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Carrier Screening for Single-Gene Disorder

Published on May 27, 2022 and last reviewed on Aug 11, 2022   -  4 min read

Abstract

Carrier screening is a test done before or during pregnancy to check genetic disorders. This article explains carrier screening and its advantages.

What Is Carrier Screening?

Carrier screening is a test done to check whether the mother or the father carries a gene mutation that could cause severe disease in the baby. The carrier screening can be done before becoming pregnant or during pregnancy. Ideally, it is recommended to perform carrier screening before becoming pregnant, allowing time to consider the options and make decisions.

What Are Single-Gene Disorders?

Single-gene disorder is caused by changes in DNA in a particular gene. It often has a predictable inheritance pattern. More than 10,000 human disorders are caused by changing the structure of a gene, referred to as mutation in a single gene. The structurally changed gene responsible for the disorder is referred to as mutants. These disorders can be easily traced within the families due to the involvement of only a single gene. Hence, the risk of them occurring in later generations can be predicted. Single gene disorders are divided into different categories:

  • Dominant.

  • Recessive.

  • X-linked.

Dominant Disease:

They are single-gene disorders that occur in the heterozygous state (when an individual has one healthy version and one mutant version of the relevant gene). The effects of the mutant copy of the gene (allele) overrule the impact of the healthy copy of the gene. Thus, the mutant allele causes disease symptoms even if a healthy gene is present. Huntington's disease is an example of a dominant single gene disorder, which is a nervous system disease.

Recessive Disease:

Recessive disease is a single-gene disorder that occurs when the gene is in the homozygous state (when an individual carries two mutants of the relevant gene). The mutant allele does not show any disease symptoms when accompanied by a healthy allele. Hence, when a parent inherits two mutant alleles, there are no healthy alleles so that the mutant allele can exert its effect. When two closely related people have a child together (consanguinity), there is an increased risk of having a recessive disorder. This is because there are more chances that the same mutant gene will be present in related parents.

X linked Disorder:

X-linked disorders are single-gene disorders that result from the presence of a mutated gene on the X chromosome. These disorders are more seen in males because females (XX) have two copies of the X chromosome, but males (XY) only have one copy. Therefore, if a male's single copy on the X chromosome is mutated, he has no healthy copy to restore healthy function. The X-linked disorder is of two types:

  • X-linked Recessive Disorders: These are much more common in males than females because two copies of the mutant allele are required for the disease in females, while only one copy is required in males. Examples of X-linked recessive disorders include red-green color blindness and hemophilia.

  • X-linked Dominant Disorders: The frequency of disorder is similar and both males and females, X-linked dominant disorders are very uncommon. Examples include Rett syndrome and some inherited forms of rickets.

What Is A Carrier?

A carrier is a person who has disease characteristics but does not have any disease symptoms. A carrier has a mutation on the recessive gene. The dominant gene overrules the recessive gene. While a carrier does not develop the disease, a carrier can pass the mutant gene to their children.

Who Should Have Carrier Screening?

Carrier Screening helps a couple know if they are carriers of a recessive disease and are at risk for having a baby with that recessive condition. It is recommended for all couples, and it is highly recommended if,

  • Married to relatives.

  • History of multiple miscarriages.

  • Any of the partners has a genetic disorder.

  • Has a child who has a genetic disorder.

  • Family history of a genetic disorder.

  • Belonging to a particular ethnic race. Ethnicity has a high risk of certain carrier disorders.

What Are Common Single-Gene Disorders?

Spinal Muscular Atrophy:

Spinal muscular atrophy, also known as SMA, causes degeneration of spinal cord motor neurons leading to atrophy of skeletal muscle and overall weakness. The disorder is caused by a gene mutation known as the survival motor neuron gene (SMN1), which is responsible for producing a protein essential to motor neuron function.

Cystic Fibrosis :

Cystic fibrosis is the most common life-threatening, autosomal recessive condition causing significant pulmonary disease and pancreatic insufficiency. It is a progressive disease and affects multiple organs like the pulmonary, gastrointestinal, and pancreatic systems but does not affect intelligence.

Fragile X Syndrome :

It is a common inherited form of intellectual disability. Fragile X syndrome is intellectual disability or impairment from borderline, including learning disabilities, to severe cognitive and behavioral disabilities. It is a commonly known cause of autism or autism spectrum disorder behaviors with intellectual disability.

What Do the Results of Carrier Screening Mean?

  • If only a parent is a carrier - The child has a 25 % chance of being a carrier for that disorder and a zero percent chance of having the disease.

  • If both the partners are carriers of the same gene mutation - Then the child has a 25 % chance of inheriting the affected gene from each of the parents and is born with the disease or has a 50 % chance of being a carrier or has a 25 % chance of not having any disorder and not being a carrier.

  • If one parent is a carrier and the other is affected - The child has a 50 % chance of being affected or a 50 % chance of being a carrier. These risks remain the same for each pregnancy.

What Are the Options If the Couples Find Out, They Are the Carriers?

Suppose the tests show that both partners are carriers. In that case, there are several options that the couples can choose to become pregnant and have prenatal diagnostic testing to find out whether the developing baby has the disorder or not. The couples can choose in vitro fertilization by selecting another donor's sperm or eggs. Preimplantation genetic diagnosis, a special test of the embryo, is performed before it is implanted. Some couples choose to adopt a child or not have children. Early screening has many advantages; knowing the baby is affected before birth makes it more helpful for early treatment by choosing the right medical specialists, who can be on hand to start treating the baby right after delivery.

Conclusion:

A carrier screening test is done on a person without signs or symptoms to determine whether they carry a gene for a genetic disorder. It is recommended that every couple goes through this test before or during pregnancy to avoid passing genetic diseases to their offspring. If you are planning for pregnancy or are pregnant, it is high-time you get genetic counselling. Seek help online!

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Last reviewed at:
11 Aug 2022  -  4 min read

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