Transcript Human Pedigrees - Downtown Magnets High School

Human Genetic Pedigrees
What is a Genetic Pedigree?


A genetic pedigree is an easy way to track your family
traits. It looks like a family tree, but also contains
information about your genetic history.
A doctor or geneticist might draw one for your family if
you had a family history of a particular disease. With
this information they could see how the disease is
inherited and calculate your probability of passing on
the disease to your future children.

Generations
I


1
2
– Notice it restarts at 1 every new
generation.
– When possible, older siblings are on
the left and younger siblings are on the
right in descending order.
II
2
1
3

III
1
This is an example of a family tree
showing 3 generations of family
members.
The roman numerals (in red) on the
left indicate the generation each
person belongs to.
Each individual in a generation is
then numbered (in green).

Using this system, the individual at
the bottom of this pedigree is III:1.
Age does not matter in determining
which generation an individual is
in. Children are always in the next
generation after their parents.
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Symbols
I
1
2


II
2
1
III
1
3
Each of the individuals
indicated by a circle is a woman
and each of the squares
represents a male family
member.
We now know that individual
III:1 is a male.
Occasionally, the sex of an
individual may not be known.
Common reasons for this would
be, miscarriages or early death,
babies given up for adoption, a
child that has not been born yet,
or distant family members.
 These individuals can be noted by
using a diamond symbol
instead of a square or circle.
“Marriage Lines”

I
1
2
II
2
1
III
1
3

The lines highlighted in red
indicate individuals that
have had children together.
Even though we call them
“marriage lines” it does not
matter if they are married,
were married, or were
never married.
It is important to realize
that time has no meaning
on a genetic pedigree,
therefore we do not usually
indicate if someone has
died or been divorced.

“Children Lines”
I
1
2
II
The lines highlighted in red are
“children lines”
– The marriage line that they are
connected to from above indicates who
gave them their genetic traits rather than
who raised them.
– If a couple has more than one child
together then we split the child line as
the green highlighted line shows. More
siblings would simply require a longer
line with more lines coming down from
it.

2
1
III
1
3
Thus II:2 and II:3 are children of I:1 and
I:2, but II:1 married into the family and
has different parents. We also know that
II:2 is older than his sister (read left to
right). However, we don’t know
anything about the relative age of II:1
even though she is on the left since she
married into the family.
 Be careful when you draw siblings to
show lines coming in from the top rather
than using a “marriage line” to connect
them.
Self Check 1
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I
1
2

II
2
1
III
1
3

For each of the individuals in
the family tree to the left,
indicate how they are related to
I:1. Be specific (for example,
use grandson or granddaughter
instead of grandchild.)
Write your answers on your
paper and label this section self
check 1.
When you and your partner are
done check your answers on the
next page.
Self Check 1
Answers
Wife
I
1
Daughter In-law
II
Son
(oldest child)
2
1
III
2
Daughter
(youngest child)
3
Grandson
1
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More Pedigree Notations

Since every family is unique, the
information on the previous pages may not
cover all families. The following are some
commonly needed notations in drawing
family trees.
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This is an example of how to show
a parent who has had children with
more than one person. It can also
be used to show remarriage. It
does NOT mean that they are
married to more than one person at
the same time. Remember, time
has no meaning in a pedigree.

In this example, II:1 and II:2 are
half brother and sister. They share
the same mother, but different
fathers. Typically, the older
sibling is still put first, but this is
not always possible.
Remarriages
Half Siblings
I
1
2
3
II
1
2
Remarriages
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Step Siblings
I
II
1
2
1
Step brothers and sisters are not
related by blood. In the example
below I:2 and I:3 each had
children from a prior relationship.
This means that II:1 and II:3 are
step brothers since they don’t
share the same biological parents.
3
4
2
3
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If your family has a situation with a more
complex marriage/sibling situation, ask and I
will show you how to draw it.
Adoptions

I
1
2
II
1
Adoptions are also fairly
common, but need to be
shown differently from
biological children since they
did not inherit their genetic
information from the parents
who raised them.
– The red line shows how to use
dashed “children lines” to
denote a child that is not
related biologically (adopted).
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In this example, the couple
adopted a son.
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Twins
I
1
2
II
1
2
3
4
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Twins are another fairly common
occurrence. However, there are
two kinds and from a genetic
standpoint it is very important to
know the difference.
– In the case of identical twins, the two
siblings have the same DNA. To
show this we split the sibling line at
an angle. The red highlighted line is
an example of this.
– In the case of fraternal twins,
although born at the same time, the
siblings are no more related than any
other siblings. Thus, they are drawn
the same as any siblings. The green
highlighted lines show this.
Tracking Traits

So far we have only looked at how to draw family
relationships. To make a family tree into a genetic
pedigree we need to be able to track a particular
trait.
– When we track traits we first put phenotypes into our
pedigrees. We then use these phenotypes to determine
possible types of inheritance of the trait and their
corresponding genotypes.
Step 1: Tracking
Phenotypes
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I
1
2
II
2
1
III
3
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Regardless of what the trait is,
geneticists shade in individuals
that have the phenotype of the
trait you are interested in
tracking.
For example, in this pedigree, 4
family members have the trait
of interest: I:1, II:1, II:3, and
III:1. In this case the key at the
bottom tells us that these four
individuals are nearsighted,
which is a known recessive trait
1
= nearsighted
= normal
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Step 2:
Recessive
Traits
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
I
1
2
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II
2
1
III
1
= nearsighted (n)
= normal (N)
3
The next step is to put in the
genotypes of each family
member.
The way to start putting in
genotypes is to start with the
recessive trait and put in all
their genotypes first.
In this case, since nearsighted is
known to be a recessive trait we
start with all the shaded
individuals. Remember, a
recessive phenotype means that
individual has two recessive
genes. (Click to put in the
genotypes)
Step 2:
Recessive
Traits
1
2
nn

nn
2
1
III

nn
I
II

nn
1
= nearsighted (n)
= normal (N)
3
The next step is to put in the
genotypes of each family
member.
The way to start putting in
genotypes is to start with the
recessive trait and put in all
their genotypes first.
In this case, since nearsighted is
known to be a recessive trait we
start with all the shaded
individuals. Remember, a
recessive phenotype means that
individual has two recessive
genes.
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Step 3:
Dominant Traits
nn
I
1
II
nn
nn
2
1
III
2
nn
1
= nearsighted (n)
= normal (N)
3
The next step is to put in the
known N gene for all
individuals with a dominant
phenotype. (Remember
someone showing the dominant
trait can be homozygous
dominant or heterozygous.)
(Click to put in the genotypes)
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Step 3:
Dominant Traits
nn
I
1
II
III
N
The next step is to put in the
known N gene for all
individuals with a dominant
phenotype. (Remember
someone showing the dominant
trait can be homozygous
dominant or heterozygous.)
2
nn
N
1
2
nn
1
= nearsighted (n)
= normal (N)
nn
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3
Sometimes we can tell who
should be NN and who is Nn if
we look at their parents or their
children’s genotypes. However,
sometimes one still cannot tell
and we put in N_, where the
line represents either N or n.
Step 4:
Determining the
2nd gene
nn
I
1
II
III
N
2
nn
N
1
2
nn
1
= nearsighted (n)
= normal (N)

In this case look at I:2. We
know that she had a recessive
daughter II:3. II:3 has nn so she
had to have gotten one from
both parents. This means that
I:2 must have a little n as her
second gene. (Click to put in the
genotypes)
nn
3
Step 4:
Determining the
2nd gene
nn
I
1

Nn
2

II
III
nn
N
1
2
nn
1
= nearsighted (n)
= normal (N)
nn
3
In this case look at I:2. We
know that she had a recessive
daughter II:3. II:3 has nn so she
had to have gotten one from
both parents. This means that
I:2 must have a little n as her
second gene.
In the case of II:2 we can also
tell their second gene is a little n
by looking at either his father or
at his son who both have nn
genes. (Click to put in the genotypes)
Step 4:
Determining the
2nd gene
nn
I
1

Nn
2

II
III
nn
Nn
1
2
nn
1
nn
3
In this case look at I:2. We
know that she had a recessive
daughter II:3. II:3 has nn so she
had to have gotten one from
both parents. This means that
I:2 must have a little n as her
second gene.
In the case of II:2 we can also
tell their second gene is a little n
by looking at either his father or
at his son who both have nn
genes.
– Remember each parent always
passes one of their two genes to
every child they have. This also
means that every child receives
one gene from each parent.
= nearsighted (n)
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= normal (N)
Determining trait inheritance:
dominant or recessive
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Sometimes you need to determine if a trait is
dominant or recessive just by looking at the
pedigree. In general assume dominant if you
cannot find the…
Recessive clue: Anytime that a child has a trait
and neither parent has it, that trait must be
recessive! (Using the pedigree shown here, can
you and your partner explain why? Hint: put in
the genotypes.)
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Self Check 2: Trait Inheritance
& Genotypes
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c)
A: For each of the three
pedigrees determine if the
shaded trait is dominant or
recessive.
B: Then determine the
genotypes of the following
individuals (Use A & a).
– a) II:3, II:4, III:4, III:6
– b) III:3, III:4, IV:1, IV:2
– c) I:1, I:2, II:4
Self Check 2 Answers: Trait
Inheritance & Genotypes
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A:
– a) recessive
– b) recessive
– c) dominant
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c)
B: Then determine the
genotypes of the following
individuals.
– a) Aa, Aa, aa, A_
– b) Aa, Aa, aa, A_
– c) Aa, aa, aa
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