What is X-linked inheritance ?

CEN4GEN inheritance patterns

Carrier screening was developed to help people learn about their risks of having children with certain genetic conditions. Most carrier screens focus on conditions that are inherited in an autosomal recessive fashion, but now will be the focus on X-linked inheritance.

How do X-linked conditions work ?
The rod-like structures in our cells that contain our genes and DNA are called chromosomes. Each cell has 23 pairs for a total of 46 chromosomes that contain thousands of genes that are also in pairs. One chromosome from each pair (and therefore all of the genes that are on that chromosome) comes from the mother, while the other chromosome comes from the father. In turn, each person passes on one copy of each of his/her pairs of chromosomes to his/her children, and so on. The first 22 chromosome pairs are the same in females and males, and they are called the autosomes. The last pair of chromosomes is called the sex chromosomes, and these chromosomes are different in men and women. Women have two X chromosomes, and men have one X chromosome and one Y chromosome. X-linked conditions get their name because the genes involved are only present on the X chromosome.

 
CEN4GEN X-linked inheritance

If a woman has a DNA change in a copy of a gene on one of her X chromosomes but the second copy of the gene is working, she will usually not show any symptoms of the condition because the working copy will balance out the copy that is not working. She would be called a carrier. A woman would need to inherit a non-working copy from each parent in order to be affected with the condition (which is rare). However, if a man’s one X chromosome has a non-working gene copy, he will be affected since he does not have the second working X copy to compensate. For this reason, women are the unaffected carriers for X-linked conditions, but men are usually the ones affected. A carrier female would be at risk for having carrier daughters and/or affected sons.

What does a negative result mean ?
Because no genetic test can look at every possible DNA change in a gene, the risk for being a carrier is never zero, but a negative test will significantly reduce your chance of being a carrier. CEN4GEN should be able to tell you how much lower your risk is with the negative result.

In most cases, there is no need to repeat this test in the future as our DNA does not change over time. However, not all carrier screens are the same. The number of conditions and the types of DNA changes included on each screen will differ between laboratories and even over time as more is learned about genetics. If your partner is a carrier of a genetic condition or if your medical history or that of your family members’ changes, you will want to discuss this with your healthcare provider in order to make sure you have had the most appropriate carrier screen to assess your risks given this new information.

What does a positive result mean ?
If you test positive for being a carrier of an X-linked condition, this does not usually mean that you are at risk for developing symptoms of the condition. It is recommended, however, that you talk with your healthcare provider to discuss the condition and what it means to be a carrier, as there are rare occurrences when female carriers may show mild symptoms.

Being a carrier of an X-linked condition means you are at an increased risk for having a child with that condition. There is a 50% (1 in 2) chance with each male pregnancy that he will inherit a non-working gene copy and be affected. There is also a 50% chance with each female pregnancy that she will inherit a non-working gene copy and likely be an unaffected carrier like the mother.

What else should I consider ?
If you are currently pregnant, there are tests available (called chorionic villus sampling and amniocentesis) to find out the gender of the child you are carrying and whether he/she has inherited a non-working gene copy.

You may also wish to discuss these results with other family members as they may also be at increased risk for being a carrier. It is helpful to share a copy of your test results with your family members so they can bring them to their own doctor to determine if carrier testing is right for them.