
6. Report your results to the class and record the class results in Table 5. Calculate the allele frequencies for
each generation based on the class results.
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Data Sheet
Name ___________________________
Lab #5: Inheritance
Table 1: Human Genetic Traits
Trait
Your Phenotype
Class Data
Mid-digital Hair
No Mid-digital Hair
Tongue Rolling
No Tongue Rolling
Widow's Peak
Straight Hairline
Free Earlobes
Attached Earlobes
Dimples
No Dimples
PTC Taster
Non-Taster
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Frequency
Data Sheet
Name ___________________________________
Lab #5: Inheritance
Table 2: F1 and F2 Generations
Make a tally mark in the appropriate box to keep count of each genotype produced by your matings.
Number of Offspring
Homozygous Dominant
(AA)
Heterozygous
(Aa)
Homozygous Recessive
(aa)
No. of your F1
offspring:
No. of your F2
offspring:
Total No. of class F1
offspring:
Total No. of class F2
offspring:
Table 3: Your Own Monohybrid Cross
Predictions
Genotype Ratios ___________________
Phenotype Ratios______________________
Number of Offspring
Homozygous Dominant
( )
Heterozygous
( )
No. of your F1
offspring:
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Homozygous Recessive
( )
Data Sheet
Name ___________________________________
Lab #5: Inheritance
Table 4: Your Individual Genotypes
Your Genotype
Original
1st Generation
2nd Generation
3rd Generation
4th Generation
5th Generation
6th Generation
Table 5: Class Genotypes
No. of people with genotypes:
AA
Aa
aa
No. of Individual
Alleles
A
a
Original
Genotype
Genotype of 1st
Generation
Genotype of 2nd
Generation
Genotype of 3rd
Generation
Genotype of 4th
Generation
Genotype of 5th
Generation
Genotype of 6th
Generation
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Gene Frequency
A
a
Lab Report Questions: (Type your answers on separate pages)
Part II: Monohybrid Cross
1. Briefly describe all of the genotypes and phenotypes that were produced in both the F1 and F2
generations using the results from table 2 (be sure to include the allele letter combinations and the
correct name for each camobination).
2. Explain why the recessive phenotype does not show up in the F1 generation, but does show up in the
F2 generation.
3. Describe how your small group results for the F2 generation differed from the combined results of
the class (be sure to include the quantitative data from table 2 in your answer) and explain why our
results did not match the expected 1:2:1 genotype ratio.
4. Describe the genetic trait you created in Activity C of Part II (be sure to include the letter symbols for
each allele and state which allele is dominant and recessive). What did this activity teach you about
genetics and genetic inheritance?
Part III: Allele Frequency
5. Using the data from table 5, describe how the number of individuals with each genotype changed in
each successive generation. Was there a trend? What do the results tell you about natural selection
in the case of our activity?
6. Looking at the pool of alleles in the population, describe how the frequency of each allele changed in
each subsequent generation. Was there a trend? What do the results tell you about evolution in the
case of our activity?
7. Why doesn't natural selection against aa individuals (those that die due to the disease during
childhood) lead to the complete loss of the "a" allele from the population? Does keeping the recessive
allele in the population have any potential benefit?
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