Biology 11A Lab #5
Genetics and Inheritance
Pre-LabQuestionsCompletesentencesasalways!
1. Give an example of the following terms. You are not writing a definition.
a.
b.
c.
d.
e.
Genetic trait
Genotype
Phenotype
Dominant trait
Recessive trait
2. Make a drawing that includes these labels:
a. Gene
e. homologous pairs of chromosomes
b. Allele
c. chromosome
d. chromatid
Movie Discussion: The Human Race PBS
Lab #5: Genetics & Inheritance
Work in groups of two
This lab is designed to demonstrate genetics, or the study of how heritable characteristics are passed from
generation to generation. Genetic traits are determined by genes, or small segments of DNA carried on
chromosomes that determine physical characteristics. This exercise demonstrates how different alleles of the
same gene segregate and reassort (separate and come together) in a population.
human genetic traits to illustrate the simple
dominant-recessive relationship between alleles in
humans.
Summary of Activities
1. Demonstrate some of Mendel’s principles of
genetics by simulating different types of mating
crosses.
2. Examine at the effect of natural selection on the
frequency of alleles in a population.
3. Observe some single-gene genetic traits in
humans.
Gene 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.
§
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 many
other human traits, like skin color or height, is much
more complex and the genetics cannot be easily
studied. We will look at a few easily observed
1. Mark your appropriate phenotype for each trait
on the chart.
2. Record the data for the class.
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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.
J.
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.
II. Mendelian Genetics: Monohybrid Crosses
A monohybrid cross is the genetic transmission of a
single trait. For this exercise, each person will
receive colored beads that represent different alleles
of a gene. Red will represent the allele for the
dominant characteristic (designated in writing as
“A”), while yellow will represent the allele for the
recessive characteristic (written as "a"). The
combination of two beads determines the genotype.
Since gametes contain only one copy of each gene, a
single bead also represents a gamete.
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.
You will “mate” (exchange gametes) with one lab
partner and determine the genotypes and phenotypes
of the offspring that result from each mating, or
“cross”.
E. Eye Color: Brown pigmented irises are
dominant. The absence of brown pigment
results in blue eyes, which is recessive. Hazel or
green eye color is the result of a second gene
that produces a yellow pigment. Hazel eyes
have both brown and yellow iris pigment, while
green eyes have both the recessive blue iris and
dominant yellow pigment. For our purposes,
assess only the presence of absence of brown
pigment.
A. F1 Generation: Work in pairs
1. For the first mating you will have a homozygous
genotype. One lab partner will use two red
beads, representing the dominant genotype. The
other lab partner will use two yellow beads,
representing the recessive genotype.
F. Facial Dimples: Dimples, or indentations, at
the corner of the mouth are a dominant trait.
2. Mate with your lab partner by shaking your
beads in your cupped hands and selecting one
without looking. Pair it with your partner's
randomly chosen allele; these two beads
represent the two alleles carried by your
offspring. Record the genotype of your
offspring in Table 1 of the attached data sheet.
G. 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.
3. Mate with your lab partner 9 more times to
produce a total of 10 offspring. Record the
genotype of each offspring in Table 1.
H. Hitchhiker's Thumb: The ability to bend the
thumb backward at least 45o is a dominant trait.
The proper term for this is distal
hyperextensibility.
I.
Palmaris Longus Muscle: The presence of this
muscle is a dominant genetic trait. If you have
this muscle you will have three wrist tendons.
To determine this, clench your fist tightly and
flex your hand toward you. If you can see or
feel three tendons in your wrist, you have the
long palmar muscle and are dominant. If you
have only two tendons, then you are recessive
for this trait.
4. Report your results to the class and determine
the class total for each genotype. Answer the
following questions in your notebook.
Relative Length of the Big Toe: If your big toe
is shorter than your second toe, you are
dominant for this trait.
B. F2 Generation:
2
This mating involves self-fertilization of the first
generation produced by the first cross.
B. Selection against the aa genotype (Deleterious
allele)
1. Each person of the pair will now have the
genotype of the F1 offspring.
We have just seen how two alleles of the same gene
can be passed on if both genes 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 which results in
death during childhood.
2. Mate with your lab partner 10 times to produce
ten offspring. Record the genotypes of your
offspring in Table 1.
3. Report your results to the class and determine
the class total for each genotype. Answer the
following questions in your notebook.
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.
III. The Gene Pool: Allele Frequency
2. Instead of mating with your lab partner, roam
around the room and find another person in the
class AT RANDOM (without respect to charm,
appearance, or even gender) to mate with. Mate
twice. These two offspring will replace you and
your partner in the next generation. If one
offspring has the genotype is "aa", it dies. One
of you cannot reproduce again. If both offspring
are “aa”, then neither of you can reproduce
again. Record your offspring's genotype in
Table 3.
Allele frequency refers to how often a particular
allele appears 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. At first you saw how natural
selection influenced changes in the frequency of a
particular phenotype. This exercise will illustrate
how natural selection influences genotype.
A.
How To Calculate Allele Frequency
Stop at this point until everyone in the class has
finished mating.
Think of a population as a pool of genes instead of
individuals, as though each of the individuals pulled
out his or her genes and threw them in a big pot.
3. You will now assume the identity of your
offspring and take on its genotype. Remember, if
you were aa, you cannot mate again. If you are
“AA” or “Aa”, replace your beads with the
appropriate colors.
Suppose we have a population of 100 individuals:
•
•
•
25 of which are "AA"
50 of which are "Aa"
25 of which are "aa"
4. Find a new partner to mate with and produce
two offspring. Record the genotypes. Again, if
you produce an "aa" offspring, you cannot mate
in the next generation.
This population contains 200 total alleles, because
100 individuals with 2 alleles each = 200 alleles
total.
5. Repeat the mating four more until a total of six
generations have been completed. Every time
that a new generation is mating, ALL THE
CLASS must be on the same generation.
Counting up all the "A's" in the population, we find
there are 100 because:
+
2 X 25 =
1 X 50 =
50 "AA"
50 "Aa"
100 "A" genes
6. Report your results to the class and record the
class results in Table 4. Calculate the allele
frequencies for each generation based on the
class results. Answer the following questions in
your notebook.
Allele frequency of "A" is 100/200 = 0.5
(or 50%).
Similarly, the allele frequency for "a" in this
population is 0.5 (or 50%).
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Lab #5: Genetics & Inheritance
Data Sheets
Name _____________________
The dominant trait is the first listed of each pair. Table 1
Trait
Your Phenotype
Mid-digital Hair
No Mid-digital Hair
Tongue Rolling
No Tongue Rolling
Widow's Peak
Straight Hairline
Free Earlobes
Attached Earlobes
Brown Eye Color
Blue Eye Color
Dimples
No Dimples
PTC Taster
Non-Taster
Hitchhiker's Thumb
Straight Thumb
Longer Second Toe
Longer Big Toe
Palmaris Longus Muscle
Two Wrist Tendons
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Class Data
NAME _______________________
Lab am or pm ____________
I. Monohybrid Cross:
Make a hatch mark to keep count of each genotype that is produced during your matings.
Table 2
Number of Offspring
Genotype of Mating
Pair: AA X aa
Homozygous Dominant
(RR)
No. of your F1
offspring:
No. of your F2
offspring:
Total No. of CLASS
F1 offspring:
Total No. of CLASS
F2 offspring:
Your genotypic ratio:
Class’s genotypic ratio:
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Heterozygous
(Rr)
Homozygous Recessive
(rr)
Natural Selection Against the Recessive Phenotype
Table 3: Your Individual Genotypes
Your Genotype
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
Table 4: Class Genotypes
No. of people with genotypes:
RR
Rr
rr
No. of Individual Alleles
R
r
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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Allele Frequency
R
r
Problems for Inheritance
1. For each phenotype, list the genotypes. (Remember to use ONE letter, that of the dominant
trait)
Straight hair is dominant to curly.
____________ straight
____________ straight
____________ curly
Pointed heads are dominant to round heads.
____________ pointed
____________ pointed
____________ round
2. In guinea pigs, the allele for short hair is dominant. Pick a letter
different from problem #1
What genotype would a heterozygous short haired guinea pig have?
________________
What genotype would a purebred short-haired guinea pig have? ________________
What genotype would a long-haired guinea pig have? ________
Simple Monohybrid Cross
3. You cross a heterozygous short-haired guinea pig with a longhaired guinea pig. Write out the
Punnett square for this cross. What % of guinea pig offspring would be long-haired?
4. Let’s say your Mom was homozygous dominant for tongue-rolling, and your Dad is can’t roll
his tongue at all. What percentage of the offspring will be able to roll their tongues? Write out
the Punnett square.
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5. Second toe longer than the first toe is a dominant trait in humans. If one of your parents had a
second toe longer than the first and the other parent did not, what would their genotypes be?
__________ What percentage of their offspring would have the second toe longer than the first?
Write out the Punnett squares.
Sex-linked problems
6. A human female "carrier" who is heterozygous for the recessive, sex-linked trait causing redgreen color blindness, marries a normal male. What proportion of their male progeny will have
red-green color blindness? Write out Punnett square.
7. Hemophilia in humans is recessive, and due to an X-chromosome mutation. What will be the
results of mating between a normal (non-carrier) female and a hemophiliac male? How many
daughters would have hemophilia? How many sons would have hemophilia? Write our Punnett
square.
Red
Incomplete Dominance
7. In a certain fish, blue scales (b1b1) and red scales (b2b2) have
incomplete dominance. When a fish has the genotype b1b2, it has a
patchwork of blue and red scales. What happens if you breed this fish
(b1b2) with a fish that only has blue Scales? What percentage of
offspring will be Red? What percentage will be “patchwork”? Write out Punnett square.
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Blue
8. Sneetches, because of incomplete dominance, come in three sizes: Small,
medium or tall. If a small sneetch Mom mates with a medium sneetch Dad,
what percentage of tall sneetches will be born from that
couple?______________
What percentage of offspring will be medium? _____________
Dihybrid Cross
9. In rabbits gray hair is dominant to white hair. Also black eyes are dominant to red eyes.
GG = gray hair
BB = black eyes
Gg= gray hair
Bb = black eyes
gg= white hair
bb = red eyes
A male rabbit with the genotype GGbb is crossed with a female
rabbit with the genotype ggBb. Figure out the genotype and
proportions in the offspring.
How many out of 16 have gray fur and black eyes?
____________
How many out of 16 have gray fur and red eyes?
______________
How many out of 16 have white fur and black eyes?
___________
How many out of 16 have white fur and red eyes?______________
10. This time you set up the square. Show me the set up. Dihybrid squares are trickier.
Suppose that black hair (B) is dominant over blonde hair (b) and brown eyes (E) are dominant
over blue eyes (e).
The father has black hair (heterozygous) and brown eyes (heterozygous) and the mother has
blonde hair and blue eyes. Write out the Punnett square.
a. What percent of the offspring will be totally heterozygous?
b. What is the phenotype ratio?
c. What percent of the offspring will have blonde hair and blue eyes?
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Lab Report: Answer these questions. (If you don’t have complete sentences
as answers, no points! L) Type for next week, double spaced.
Q1 (From page 4) Which of the phenotypes investigated in the class showed that the dominant
trait was not the most frequent trait?
Q2 Why is the dominant trait NOT always the most frequent? Explain.
F1 Generation
Q3. (From page 5) What is meant by this F1 mating? How many possible genotypes are
produced by this mating and what are they?
Q4. How many different phenotypes are possible resulting from this F1 mating and what are
they?
Q5. What happened to the recessive phenotype? Explain.
F2 Generation
Q6. (From page 5) What is meant by tis F2 generation? How many possible genotypes are
produced by this mating and what are they?
Q7. How many different phenotypes are possible and what are they?
Q8. What happened to the recessive phenotype? Why?
Q9. How do the results from your group compare with the combined results of the entire class?
(Give numbers that compare the two here) Explain why some matings do not result in the
classic 1:2:1 genotypic ratio.
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Deleterious allele (Keep using complete sentences!)
Q10. (From page 6) How does the frequency for each allele change over the generations? Give
me percentages here. Is this always expected?
Q11. How does the frequency of each genotype change over the generations? Give me
percentages.
Q12. Explain the effect that natural selection appears to have on allele frequency in a
population.
Q13. Why doesn't selection against aa individuals lead to the complete loss of the "a" allele from
the population? Explain.
Q14. What is incomplete dominance?
Q15. What is codominance?
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MORE PRACTICE ! ! ! ! ! These are extra practice problems for you to do if you feel you
need the practice. They are not to be turned in. Our next lecture exam has many genetics
problems on it so it pays to practice.
Rabbits can have solid or spotted coats, and the base color can be either brown or black. Spotted
coat color is dominant to solid, and black is dominant to brown. Complete the following problem
by answering the following questions.
1. Write down letter designations for the alleles. Be sure to distinguish between the two traits,
and be sure to consider which alleles are dominant and recessive. For example, Mendel
used P to designate the purple allele, and p to designate the white allele for pea flower
color.
2. What would be the genotype of a female spotted brown rabbit whose mother was solid black?
3. What would be the genotype of a male solid black rabbit whose parents were homozygous for
the black allele?
4. If you cross these two rabbits, what kinds of gametes would be produced by the spotted brown
rabbit?
5. What kinds of gametes would be produced by the solid black rabbit?
6. Draw a Punnett square showing the mating of this pair of rabbits.
7. What proportion of the offspring of this cross would you expect to be spotted brown? Solid
black?
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