Genetic Carrier Screening

Most people do not know they are a carrier for an inherited genetic disease until they have a child with the condition. Everyone has the chance to be a carrier for a genetic condition regardless of ethnicity, family background, or family history. It is common to be a carrier for at least one recessive genetic disease.

Genetic carrier screening can help you and your partner learn about the chance to have a child with a genetic disease before or even during pregnancy. With Horizon genetic carrier screening, you can have screening for one or many genetic diseases in one simple test. Before having carrier screening, it may be helpful to learn more about genetics and inheritance of diseases.

 

Genetics: The Basics

Cells are the basic building blocks of our body. Each of our cells contains our genetic information including our thousands of genes that are packaged into chromosomes. Genes are the instructions that tell our bodies how to grow and develop. People have 46 chromosomes that come in pairs. The first 22 pairs are numbered 1 through 22. The last pair, called the sex chromosomes, are different and are called X and Y. We usually we get one copy of each chromosome from our mother and the other copy from our father.

horizon carrier screen

horizon carrier screen

Genes also come in pairs.  We inherit one copy of each gene pair from our mother and the other copy from our father. Genes are made of DNA, and DNA is made up two strands that are wound together like a twisted ladder. DNA strands are made of an alphabet of 4 letters that are repeated in a certain sequence or order. These four letters A, T, G, and C are called bases. The bases in the DNA pair together, making “base pairs” where A is usually linked with T and G is usually linked with C.

Some diseases are caused when there is a mutation in the DNA. A mutation happens when the bases in the DNA are changed or “mutated.” This gene has a mutation where the ATTGCG has been changed to ATCGCG. This gene has a missing sequence of DNA called a deletion mutation. A mutation in a gene causes the gene to work improperly or not work at all. Genetic diseases happen when a gene or genes are not working properly.

horizon carrier screen

Reproductive Options

Inheritance: Autosomal Recessive Conditions

There are different ways genetic diseases can be passed down or inherited in families. Some genetic diseases happen when only one gene in a pair has a mutation. These are called dominant genetic diseases. Horizon carrier screening does not screen for dominant genetic conditions. Other genetic diseases happen when both copies of a gene pair have a mutation.  These are called recessive genetic diseases. A carrier of a recessive genetic disease is someone who has a mutation in one of the genes in a pair. A carrier is usually healthy because the other gene copy works normally.  A carrier of a recessive disease can be a male or a female.  This person has one working copy and one non-working copy of a recessive gene. 

horizon carrier screen

horizon carrier screen

A couple can have a child with a recessive disease when both the mother and the father are carriers of the same disease.  This means they both have a mutation in the same gene.  This couple’s chance with each pregnancy is 1 in 4 or 25% to have a child with the disease.  This couple also has a 3 in 4 or 75% chance to have a child who does NOT have the disease.

Inheritance: X-linked Conditions

Male or female gender comes from the “sex chromosomes” X and Y.  Females have two copies of the X chromosome. Males have one X chromosome and one Y chromosome. This means that females have two copies of each of the genes on the X chromosome and males only have one copy of these genes. Some genetic diseases are caused by mutations in genes found on the X chromosome. These are called X-linked genetic diseases. Females can be carriers of X-linked diseases. This woman has one working copy and one non-working copy of an X-linked gene. Males are not typically carriers because they only have one X chromosome, so they will be either healthy or affected.

For X-linked genetic diseases, only the mother needs to be a carrier to have a chance of having an affected child. This woman’s chance with each pregnancy is 25% or 1 in 4 to have a son affected with the disease. She also has a 25% chance with each pregnancy to have a daughter who is a carrier. This woman also has a 75% chance to have a child who does NOT have the disease.

 

horizon carrier screen

horizon carrier screen

Recessive Conditions: Pre-Pregnancy

If you are a carrier for a specific recessive condition, your partner may want to have carrier screening for the condition ordered by a health care professional. Your doctor or a local genetic counselor can help decide which carrier screen is best for your partner.  If your partner screens positive for the same condition that you are a carrier for, different reproductive options can be considered.

Couples at risk for having a baby with a recessive disease can consider:

  • Natural pregnancy with or without prenatal diagnosis such as chorionic villus sampling (CVS) or amniocentesis     
  • Preimplantation genetic diagnosis (PGD) with in vitro fertilization (IVF) to test and then transfer embryos free of the disease that both partners carry
  • Adoption
  • Use of a sperm or egg donor 

Recessive Conditions: Currently Pregnant

If you are a carrier for a specific recessive condition, your partner may want to have carrier screening for the condition ordered by a health care professional. Your doctor or a local genetic counselor can help decide which carrier test is best for your partner. Couples at risk for having a baby with a recessive disease can consider prenatal diagnostic tests such as chorionic villus sampling (CVS) or amniocentesis.

horizon carrier screen

horizon carrier screen

X-Linked Conditions: Pre-pregnancy

You may want to speak with your doctor or a genetic counselor about your Horizon test results. Partner screening is not often recommended as a next step if you are a carrier for an X-linked condition as only the mother needs to be a carrier for the condition to pass it along to her child. A doctor or genetic counselor can discuss available reproductive options with you including preimplantation genetic diagnosis (PGD) with in vitro fertilization (IVF), adoption, using an egg or sperm donor, or prenatal diagnosis such as chorionic villus sampling (CVS) or amniocentesis.

X-Linked Conditions: Currently Pregnant

If you are pregnant and are a carrier for an X-linked disease, prenatal diagnostic tests such as chorionic villus sampling (CVS) or amniocentesis are usually available to test for the X-linked disease in pregnancy.  Partner screening is not often recommended as a next step if you are a carrier for an X-linked condition as only the mother needs to be a carrier for the condition to pass it along to her child.

horizon carrier screen

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