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Pedigree Analysis in Human Genetics: Tutorial

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  • 0:05 What is a Pedigree?
  • 1:02 How to Create a Pedigree
  • 2:14 Family Relationships
  • 4:03 Expanding the Pedigree
  • 5:34 Finishing the Pedigree
  • 6:02 Lesson Summary
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Lesson Transcript
Instructor: Joshua Anderson
One of the oldest genetic techniques is pedigree analysis, and yet it is still used every day all around the world by doctors, geneticists, and genetic counselors. Building and reading a pedigree is actually fairly easy if you know what the symbols mean. In fact, it's so easy that you can learn how to do it in less than ten minutes if you watch this video!

What is a Pedigree?

As you may recall, pedigrees are charts of family histories that show the phenotypes and family relationships of the individuals. Doctors and scientists have used pedigrees to study human genetics long before the technology existed to sequence DNA. The concept is actually pretty simple. A family history is taken and a family tree is constructed. Family members that have the trait of interest are marked, which allows the investigator to follow the trait through the family and determine the pattern of inheritance. Pedigrees use a standardized set of basic shapes to convey information and don't require writing. This means that pedigrees do not rely on any single language and are readable by anyone who knows what the symbols mean. By the end of this lesson you too will know what the symbols mean and be able to read or create a typical pedigree on your own.

How to Create a Pedigree

So let's get to it! The best way to learn how pedigrees work is to see how they're created, so we're going to start off by creating a pedigree of our own using an undefined genetic condition. Now, pedigrees are usually created after a doctor has encountered a patient that has a genetic condition.

Proband, Gender, and Phenotype

This individual is the proband, or the first affected family member that seeks medical attention. The pedigree is built around the proband, so let's start there with our pedigree. In a pedigree, a male is represented by a square and a female is represented by a circle. Let's say that our proband is a female, so we'll make the first symbol in our pedigree a circle. Now, if an individual does not exhibit the trait of interest, the circle or square is left open, but if the individual does exhibit the trait, the shape is filled in. Our female patient has the genetic condition, so we'll fill the circle in - and, to mark her as the proband, we'll use an angled arrow labeled with a capital P pointing to the filled-in circle.

Family Relationships

Now let's say that our patient has an unaffected husband. We draw an open square to represent the husband and connect the square and circle with a horizontal line to represent their marriage. This horizontal line is called a relationship line. When possible, the male partner is placed on the left end of the relationship line and the female partner is placed on the right end of the relationship line. If the couple has a child, we draw a line straight down from the marriage line and connect it to a circle for a female child or a square for a male child. If the couple has more than one child, a horizontal line is used between the generations to show the sibling relationships, and the siblings are ordered in their birth order from left to right.

Multiple Births

If the couple has twins, a vertical line is bifurcated to show that the children were the result of a multiple birth. For triplets, the line is split into three, for quadruplets, the line is split into four, and so on. If two or more siblings arise from the same zygote, they are called monozygotic. Most people use the more common term 'identical,' but the correct medical term is monozygotic because it more accurately describes the relationship between the siblings. In any case, monozygotic siblings are indicated on a pedigree by connecting the split line with a horizontal line like this. In this case, the couple has quadruplets, three boys and one girl. This horizontal line here indicates that two of the boys are monozygotic, but the third boy and the girl are not.

Expanding the Pedigree

Okay, now that we have some of the basics down, let's expand our pedigree by adding parents for our proband. We use a square to represent the father and a circle to represent the mother, and we place the square on the left side of the relationship line and the circle on the right side. Let's also give the proband some siblings: two sisters and two brothers. We use the same line structure here as we did when we added children for the proband. Now let's add spouses for all of the siblings. Because the spouses are not blood-related to the proband's parents, make sure that they're only connected to their spouses for now.

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