Download Puzzling Pedigrees - Blue Valley Schools

Name: _______________________
Puzzling Pedigrees
Essential Question: How can pedigrees be used to study the inheritance of human traits?
Studying inheritance in humans is more difficult than studying inheritance in fruit flies or pea plants. For
obvious reasons, geneticists studying humans cannot set up breeding experiments to study the resulting
offspring! Clearly, other approaches must be used when studying human genetics. Family trees (also called
"pedigrees") provide a useful way to gather evidence about inherited traits in humans.
One approach to studying genes in humans is to construct a pedigree from data on a single trait within a
family. These data can then be used for genetic analyses. Researchers use pedigrees to look for inheritance
patterns as traits are passed from one generation to the next. These patterns can provide clues to the way
the trait is passed from parents to their offspring.
The genotypes of most of the individuals in a pedigree can be determined by examining the pattern in which
the trait is inherited from one generation to the next. By applying Mendel's principles, you can figure out
whether the trait being examined is dominant or recessive. For some individuals in a pedigree, however,
there may not be enough information to determine the full genotype.
Part A – Learning How to Interpret Pedigrees
You have used Punnett squares and your knowledge of which trait is dominant to predict what fraction of
the offspring of a particular set of parents are likely to have each trait. But how do scientists find out which
trait is dominant when they cannot do breeding experiments? They analyze data provided by pedigrees.
The figure below is a pedigree, or a diagram of a family’s pattern of inheritance for a specific trait.
I
II
III
Notice that in a pedigree, each person is represented by a number and each generation is represented by a
Roman numeral. In this way, each person can be identified by a generation numeral and an individual
number (i.e. person II-6 is the daughter-in-law in this family). Males are represented by squares and females
by circles. In this pedigree, unshaded symbols (squares or circles) indicate people who display the dominant
trait. These individuals are either homozygous dominant or heterozygous for the trait. Shaded symbols
indicate people who are homozygous for the recessive trait.
In this pedigree, I-1 and I-2 are the grandparents. The horizontal line that connects them is called a
marriage line. The vertical line that extends down from the marriage line connects the children to the
parents. Children are listed in order of their births from left to right. In other words, the oldest child is
always placed on the extreme left. In this pedigree, persons II-1, II-2, II-3, II-4, and II-5 are the children of
persons I-1 and I-2.
The trait being analyzed in this pedigree is ear-lobe shape. There are
two general ear-lobe shapes, free lobes and attached lobes. The gene
responsible for free ear lobes, represented by the capital letter E, is
dominant over the gene for attached ear lobes, represented by the
lowercase letter e. People with attached ear lobes are homozygous
for the recessive trait and are represented as ee.
Persons I-1 and II-5 are homozygous recessive (ee) and have attached
ear lobes. The people represented by the unshaded symbols have two
possible genotypes: EE or Ee.
Free Ear Lobes (left) vs Attached Ear Lobes (right)
Demonstrate your ability to interpret a pedigree by answering the
questions below.
Images from Wikimedia Commons
Questions:
1. How many generations are represented in this pedigree?
2. What is the genotype of I-2? How do you know? Explain your answer.
3. What are the genotypes of II-1, II-2, II-3, and II-4? How do you know? Explain your answer.
4. What are the two possible genotypes for II-6? Why is the genotype unknown? Explain your answer.
5. If II-6 is EE, what is the genotype of her child (III-1)?
6. What sex is the oldest child in generation II?
7. Who is the daughter-in-law in this family?
Part B – Recognizing Patterns in Pedigrees
Although most human traits are the result of interactions between multiple genes and/or environmental
factors, some hereditary disorders in humans are caused by a single gene. Pedigrees from several
generations of an affected family enable scientists to determine whether such conditions are dominant or
recessive. Knowing this, scientists can predict how likely it is that a child of particular parents will have the
condition.
Phenylketonuria, or PKU, is an example of such a genetic condition. Individuals with PKU lack the ability to
break down proteins normally. This causes the build-up of a chemical in the brain that can lead to mental
disabilities. If PKU is diagnosed shortly after birth, the child can be given a special diet. Typically, children
with PKU who are placed on this special diet for the first 10 years of life do not develop symptoms of the
condition. In most of the United States, newborns are routinely tested for PKU within a few days after birth.
Analysis of pedigrees indicates that PKU almost always appears in children of people who do not have the
condition. The figure below shows a family in which two grandchildren inherited PKU and five did not. These
numbers vary from family to family.
Family with PKU
I
II
III
Questions:
1. Is PKU likely to be a dominant or a recessive trait? Use evidence from the pedigree to support your
answer.
2. What are the most likely genotypes of persons II-1 and II-2? Explain your answer.
In the study of genetic diseases, a person who is heterozygous for a recessive genetic condition is called a
“carrier.” Such a person does not have the condition, but can pass on an allele for it to his or her children.
The recessive allele is hidden, or masked—until it shows up in a homozygous individual who has the
condition. A person who has a recessive condition is not called a carrier. In some pedigrees, symbols with a
dot in the middle are used to represent carriers. Draw dots inside the shapes of the known carriers in the
“Family with PKU” pedigree above.
We have just observed that recessive conditions can skip a generation, even though the allele is still present
in carriers. Is this pattern the same for all other hereditary diseases?
Consider polydactyly, which causes individuals to have an extra finger on their hands or extra toes on their
feet. This condition is not life threatening, but it does have a tendency to run in families. The pedigree
below shows a typical family history of a family with polydactyly. As you can see, one grandchild in this
family has polydactyly, but her siblings do not. Proportions of offspring with polydactly vary from one family
to the next.
Family with Polydactyly
I
II
III
Questions:
1. Is polydactyly likely to be a dominant or a recessive trait? Use evidence from the pedigree to support
your answer.
2. What are the most likely genotypes of persons I-1 and I-2? Support your answer with evidence from the
pedigree.
3. There is a small village in a mountain valley in Spain where a large number of people are polydactyl.
Why do you think this trait is so prevalent in this village? Explain your answer.
Another inheritance pattern that occurs in humans involves recessive alleles that are sex-linked. As you
learned earlier, sex-linked alleles are those located on one sex chromosome but not on the other.
Remember that, in humans, females are XX and males are XY. Most human sex-linked alleles are located on
the X chromosome. Since the Y chromosome is always inherited from the father, males only receive X-linked
alleles from their mothers. A male therefore needs only one copy of a sex-linked recessive allele to exhibit
the recessive trait. In contrast, a female must inherit two such recessive alleles—one from each parent—to
exhibit the trait.
Red-green colorblindness is a common sex-linked disorder that involves a malfunction of light-sensitive cells
in the eyes. Affected individuals often have difficulty distinguishing reds from greens. The pedigree below
shows the occurrence of red-green colorblindness in four generations of a family. The circles with a dot
represent carriers. Circles representing females whose genotypes are unknown are marked with a question
mark.
Family with Red-Green Colorblindness
I
II
III
IV
Questions:
1. Why is the genotype of person II-2 unknown? Could the genotype ever be determined? How?
2. It is rare—but not impossible—for females to exhibit sex-linked (X-linked) recessive traits. Describe a
scenario that could produce a female offspring with an X-linked recessive trait.
3. Draw a pedigree representing the following family history. Be sure to indicate the gender, condition, and
genotype of each individual. Use shaded symbols to represent individuals with the recessive trait. Use
symbols with a dot to represent carriers. Below your pedigree, write a brief paragraph explaining how
you were able to determine the gender, condition, and genotype of the children and grandchild.
Family History
A man and woman marry. The woman is a carrier of hemophilia, a recessive X-linked disorder (XHXh). The
man does not have hemophilia (XHY). They have 2 boys and 1 girl. The mother passed the gene on to the
two boys who died in childhood. The daughter grows up and marries a man without hemophilia. Their first
child has the disorder.
Part C – Creating a Pedigree Puzzle
Now is your chance to create a fictional pedigree that you will use to challenge your classmates. Your
Pedigree Puzzle will be posted online using social media so that others can attempt to meet your challenge.
Objectives:
 Create a pedigree puzzle that tracks the inheritance of a certain human trait, leaving at least one
unknown individual whose genotype can be correctly inferred using the information provided.
 Use social media to challenge your classmates to solve your puzzle.
 Interpret pedigrees created by others and use scientific argumentation to support a prediction about
the likelihood of the inheritance of certain genetic trait.
An example Pedigree Puzzle is shown below.
Example:
Pedigree Puzzle Requirements:
Your Pedigree Puzzle…
 must trace one of the single gene traits listed on the GENETIC TRAIT GUIDE from the Baby Lab.
 must include a key identifying the genotypes and phenotypes represented by each symbol.
 must include at least ten (10) individuals.
 cannot include more than four (4) generations.
 must include at least one unknown individual whose genotype can be correctly inferred using the
information available in the pedigree.
 must be accompanied by at least one question with a single, correct, evidence-based answer.
 must be pre-approved by your instructor prior to posting on social media.
 will require you to respond to your classmates’ attempted answers with “Correct!” –or- “Try Again!”
Rules for Solving Pedigree Puzzles:
Your Pedigree Puzzle Answers…
 must specifically address the question posed in the puzzle.
 must include supporting evidence from the pedigree.
The first to initially solve three different Pedigree Puzzles will be the class winner!
Tips for Creating Your Pedigree Puzzle:
1. In order to post your Pedigree Puzzle online, it will need to be a digital image (preferably a “.jpg” file).
One way to do this is to draw your pedigree on paper and photograph it. Alternatively, you could use
digital tools to construct your pedigree and convert it to a digital image. One such digital tool is the
Progeny Online Pedigree App found at the following URL:
http://www.progenygenetics.com/online-pedigree/launch.html
2. Your Pedigree Puzzle should be challenging, yet solvable. In other words, try not to make the answer
obvious but also make sure that there is a single correct answer. You may need to work through several
different scenarios before you find one that works.
3. Your teacher will explain the process for posting your Pedigree Puzzle online using social media.
Carefully follow the guidelines for posting and solving the Pedigree Puzzles.
Tips for Solving Your Classmates’ Pedigree Puzzles:
There are several ways to identify what pattern is represented in a Pedigree Puzzle. Below are some tips for
how to recognize four of the most common patterns of inheritance.
How to Recognize Four Major Patterns of Inheritance
Autosomal Dominant
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Males and females are equally likely to be
affected.
There are affected people in every
generation (generally).
There is male to male transmission of the
disease.
DD and Dd are affected, dd is not.
Autosomal Recessive
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X-linked Dominant
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All daughters of a male who has the disease
will also have the disease
There is no male to male transmission.
A female who has the trait may or may not
pass the gene for the disease to her son or
daughter.
XDXD and XDXd are diseased females; XdXd are
females without the disease.
XDY are affected males; XdY are males
without the disease.
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Males and females are equally likely to be
affected.
Disease often skips generations.
Disease may appear in siblings without
appearing in their parents.
If a parent has the disease, those offspring
who do not have the trait are heterozygous
carriers.
dd is affected, DD and Dd are not.
X-linked Recessive
The disease is far more common in males
than in females.
All daughters of a male who has the disease
are diseased or heterozygous carriers.
The son of a female carrier has a 50% chance
of having the trait.
There is no male to male transmission.
Mothers of males who have the trait are
either heterozygous carriers or have the
disease.
Daughters of female carriers have a 50%
chance of being carriers.
XDXD and XDXd are normal females; XdXd are
females with the disease.
XDY are normal males; XdY are males with the
disease.