12/8/2020 Chapter 14
Chapter 14
Due: 11:59pm on Monday, December 14, 2020
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Mendel's Law of Segregation
This tutorial introduces Mendel’s law of segregation of alleles using monohybrid crosses, the addition and multiplication rules, conditional probability, and binomial
probability.
In this tutorial you will investigate how alleles segregate during meiosis. You will also explore how this segregation, in conjunction with random gamete fusion,
allows geneticists to predict the outcome of genetic crosses using simple rules of probability.
Part A - Allele segregation and gamete formation
One character in peas that Mendel studied was yellow versus green seeds. A cross between a homozygous yellow line (YY) and a homozygous green line (yy)
will result in F1 plants that are heterozygous (Yy) for this trait and produce yellow seeds.
When an F1 plant undergoes meiosis, what gamete types will it produce, and in what proportions?
Hint 1. How do alleles segregate during gamete formation?
According to Mendel’s law of segregation, a gamete
ANSWER:
always receives both of the parent’s alleles for each gene.
always receives only one of the parent’s alleles for each gene.
may receive either one or two of the parent’s alleles for each gene.
Hint 2. Does allele segregation determine gamete frequencies (i.e., proportions)?
According to Mendel’s law of segregation,
ANSWER:
dominant alleles segregate into gametes more frequently than recessive alleles.
allele frequencies (i.e., proportions) in gametes cannot be predicted in advance.
alleles segregate into gametes with equal frequency.
ANSWER:
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,12/8/2020 Chapter 14
Yy Yy
Y y
Y y
YY yy
Y y
Correct
Mendel’s law of segregation states that allele pairs segregate equally into gametes during meiosis. This means that a gamete will have only one
allele of any given gene, and that the probability of a gamete having one allele or the other is equal (and therefore ½, or 50%, for either allele).
Part B - Punnett square
Punnett squares are convenient ways to represent the types and frequencies of gametes and progeny in experimental crosses.
This Punnett square shows the results of a Yy x Yy cross to form F2 progeny.
Use your understanding of Mendel’s law of segregation and the rules of probability to complete the Punnett square for this cross.
First identify the gametes. Use pink labels to identify the male and female gamete types and white labels to identify the gamete
frequencies.
Then identify the F2 progeny. Use pink labels to identify the progeny genotypes and white labels to identify the progeny frequencies.
Hint 1. Setting up a Punnett square
To create a Punnett square, list the possible gametes from one parent down one side and the possible gametes from the other parent across the top.
Each gamete is assigned a frequency according to Mendel’s law of segregation (as explained in Part A).
The boxes inside the Punnett square represent the genotypes of the progeny that result from random gamete fusion. Progeny frequencies are then
calculated using the frequencies of the gametes that formed them. (See Hint 2.)
Hint 2. How do I calculate the frequencies of the F2 progeny?
Which statement correctly explains how to determine the frequencies of the F2 progeny?
ANSWER:
The progeny frequencies are determined by the addition rule (adding the gamete frequencies together).
The progeny frequencies are determined by the multiplication rule (multiplying the gamete frequencies together).
The progeny frequencies cannot be predicted from gamete frequencies.
ANSWER:
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Y y YY Yy yy
Y y
Y Yy
y Yy
Correct
A Punnett square is a convenient method for representing Mendel’s law of segregation in a visual form. Using a Punnett square allows one to easily
see gamete types and frequencies, as well as the genotypes and frequencies of progeny formed by random gamete fusion.
The genotype frequencies inside a Punnett square are calculated using the multiplication rule: The probability of two independent events occurring
simultaneously is the product of their individual probabilities.
In this example, the genotype frequencies inside the square (¼) are the product of the gamete frequencies that led to their formation (½ x ½ = ¼).
Part C - Using the Punnett square to make predictions
Use the completed Punnett square in Part B to answer the questions below about the F2 generation.
Drag the probabilities on the left to the blanks on the right to answer the questions. Terms can be used once, more than once, or not at all.
Hint 1. How to choose between the addition rule and the multiplication rule
The addition (or sum) rule is used when there is more than one way to fulfill the requirements of the event of interest. Often the
word “or” acts as a hint that the addition rule is needed.
For example, you might say, “Yy individuals in the F2 can be formed by either a Y from the female parent and a y from the male parent OR
a y from the female parent and a Y from the male parent." This indicates that there are two ways to fulfill the requirement of “an F2
progeny with genotype Yy” in this case, and tells you that the addition rule should be used.
The multiplication (or product) rule is used to determine the probability of simultaneous events with individual probabilities.
Often the word “and” acts as a hint for the multiplication rule.
For example, you might say “YY individuals in the F2 require a Y from the female parent AND a Y from the male parent." In this case,
there is only one way to achieve the desired event. Therefore, the addition rule is not needed, and the probability of the YY genotype in
the F2 is determined with the multiplication rule alone.
Hint 2. For question 2, how do you account for conditional probabilities?
Conditional probabilities apply when the pool of possible outcomes is reduced in some way by additional criteria. For example, determining probabilities
for genotypes within only the yellow seeds in the F2 restricts the total number of possibilities because it excludes green seeds by definition.
Use your knowledge of conditional probabilities and the Punnett square in Part B to answer this question.
What is the probability that an F2 seed chosen at random from among only the yellow seeds will have a genotype of Yy?
ANSWER:
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3/4
2/3
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Hint 3. How to approach questions 3 and 4
For questions 3 and 4, you need to consider all possible groupings and orders of three F2 seeds and their probabilities (determined using the
multiplication rule).
The table below shows all the possible groupings of three F2 seeds. Two of the probabilities have been filled in for you. Complete the table by
calculating all of the other probabilities.
Possible grouping Probability
1. Green, Green, Green ?
2. Yellow, Green, Green
3. Green, Yellow, Green ?
4. Green, Green, Yellow ?
5. Yellow, Yellow, Green ?
6. Yellow, Green, Yellow ?
7. Green, Yellow, Yellow ?
8. Yellow, Yellow, Yellow ?
Once you’ve filled in the table, you will be able to calculate the probability of any defined outcome, employing the multiplication and addition rules as
needed. (See Hint 1.) For example, to determine the probability of getting two green seeds and one yellow seed in a three-seed grouping, there are
three groupings that qualify:
Yellow, Green, Green
Green, Yellow, Green
Green, Green, Yellow
After you've calculated the individual probabilities of these three groupings (using the multiplication rule) you can then employ the addition rule to
calculate their combined probability (because any of these three outcomes meets the stated criterion of having one yellow seed and two green seeds).
ANSWER:
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