Genetic Pedigree: crucial part of Genetic Counseling

Although taking family history and drawing family history to perform pedigree analysis will be a part of genetic counseling graduate program, it is always good to know the basics for our own understanding of what Genetic counseling involves and also to be well prepared as an applicant for the course.

Why take Family History?

We all inherit our genome from our parents, which then gets passed onto almost every cell of our body. This also means any variations can also be passed on including the ones that cause genetic disorders/health issues. Understanding how this pattern is carried forward to generations is important not only for the health professional but also for the patient and their family so we can be well aware and perhaps take prevention.

Recording Family History:

  • During the consultation session of genetic counseling, family history is collected by asking a series of questions related to different family members in a systematic order and this information is recorded pictorially as a family tree or otherwise called a “Pedigree”.
  • A genetic pedigree is a visual representation of several generations in a patient’s family. It shows how family members are related to each other and notes any medical conditions they may have along with any other pertinent information. For example, a family’s ethnic background may be relevant, as this could indicate whether certain tests should be considered based on the frequency of conditions in different populations.
  • Standardised symbols and lines are used to represent the family members and their relationships.
  • By analyzing a pedigree, we can determine genotypes, identify phenotypes, and predict how a trait will be passed on in the future. The information from a pedigree makes it possible to determine how certain alleles are inherited: whether they are dominantrecessiveautosomal, or sex-linked.

To start reading a pedigree:

  1. Determine whether the trait is dominant or recessive. If the trait is dominant, one of the parents must have the trait. Dominant traits will not skip a generation. If the trait is recessive, neither parent is required to have the trait since they can be heterozygous.
  2. Determine if the chart shows an autosomal or sex-linked (usually X-linked) trait. For example, in X-linked recessive traits, males are much more commonly affected than females. In autosomal traits, both males and females are equally likely to be affected (usually in equal proportions).

Below example shows pedigree of a family with four generations. The person giving the information is Julie Smith, as noted by the small arrow. Julie, her mother Mollie, her grandmother Alice, and her cousin Mary, have all been affected the same medical condition, indicated by the shaded circles:

Symbols and lines:

It is of great importance for any health professional to understand and be able to read the genetic pedigree and so internationally recognised symbols and lines are used as proposed by National Society of Genetic Counselors.

  • Below are the symbol and lines used to form a family pedigree:
  1. How do we record required history on a pedigree?

For each person, we record the following information:

  • Name
  • Date of birth (if known, the date of birth is preferable to current ages or estimates)
  • Relevant symptoms and/or diagnoses and age at diagnosis (if known)
  • Cause of death and age at death (if known)

Depending on the clinical question, you might also want to record:

  • Pregnancy and birth history
  • Occupational/environmental exposures
  • Medications

2. Can this family history pedigree be shared with every member of the family?

only if explicit consent is provided are we to share the information with only those family members that the consent is provided for. This area has been in debate on implications of sharing or not sharing important medical history.

3. How do we know how many generations to gather?

This is subjective to the case and medical condition that we are dealing with. however, most commonly it would be up to 3 generations of family history that we try to obtain. In some scenarios it might be only specific condition history that we will be looking for.

4. What if we need to amend or make changes to an already completed genetic pedigree?

Changes can be made at a later date, however it is important to clearly document who made the changes as in the name, date as well as use a different color pen to highlight the changes or to upload the pedigree as a separate document in the electronic system. Also this process can usually have different rules at different practice locations or countries.

Practice pedigree:

The pedigree below tracks Duchenne Muscular Dystrophy (DMD) through several generations. DMD is an X-linked recessive trait.

If individual II-3 has a child with a carrier woman, what is the percent chance that the child will be a daughter with DMD?

choose 1 answer:

a. 0%

b. 25%

c. 50%

d. 100%

Practice 2:

The pedigree below tracks the presence of attached earlobes through a family’s generation. Having attached earlobes is an autosomal recessive trait.

What is the genotype of individual II-3?

Choose 1 answer:

a. XeY

b. XeY

c. ee

d. EE

e. Ee

– Common mistakes and misconceptions

  • The presence of many affected individuals in a family does not always mean that the trait is dominant. The terms dominant and recessive refer to the way that a trait is expressed, not by how often it shows up in a family. In fact, although it is uncommon, a trait may be recessive but still show up in all generations of a pedigree.
  • You may not always be able to determine the genotype of an individual based on a pedigree. Sometimes an individual can either be homozygous dominant or heterozygous for a trait. Often, we can use the relationships between an individual and their parents, siblings, and offspring to determine genotypes. However, not all carriers are always explicitly indicated in a pedigree, and it may not be possible to determine based on the information provided.

Answers to the practice scenarios will be posted on my Instagram story 🙂

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