SAN DIEGO MESA COLLEGE
School of Natural Sciences & Mathematics
Intro Molecular & Cellular Biology Lab (BIO 210A); Instructor: Elmar Schmid, Ph.D.
H
Huum
maann P
Peeddiiggrreeee A
Annaallyyssiiss
_______________________
(Student Name)
Assignments:
Example A.
In our first example let’s use a hypothetical family history that shows the inheritance of the
cystic fibrosis (CF) trait (= excessive build-up of mucus in the lungs of affected human
individuals) through three generations and which pedigree is charted in the Lab
Assignment Sheet 1 (retrievable via the Website).
Look at the family pedigree shown in the Assignment Sheet 1 and then answer the
following questions:
1. Which type of inheritance is observed?
Hints:
- to do this you need to examine the children who have the trait and then look at
the their parents, and, if possible at their grandparents
- in our first case, the parents of affected individuals II-3 and III-3 (CF patients) do
NOT have the trait but pass it on to their children, an important observation
which rules out the possibility of a dominant allele/gene. Why?
________________________________________________________________
________________________________________________________________
-
-
we can further assume that the trait is most likely NOT X-linked inherited
because the biological fathers of individuals II-3 and III-3 should be affected by
CF (= have the trait) in order for it to appear in their daughters
based on these observations and the limited evidence in this pedigree it is
reasonable to assume that the analyzed trait (which is neither dominant nor Xlinked inherited) is controlled by a
_______________________________ allele/gene
2. Knowing that the trait of pedigree example A is _______________________
inherited, now write down the genotypes for each individual into the blank
lines underneath the symbols of each family member in the family pedigree
- use the letter A abbreviation for the dominant (normal CF allele) and the letter
a for the recessive “cf” allele (abnormal mucus formation in lungs)
- AA = homozygous dominant
- aa = homozygous recessive
- Aa = heterozygous
3. Determine the probability of the two individuals III-1 (male) and III-5 (female) –
which wish to marry and have children – of having a child with the cystic
fibrosis (CF) trait. You will be able to answer this question by carefully
studying the example below.
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SAN DIEGO MESA COLLEGE
School of Natural Sciences & Mathematics
Intro Molecular & Cellular Biology Lab (BIO 210A); Instructor: Elmar Schmid, Ph.D.
Probability p =
______________________
Calculation:
How to determine the probability that a certain marriage between members of a
pedigree will produce children with a certain trait, e.g. a disorder.
- to do this we first must make ourselves familiar with the laws of probability that
relates to the simultaneous occurrence of independent events (for a review see
statistics and the laws of probability)
- to answer the question 3# above you have to apply the probability product law
which is stated as follows:
If A1 and A2 are two independent events, the probability p that they will occur
together, is the product of their individual probabilities p1 and p2.
p = p1 x p2
Example: if you roll one die, the probability of obtaining a 4 is 1/6; the same
probability exists for a second die to obtain a 4; the probability of obtaining two
fours when rolling two dice simultaneously, is therefore:
1/6 x 1/6 = 1/36
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-
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To determine the probability of the couple in Question #3 above of having a CF
child, we will apply the same principle. We first try to answer the following
questions:
What is the probability that each intended parent is heterozygous regarding the
gene/allele?
In the case of the intended father (III-1), the probability that his father (II-2) is
carrying the CF allele is 2/3 (it’s not 2/4 because we know his father is not
suffering from cystic fibrosis (CF), has therefore NOT the “aa” genotype)
Assuming that his mother (II-1) is homozygous dominant (“AA”) regarding the
CF allele (remember: we are to assume that anyone marrying into the family
does not carry the recessive “a” allele, is therefore homozygous dominant)
This means that the son (III-1) has a probability of ½ of carrying the trait, since
his father (II-2) may be heterozygous (“Aa” genotype); the Punnett square
shown in Figure 1 below summarizes these possibilities.
In the case of the mother of III-5, we are certain that she (II-6) is heterozygous
regarding the “a” allele (“Aa” genotype), since her father (I-4) has the trait – is
suffering from CF
Again, we assume that her father (II-7) – who married into the family – is
homozygous dominant (“AA” genotype); the daughter (III-5), then, has a
probability of ½ of carrying the CF allele; the Punnett square shown in Figure 1
shows this cross again.
The final step is to use the product law of probability to the determine the
probability p that an CF child will result from the marriage between III-1 and III-5
following the calculations as shown in Table 1 below
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SAN DIEGO MESA COLLEGE
School of Natural Sciences & Mathematics
Intro Molecular & Cellular Biology Lab (BIO 210A); Instructor: Elmar Schmid, Ph.D.
-
Based on the calculations below, the chances are 1 in 24 that the couple III-1
and III-5 will have a child being diagnosed with cystic fibrosis (CF)
Figure 1: Punnett squares showing calculations that intended father carries
recessive allele
1. Calculations for intended father’s lineage with help of Punnett squares
I-1
A
II - 1
A
a
AA
A
I-2
A
A
A
AA
AA
a
Aa
Aa
II - 2
a
A
aa
p that intended father’s father (II-2)
is heterozygous (Aa) = 2/3
p that intended father (III-1)
is heterozygous (Aa) = 1/2
2. Calculations for intended mother’s lineage with help of Punnett squares
II - 7
A
A
A
AA
AA
a
A
Aa
II - 6
p that intended mother (III-5)
is heterozygous (Aa) = 1/2
Table 1: Calculation of chance of couple III-1 & III-5 to have a child with cystic
fibrosis (CF)
Probability p
that
Father’s father
is carrying the
“a” allele
II-2
2/3
2/3
x
Intended father
is carrying the
“a” allele
III-1
1/2
1/2
x
Intended mother Intended
is carrying the
parents will
“a” allele
produce CF
child
III-5
III-1 x III-5
1/2
1/4
1/2
x
1/4
Total
probability p
that
Couple will
produce CF
child
=
1/24
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SAN DIEGO MESA COLLEGE
School of Natural Sciences & Mathematics
Intro Molecular & Cellular Biology Lab (BIO 210A); Instructor: Elmar Schmid, Ph.D.
4. Look at the following pedigree Example B (= Lab Assignment Sheet 2) which
shows the inheritance of a trait in four generations of a family and answer the
following questions.
a. How is this trait inherited? (Mark your answer in the Table below)
Dominant
Recessive
Autosomal
X-linked
b. List the genotypes for each individual of this pedigree on the blank lines
under the symbols for male and female, using the letters “B” (dominant
allele) and “b” (recessive allele)
c. What is the probability that the children from the following marriages will
show the trait?
III-1 x III-12
p = ___________
IV-2 x IV-3
p = ___________
IV-1 x IV-4
p = ___________
III-6 x III-13
p = ___________
d. If we assume that the frequency of heterozygous persons for this trait in the
normal population is 1/71, then the chances of two heterozygotes marrying
and producing a child with the trait are:
heterozygous
parent
___________
heterozygous
parent
X ___________
X
probability p of
having child with trait
____________
= ______
e. Use the information in parts (c) and (d) above to make a statement about the
problem associated with consanguineous (first cousins) marriages.
5. The next pedigree Example C (= Lab Assignment Sheet 3) shows the
inheritance of a sex-linked trait - which leads to the disorder M in affected
human individuals - over three generations of a family. Study the pedigree
carefully and, if necessary, review the section in your textbook or my website
concerning sex-linked inheritance and X-linked inheritance patterns before
starting to solve this problem.
a. Use the symbols XM (normal allele) and Xm (mutated allele) for this trait “M” and
list the genotypes for each individual on the blank lines under the symbols for
the male and female family members.
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SAN DIEGO MESA COLLEGE
School of Natural Sciences & Mathematics
Intro Molecular & Cellular Biology Lab (BIO 210A); Instructor: Elmar Schmid, Ph.D.
b. Which of the parents in generation I is heterozygotic (hemizygotic)?
________________
c. From whom must individual III-1 have received the m allele? ______________
d. What is the probability p that:
II-4 is a carrier for the trait “M”?
p = ___________
II-6 is a carrier for the trait “M”?
p = ___________
(show your calculations!)
e. What is the probability p that the male children of a marriage between III-4 and
III-5 will have the “M” trait? (Show your calculations!)
p = _____________
Calculations:
6. Carefully study the next pedigree Example D (= Lab Assignment Sheet 4)
which shows the inheritance of a trait - which leads to the disorder “K” in
affected human individuals - over four generations of a family and answer the
following questions below.
a. What is the inheritance pattern for this genetic disorder “K”?
________________________________________________
b. What is the genotype of the following individuals of this family?
I-1
_______
II-8
_______
III-1
_______
IV-1
_______
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SAN DIEGO MESA COLLEGE
School of Natural Sciences & Mathematics
Intro Molecular & Cellular Biology Lab (BIO 210A); Instructor: Elmar Schmid, Ph.D.
c. What is the probability p that the male children of a marriage between IV-3 and
IV-7 will have the “K” trait? (Show your calculations!)
p = _____________
Calculations:
7. The last pedigree Example E (= Lab Assignment Sheet 5) shows the
inheritance of a human trait - which leads to the disorder “F” in affected
human individuals - over five generations of a family. Study the pedigree
carefully and answer the questions below.
a. What is the inheritance pattern for this genetic disorder “F”?
________________________________________________
b. What are the genotypes of the following individuals of this family? (Write it onto
the lines underneath the symbols in your Lab Assignment Sheet)
II-1 ______ :
II-7
______ :
III-4
______ :
IV-3 ______ :
IV-7
______ :
V-1
______ :
c. Which members of the family in Example E deceased due to complications
connected with the heritable disorder “F”?
____________________________________________________________
d. What is the probability p that the child (V-1) of the marriage between IV-5 and
IV-6 will have the “F” trait? (Show your calculations!)
p = _____________
Calculations:
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