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

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).– 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.

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.

I

II

III

1

2

2

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1

Symbols 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.

I

II

III

1

2

2

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1

“Marriage Lines” 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.

I

II

III

1

2

2

31

1

“Children Lines”

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.

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.

1

2

2

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1

I

II

III

Self Check 1 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.

I

II

III

1

2

2

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Self Check 1Answers

WifeI

II

III

Daughter(youngest child)

1

2

2

3

Daughter In-law

Grandson

1

1

Son(oldest child)

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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.

RemarriagesHalf Siblings

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.

I

II

1

2

2 3

1

RemarriagesStep Siblings

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.

If your family has a situation with a more complex marriage/sibling situation, ask and I will show you how to draw it.

I

II

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1

2

2 3

1

4

3

Adoptions 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).

In this example, the couple adopted a son.

I

II

1 2

1

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Twins

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.

I

II

1 2

1 2 3 4

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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

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

I

II

III

1

2

2

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1

= nearsighted= normal Table of Contents

Step 2: Recessive

Traits

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)

I

II

III

1

2

2

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1

= nearsighted (n)= normal (N)

Step 2: Recessive

Traits

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.

nnI

II

III

nn

1

2

2

3

nn

nn

1

1

= nearsighted (n)= normal (N)

Step 3: Dominant Traits

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)

nnI

II

III

nn

1

2

2

3

nn

nn

1

1

= nearsighted (n)= normal (N)

Step 3: Dominant Traits

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.)

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.

NnnI

II

III

nn

1

2

2

3

nn

nn

1

1

N

= nearsighted (n)= normal (N)

Step 4: Determining the

2nd gene 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)

NnnI

II

III

nn

1

2

2

3

nn

nn

1

1

N

= nearsighted (n)= normal (N)

Step 4: Determining the

2nd gene

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)

NnnnI

II

III

nn

1

2

2

3

nn

nn

1

1

N

= nearsighted (n)= normal (N)

Step 4: Determining the

2nd gene

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.

NnnnI

II

III

nn

1

2

2

3

nn

nn

1

1

Nn

= nearsighted (n)= normal (N) Table of Contents

Determining trait inheritance: dominant or recessive

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

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

c)

Self Check 2 Answers: Trait Inheritance & Genotypes

A: – a) recessive– b) recessive– c) dominant

B: Then determine the genotypes of the following individuals.– a) Aa, Aa, aa, A_– b) Aa, Aa, aa, A_– c) Aa, aa, aac)

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