The CHOP PediSeq Project

The Pediatric Clinical Sequencing Project

All the genetic material in a person is referred to as their genome. Whole genome sequencing is a type of genetic test that looks at the sequence of all of the DNA in a person’s body. The portion of the DNA that is needed for a person’s body to make proteins is referred to as that person’s exome. The vast majority of known genetic diseases are caused by mutations in the exome. Whole exome sequencing only looks at the portion of DNA that is used to make proteins. Whole genome sequencing and whole exome sequencing may also be referred to as just genome or exome sequencing. Using exome and genome sequencing without the term “whole” may be more appropriate, since these tests do not usually look at a person’s whole sequence.

Genome or exome sequencing is usually done to try to figure out the cause of a condition that seems to be genetic. It is often performed when there are many possible genes that could be causing a patient’s condition. It might also be performed when standard genetic testing has not identified the cause or a diagnosis. Samples from a person’s family members, especially parents, may also be requested when a person has exome or genome testing. This allows the laboratory to compare the DNA sequences of the patient and family members in order to better understand which genetic variants are inherited. Because genetic variants are often inherited from an individual’s parents, sequencing may identify information that is relevant to family members. For example it could identify a chance that they or their child may have or develop a certain medical condition.

 


 

What kinds of results are available from whole genome and whole exome sequencing?

Whole genome and whole exome sequencing will find many variants or mutations in your genome. There are two main categories of results.

  1.  Results related to the condition that prompted sequencing (“primary findings”); and
  2.  Results unrelated to the condition that prompted sequencing (“incidental findings”).

Click here to learn more about whole exome sequencing results.

 


 

What are the benefits, limitations, and risks of whole genome and whole exome sequencing?

Learn more about the benefits, limitations, and risks of whole genome and exome sequencing here.

 


 

What do my sequencing results mean for risk of genetic disorders in my family?

The risk to family members will depend on the inheritance pattern for the condition. Genetic disorders can be inherited in one of four different ways. Click on the type of inheritance pattern to learn more.

A genetics healthcare provider will be able to tell you more about the inheritance pattern and risk to family members for a given condition. Please visit our resources page for more information.

 

What is DNA?

DNA (deoxyribonucleic acid) is the basic material of heredity. It is a chemical made of four kinds of building blocks called bases. The four types of bases are: adenine (A), cytosine (C), guanine (G), and thymidine (T). These four bases are strung together in different combinations or sequences. DNA sequences are mostly the same in everyone. In fact, about 99% of the DNA sequence is identical in all humans. The 1% of the DNA that differs between people is what makes us each unique. The differences in the DNA between people are called variants.

 


 

What are genes?

Genes are segments of DNA that we inherit from our parents and pass on to our children. Most genes tell our body how to make products called proteins. Proteins are components of our body that perform a specific function so that our bodies work properly. We have about 23,000 genes that instruct our body on everything from how our heart will form and beat, to what color our eyes will be.

You can think of a gene as a book that contains instructions for how to create a specific protein. The order of the bases in DNA is like the letters that make up a sentence in a book. If there is a “typo” or a change in a letter in a word, it can alter the meaning of the sentence. For example, changing the letter “T” in the word tag to the letter “G” would result in the word gag, changing our understanding of the sentence. Similarly, a variant or mutation in a gene can alter the instructions a gene provides to the body, causing the body not to make a working protein. Such variants may lead to problems with a person’s growth and development, or cause disease. These variants are called mutations.

 


 

What are chromosomes?

Chromosomes are the structures in our cells that are made up of DNA. Our chromosomes are contained in the nucleus of the millions of cells that make up our body. We have 23 pairs of chromosomes (46 total). For every pair of chromosomes, one chromosome comes from our mother and one from our father. One pair of chromosomes is the sex chromosomes called the X and Y chromosomes. These two chromosomes determine if we will be male or female. Females have two X chromosomes and males have an X and a Y. Except for genes carried on the sex chromosomes, we inherit two copies of every gene, one from each parent.

The diagram below shows how our bodies are made of millions of cells that contain DNA packaged in chromosomes.

 

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