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MCDB 1041
Activity 2: Using information from pedigrees to solve genetics problems
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Objectives:
• Be able to determine the modes of inheritance from both autosomal and X-linked pedigrees
• Be able to calculate the probabilities of inheritance from information in a pedigree
• Be able to calculate probabilities over several generations
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PART I. Warm-up problems: interpreting pedigrees showing the inheritance of one trait.
1. The pedigree shows inheritance of deafness, an autosomal recessive disorder.
I.
II.
1
1
2
dd
III
2
dd
3
Dd
1
If II-2 and II-3 just had another baby boy. What is the chance that he is deaf given the information on the
pedigree?
2. Tanya is worried about her baby (not yet born). Will the baby have sickle cell disease (also called sickle
cell anemia)?
Rasheed
?
Tanya
A. First let’s think about Rasheed. What is the probability that he is a carrier? Note, we KNOW he does
not have sickle cell anemia, so we need to adjust the probability accordingly. What are his possible
genotypes?
B. There is no history of sickle cell anemia in Tanya’s family. Can the baby be born with this disease?
C. What’s the probability of the baby also being a carrier, like Rasheed? To calculate this chance, we have
to take into account both Rasheed’s chance of being a carrier, and then the chance of him passing on the
disease allele. Show your work below.
Summary: If you know that someone cannot have the disease genotype (or the homozygous dominant, normal
phenotype), you EXCLUDE that as a possibility in the probability calculation. And, when calculating the
probability for an offspring of somebody (ie, calculating over two generations), you have to multiply the chance
that they have the allele by the chance they have of passing on that allele.
3. Deafness runs in the two families shown below. Using the pedigree information, what are the chances that
II-1 from family 1 and II-1 from family 2 will have a deaf child together?
family 2
family 1
I.
II.
III
Dd
Dd
1
1
Dd or
DD
I.
Dd
1
2
dd
Dd
2
3
dd
1
II.
III
Dd
1
2
dd
2
3
dd
Dd or
DD
1
Part II. Interpreting pedigrees that show autosomal inheritance of two different alleles.
Below is the pedigree of the Roloff family from the reality TV show “Little People, Big World” (filled
symbols indicate that the person is short in stature). This is an interesting example of inheritance, because there
are two different diseases with similar phenotypes (ie, dwarfism) shown on the same pedigree. The short
stature can be caused by either of two different genes located on non-sex chromosomes. The two diseases have
slightly different phenotypes. Matt has diastrophic dysplasia and Amy has achondroplasia. There are carriers
in this pedigree who are not marked; you have to figure out who they are.
Background Information
Diastrophic dysplasia
- For diastrophic dysplasia: “D” represents the normal version of the gene and “d” represents the mutant
version of the gene.
- Diastrophic dysplasia is inherited in an autosomal (non-sex chromosome) recessive manner. There must be
a mutation in both copies of the gene in order to be short in stature. (Matt has this genotype: dd)
- There is no history of diastrophic dysplasia in Amy’s family so you can assume that Amy and her parents
are not carriers (i.e. not Dd).
Achondroplasia
- For achondroplasia: “A” represents the mutant version of the gene and “a” represents the normal version of
the gene
- Achondroplasia is inherited in an autosomal (non-sex chromosome) dominant manner, which means that
there only needs to be a mutation in one copy of the gene in order to be short in stature.
- Amy is Aa. Please note: for this disease, AA individuals die as embryos.
Ron
Jeremy
Peggy
Gordon
Matt
Amy
aadd
AaDD
dd
Zach
Molly
Pat
1. In the blank lines next to each person on
the pedigree above, fill in the person’s
genotype. Matt and Amy’s genotypes are
already indicated.
Jacob
2.
Amy has autosomal dominant achondroplasia, but both of her parents are normal height. Ruling out infidelity,
there are several explanation described below for how this might happen. Which of these scenarios offers the
best possible explanation?
a. There was a mutation in a somatic (body) cell in one of Amy’s parents.
b. There was a mutation in a somatic (body) cell in both of Amy’s parents.
c. There was a mutation in a germline cell (sperm or egg) in one of Amy’s parents.
d. There was a mutation in a germline cell (sperm or egg) in both of Amy’s parents.
Explain your choice.
3. Does Zach have diastrophic dysplasia or achondroplasia? How do you know that Zach has only one of the
two little people phenotypes described? Describe how you were able to deduce this from the pedigree.
4. Jeremy marries Sue, a woman with diastrophic dysplasia but not achondroplasia. What is the chance that
their daughter will have diastrophic dysplasia?
5. Molly marries Greg who has achondroplasia and has no family history of diastrophic dysplasia (Greg is not a
carrier).
a. What is the chance their son will have achondroplasia?
b. What is the chance their son will have achondroplasia and be a carrier for diastrophic dysplasia?
PART III. Interpreting pedigrees that show X-linked inheritance.
When men make sperm, 50% of the sperm have an X chromosome and 50% of the sperm have a Y
chromosome. In females, 100% of the eggs have an X chromosome. Please note: sperm and egg, also called
gametes, have ½ the genetic content of an embryo. An embryo must have 2 copies of every somatic
chromosome and either an XX or an XY. Sex chromosomes (X and Y) are inherited in the following manner:
sperm
X
eggs
X
X
Y
XX
XY
female male
XX
XY
female male
1. Can men pass on an X chromosome to their sons?
2. If a man has an X-linked disease, what is the chance that his sons will inherit this disease from him?
An X-linked example:
X-linked ichthyosis is a skin condition that causes the appearance of scaly skin and clouding of the
cornea, and is caused by a mutation in the steroid sulfatase (STS) gene. The STS gene is on the X chromosome.
An X chromosome with a non-mutant STS gene is symbolized by XI and an X chromosome that has a mutation
in the STS gene is symbolized by Xi.
Ichthyosis is inherited in a recessive manner. Women who are XIXi do not have ichthyosis, but men
who are XiY do have ichthyosis because they do not have another X-chromosome with a normal version of the
STS gene. Ichthyosis runs in the family shown below. Filled symbols indicate people with ichthyosis; there are
carriers in this pedigree that are not marked; you have to figure out who they are.
3. In the blanks provided below, fill in the genotypes of the individuals with respect to the STS gene. Shade in
half-way the individuals you determine to be carriers. Ruth and Kathy’s genotypes are already shown to help
you get started.
William
XIXI Ruth
Kenny
Charlie
Debbie
Jean
John
Eileen
Kathy XIXI
Michelle
4. If Michelle and her husband Jason (who does not have ichthyosis) have a boy, what is the chance that he will
have ichthyosis? Does your answer change if it is a girl? Please explain how you figured out this chance.
Remember: men only have one X chromosome!
5. If Kenny and his wife Kim (whose family does not have a history of ichthyosis) have a boy, what is the
chance that he will have ichthyosis? Does your answer change if it is a girl? Please explain how you figured
out this chance.
6. What do you notice about the carriers in this pedigree?
7. Can a female be a carrier for an X-linked dominant disease?
Why or why not?
8. Can a woman ever have an X-linked recessive disease? If so, what are all the possible genotype
combinations for this woman’s parents?
LAST PART
Using the white boards, work with your group to create a pedigree that has 3 generations and shows a
particular inheritance pattern of your choice. Design the pedigree so that it can only be interpreted to be the
mode you have chosen.
After you are finished, trade your board with another group and see if you can figure out the mode of
inheritance they intended.
On a separate piece of paper, copy down the pedigree you created and write one or two sentences that
justify how this pedigree shows the particular mode you have chosen. Turn this in with your names on it!