If you've registered for a carrier screening through the Sarnoff Center, you may have noticed that our panel screens women, but not men, for X-linked genetic disorders. Understanding this distinction can help clarify how our screening panel works and what your results mean for your family.
A Refresher on Chromosomes
Chromosomes are structures in the cell that carry DNA. Humans typically have 23 pairs of chromosomes, including one pair of sex chromosomes. Every person inherits one sex chromosome from each biological parent, forming a pair. Females inherit two X chromosomes (XX), and males inherit one X chromosome and one Y chromosome (XY). X-linked genetic disorders are caused by mutated genes specific to the X chromosome.
Being a Carrier for an X-Linked Disorder
For the non-X-linked, autosomal conditions on our panel, being a carrier means you possess one mutated copy of a gene and one non-mutated copy of a gene. The working, non-mutated copy masks the non-working, mutated copy, allowing carriers to pass down the mutated copy of the gene to their children without being affected by the disorder themselves.
With X-linked disorders, only the X chromosome contains a copy of a gene. Females, being XX, will possess two copies of a gene, while males, being XY, will possess only one copy of a gene. Men can't be carriers for an X-linked disorder because they only have one X chromosome, and any mutation on this X chromosome won't be masked by another, non-mutated copy.
Screening is redundant for men because they either have the X-linked disorder or don't. The carrier status doesn't apply to them like it does for women, who can carry a mutated gene copy on one X chromosome and a non-mutated gene copy on the other.
What This Means for Your Family
When a woman is identified as a carrier for an X-linked disorder, there is a 50% chance her son will inherit the mutated gene and develop the disorder, as sons inherit their X chromosome from their mother. There is also a 50% chance her daughter will inherit the mutated gene and become a carrier like her mother.
Fathers pass down their Y chromosome to their sons and their X chromosome to their daughters. Fathers are unable to pass an X-linked disorder to their sons, but will pass their X chromosome, and any mutation on it, to all of their daughters.
The Sarnoff Center screens women and not men for these conditions, as it is their carrier status that largely determines the health of future children.
Have Questions?
Genetics can feel complicated, and we're here to help. If you'd like to learn more about X-linked conditions, our genetic counselor is available to answer your questions. Contact us at (312) 357-4718 or [email protected].
