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HorizonTM
Test to identify carriers of recessive genes
Genetics: The basis
Cells are the basic building blocks of our body. Each of our cells contains our genetic information including thousands of genes that are packaged into chromosomes. Genes are the instructions that tell our bodies how to grow, develop, and function. 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 get one copy of each chromosome from our mother and the other copy from our father.
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 of two strands that are wound together like a twisted ladder. DNA strands are made of an alphabet of four 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.” 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.
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. Horizon carrier screening tests for genetic diseases that 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 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. With each pregnancy, this couple’s chance 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 sex 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.
What’s next if I am a carrier for a genetic condition?
Patients who are planning to conceive in the future
If you are a carrier for a specific recessive condition, your partner may want to have carrier screening for the condition ordered by a healthcare professional. Your doctor or a 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.
If you are a carrier for an X-linked condition, 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.
For many couples, knowing their carrier status before their baby is conceived enables them to make informed reproductive decisions that can impact their child’s future. Couples at risk for having a baby with a recessive or an X-linked condition have the opportunity to consider:
- Natural pregnancy with or without prenatal diagnosis such as chorionic villus sampling (CVS) or amniocentesis
- Other reproductive options such as: preimplantation genetic testing (PGT) with in vitro fertilization (IVF) to test and then transfer embryos free of the disease that both partners carry, use of a sperm or egg donor, or adoption
Pregnant patients
If you are a carrier for a specific recessive condition, your partner may want to have carrier screening for the condition ordered by a healthcare professional. Your doctor or a local genetic counselor can help decide which carrier test is best for your partner.
If you are a carrier for an X-linked disease, partner screening is not often recommended as a next step as only the mother needs to be a carrier for the condition to pass it along to her child.
Carrier screening during pregnancy can have life-altering benefits: It can help couples decide on prenatal diagnostic testing such as chorionic villus sampling (CVS) or amniocentesis, and if necessary, help them prepare emotionally, medically, and financially for a baby with a genetic condition.