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Week 4 Day 1 Lab: MENDELIAN TRAITS and INHERITANCE
Part 1: Mendelian Traits
Alleles are alternative versions of one gene. Alleles are found at the same locus on
homologous chromosomes, but may code for different phenotypes. The genotype of a
trait is the combination of two inherited alleles at one locus (either of these alleles can be
dominant or recessive). The genotype determines phenotype, or how the trait is
expressed. Mendelian traits are those that are inherited simply and are expressed based on
the presence or absence of dominant and recessive alleles. If one dominant allele is
present in the genotype, the dominant phenotype will be expressed. In order for the
recessive allele to be expressed, both alleles must be present. If both alleles are the same
this is called homozygous (both the same). If one of the alleles is dominant and the other
recessive, this is called heterozygous
EXAMPLE: Pretend that purple hair is inherited as a Mendelian trait. Having purple hair
is a recessive trait, with the allele represented by the “p.” Having white hair is the
dominant trait, with the allele represented by the letter “P.”
An individual who has the homozygous dominant genotype is written “PP” and has the
phenotype of white hair.
An individual who has the homozygous recessive genotype is written “pp” and has the
phenotype of purple hair.
An individual who has the heterozygous genotype is written “Pp” and has the phenotype
of white hair.
A. Below is a table and list of traits that are inherited in this way. Record which
phenotype you display for each trait by marking the appropriate column with an X (for
these traits this will be presence or absence of the trait). The photographs supplied will
help you determine whether you do or do not exhibit the trait. After presence/absence
data are recorded for each student, total the number of students displaying each
phenotype in the class.
TRAIT
PRESENT
ABSENT
Dominant
Class Total
State
(Present):
1. Widow’s peak
________
________
X
Present
____________
2. Attached earlobe
________
X
________
Absent
____________
3. Hitchhiker’s thumb
X
________
________
Absent
____________
4. Mid-digital hair
X
________
________
Present
____________
5. Short hallux
________
X
________
Present
____________
X
6. Left thumb hand clasping ________
________
Present
____________
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Trait Descriptions:
1. Widow’s peak: A distinct downward point of the hairline at the center of the
forehead is called a widow’s peak (if you are a lady and have this trait you are
supposed to outlive your husband). A receding hairline due to baldness will
mask this trait, so try to remember what you may have looked like in your
younger days. The dominant phenotype for this trait is to have a widow’s
peak, while a straight hairline is recessive.
2. Attached earlobes: The inheritance of a dominant allele results in earlobes
that hang free rather than attaching directly to the side of the head. Those
with attached earlobes are homozygous recessive for the trait.
3. Hitchhiker’s thumb: Hold your hand up like you’re trying to hitch a ride. If
the end of your thumb bends backwards more than 45 degrees, you have
“hitchhiker’s thumb.” If your thumb is straight, start thinking of another way
to get a ride. Straight thumbs are dominant, while expression of the
hitchhiker’s thumb is recessive.
4. Mid-digital hair: The presence of hair on the middle segment of the fingers
is due to the presence of a dominant allele. Make fists with both hands.
Examine the surface of the middle finger segments for the presence of any
hair. Those with no hair are homozygous recessive.
5. Short hallux: Compare the length of your big toe (hallux) to your second toe.
If your big toe is shorter than your second toe, you have inherited a dominant
allele. If your big toe is equal to or longer than your second toe, you are
homozygous recessive for this trait.
6. Left thumb hand clasping: Fold your hands together by interlacing your
fingers. Which thumb is on top? Try interlacing your fingers the opposite
way. It will feel awkward and may require some thought to accomplish.
Those who place the left thumb over the right have inherited a dominant
allele, while those who place the right thumb over the left are homozygous
recessive.
B. Imagine your lab section is a random sample of the population, and use what you
have learned from this exercise to answer the following questions:
1. Were the dominant phenotypes of the traits the ones that were most often
expressed? Were any of the traits particularly rare in your sample?
After using online populations samples, it was found that about 63% of those
sampled conformed to the dominant state of the phenome type. In other words,
about 63% of the time, if the dominant state was absent or present, the sample
population were correspondingly absent or present.
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2.
What does the variation found in your sample mean for the entire population?
Given the sample sizes used and that it was from the U.S., the variation
is a good representation of the U.S. population. However, if this were a
limited classroom situation, I do not feel it would have been a good
representation of the facts as it is narrow in scope diversity.
3. Why is variation in traits, phenotypes and genotypes important for natural
selection?
Variation is important because it enables us to adapt and allows those better
equipped to make better use of their resources, eventually either out-surviving
inter-breeding these environmentally-adapted variences into a better organism
or species.
Part 2: Mendelian Inheritance
Mendel came to some amazing conclusions about heredity through observation and some
very simple cross-breeding experiments with pea plants. For instance, he was able to
identify that genes are passed down from both parents, can be expressed in different
ways, and act as discrete units. Two principles were illustrated:
 The Law of Segregation simply states that the two alleles a parent has for a
given gene will separate during meiosis so that each gamete has only one of the
two alleles.
 The Law of Independent Assortment tells us that genes coding for different
traits and found on different chromosomes, will separate independently of one
another in the formation of gametes. (i.e. chromosomes do not influence the
segregation of other chromosomes)
Punnett Squares
To help understand Mendel’s laws, we can use a Punnett Square. Punnett Squares are
designed to illustrate how alleles for a gene can be passed on from parents and combined
and inherited in offspring.
A. Punnett Squares: Monohybrid Cross (crossing one gene)
In the example below, the letters H and h represent the two alleles found for the gene
expressing mid-digital hair. Remember from earlier that having hair on your knuckles is
the dominant state, and is represented here by H. A Punnett Square is organized with the
potential gametes produced by parents (the F0 generation) above and to the side of the
square, here indicated by the sperm and eggs. Only one allele from each parent will be
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passed on in each gamete, as described in Mendel’s Law of Segregation. The interior of
the Punnett Square (shaded gray) is where you find all possible combinations of alleles
that could occur in the offspring (the F1 generation) once the gametes from both parents
find combine. Each potential offspring genotype is equally likely to occur.
QUESTIONS:
1. What is the genotype of the mother?
____________________________
Hh
2. What is the phenotype of the
mother?
Observable as Hh - trait will be visible.
____________________________
3. List all the genotypes possible in the
offspring from this mating?
____________________________
The three groups are: HH, Hh, hh
4. Out of the four zygotes produced
how many are homozygous dominant?
One is homozygous dominant
____________________________
5. Out of the four zygotes produced
how many are heterozygous?
Two are heterozygous
____________________________
6. Out of the four zygotes produced
how many are homozygous recessive?
____________________________
One is homozygous recessive.
*****This results in a genotypic ratio of 1:2:1*****
7. What are the possible phenotypes of the offspring?
____________________________
Phenotypes are observable. The only observable possibilites are Hh and hh.
8. What would the phenotypic ratio be? ____________________________
3:1 is the phenotypic ratio in this situation.
Read the explanations and answer the questions below based on what you have
learned from the Punnett Square above. Make sure you understand the basics,
because more complex squares are next.
QUESTION 9: (Part 1)
Mendel discovered that his pea plants could have either green pea pods or yellow pea
pods. G represents the dominant green color, and g represents the recessive yellow color.
Mendel found that one of his favorite plants was heterozygous for pod color.
a. What is his plant’s genotype for pod color? ________________
His plant's genotype is Gg.
b. What is this plant’s phenotype for pod color? The
________________
phenotype is observable as Gg.
c. In order for the plant to have the recessive pod color, what would the genotype
have to be?
________________
The recessive
gene the plant would need to have is gg.
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QUESTION 9: (Part 2)
You decide you want to have pea pods with both yellow and green coloration in your
garden. You begin your experiment with the seeds from a homozygous green plant and a
homozygous yellow plant. These parents are the F0 generation of your experiment.
(***Remember that green is dominant and yellow is recessive***)
a. What are the genotypes for each plant?
Yellow: ________________
The genes are bb.
The genes are BB.
Green: ________________
b. Fill in the Punnett
Square below to
illustrate a cross
between the two plants.
b
arent 2
( GG )
b
B
B
Bb
Bb
Bb
Bb
c. What is/are the possible genotype(s) for the offspring produced by crossing the
plants? This group of offspring is the F1 generation.
The only type for this generation is Bb.
________________________________
d. What is/are the possible phenotype(s) for the F1 generation?
All would be observable as Bb, the dominant trait will be visible.
________________________________
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QUESTION 9: (Part 3)
You discover that the F1 generation does not provide you with both color pea pods to
compare in your garden. You decide you may as well not let all these pea pods go to
waste, so you make a cross between
two of the F1 generation to discover
what might happen next. The product
B
b
of this second cross is called the F2
generation.
a. Fill in the Punnett Square
below to illustrate a cross
between two members of the
F1 generation
B
b
BB
Bb
Bb
bb
b. What is/are the possible genotype(s) for the F2 generation?
The possible genotypes consist of: BB, Bb, bb.
________________________________
c. What genotypic ratio does this result in?
One
Two
One
_________________
: _________________:
_________________
d. Does this second generation give you the variety that you were originally looking
for in your garden?
Yes, it should eventually provide me with the color I am looking for.
e. In your own words, explain how the Law of Segregation works when gametes
are created:
By splitting the pair of alleles, the Law of Segregation allows genes to cross
during the reproductive sequece and obtain a differing allel from the
other reproductive donor.
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