Genetic Conditions

Learning about X-linked inheritance

Genes are the instructions that tell your body how to develop and function. They are like a recipe for making cookies or the blueprints for a house. Genes are packaged into structures called chromosomes. You have 2 copies of most chromosomes. One copy comes from the egg and one from the sperm that formed you.

One set of chromosomes is called the sex chromosomes. Sex is a category that people are assigned at birth based on the appearance of their external genitals. This most often includes female, male, or X. Usually, individuals assigned female at birth have two X chromosomes, and individuals assigned male at birth have one X chromosome and one Y chromosome.

Genetic variants

We all have differences in our genetic instructions that make us different from each other. We call these variants. Many of these differences do not cause any health problems. These are called benign variants. But sometimes differences in our genetic instructions can cause health problems. These differences are called pathogenic variants or disease-causing variants.

X-linked inheritance

X-linked inheritance refers to one way that a genetic condition can happen and how it can be passed down from parent to child. The term X-linked means that the gene with a pathogenic variant involved in the condition is located on the X chromosome.

X-linked conditions

There are many different X-linked conditions that can affect our health and development. X-linked conditions can look different in males and females because they have different numbers of X chromosomes. Because males usually have one X chromosome and one Y chromosome, having a pathogenic variant in an X chromosome gene is usually more significant than it is for females, who usually have two X chromosomes.

In some X-linked conditions, both males and females will develop the condition, although the condition will generally be more severe in males. For other X-linked conditions, only males will develop the condition, while females will have no symptoms. For these conditions, females are called carriers of the X-linked condition. Female carriers can pass the pathogenic variant down to their male and female children.

It is important to talk to your healthcare team about a specific X-linked condition and how it can affect you or your children differently based on sex.

X-linked conditions can affect people at different stages of their life. Depending on the condition, signs may be present during pregnancy or at birth, in childhood, in adolescence, or in adulthood.

Genetic testing for X-linked conditions may or may not be available. In general, genetic testing may be considered for people with signs and symptoms of the condition or for people with a family history due to a known pathogenic variant.

How X-linked conditions are passed down from parent to child

In each pregnancy, the child gets one sex chromosome from each of their biological parents’ pairs of sex chromosomes. The child will get an X chromosome from the egg and an X or Y chromosome from the sperm. Male children will get the Y chromosome from the sperm and female children will get the X chromosome from the sperm.

If the female parent is a carrier (or affected by the condition) and the male parent does not have the condition

A female with two X chromosomes who is a carrier of an X-linked condition has one X chromosome gene with a pathogenic variant and one X chromosome gene without a pathogenic variant.

This means that in each pregnancy where a female parent is a carrier of the condition (or is affected by the condition) and a male parent is not affected by the condition, there is a:

• 1 in 4 chance (or 25% chance) that the child will have two X chromosomes (female) and does not inherit the pathogenic variant. They would not be at risk of having this condition and would not be a carrier of the condition.
• 1 in 4 chance (or 25% chance) that the child will have two X chromosomes (female) and inherits the pathogenic variant. Depending on the condition, they may be a carrier of the condition or they may have or develop the condition.
• 1 in 4 chance (or 25% chance) that the child will have an X and Y chromosome (male) and does not inherit the pathogenic variant. They would not be at risk of having this condition.
• 1 in 4 chance (or 25% chance) that the child will have an X and Y chromosome (male) and inherits the pathogenic variant. Depending on the condition, they may have or develop the X-linked condition.

If the male parent has the X-linked condition and the female parent is not a carrier (or not affected by the condition)

A male with an X and Y chromosome who is affected with the X-linked condition has one X chromosome with a pathogenic variant and one Y chromosome.

This means that in each pregnancy where a male is affected with the condition and a female is not a carrier (or is not affected by the condition):

• All children with two X chromosomes (female) will inherit the pathogenic variant. Depending on the condition, they may be carriers of the condition or they may have or develop the condition.
• No children with an X and Y chromosome (male) will inherit the pathogenic variant. They would not be at risk of having this condition.

Parents cannot control what genes are passed on to their children

Having a genetic condition, or having a child with a genetic condition, is not caused by anything a person or parent did or did not do. The chance of inheriting an X-linked condition is also not impacted by:

• anything a parent did or did not do before or during pregnancy
• the outcome of a previous pregnancy (all pregnancies between the same 2 people will have the same chances of a child inheriting or not inheriting the X-linked condition)

Genetic counselling

If you have any questions about X-linked conditions or inheritance, talk with your healthcare team.

Genetic counselling is available for individuals who have a genetic condition or who may be at risk of a genetic condition. A genetic counsellor can review your family history, genetic testing options, and screening options, and they can help families and individuals adjust to their diagnosis. Ask your healthcare provider if a referral for genetic counselling may be right for you.

To see this information online and learn more, visit MyHealth.Alberta.ca/health/aftercareinformation/pages/conditions.aspx?hwid=custom.ab_genetics_xlinked_inherit_inst.

For 24/7 nurse advice and general health information call Health Link at 811.

Current as of: November 25, 2024

Author: Clinical and Metabolic Genetics Program, Alberta Health Services

This material is not a substitute for the advice of a qualified health professional. This material is intended for general information only and is provided on an "as is", "where is" basis. Although reasonable efforts were made to confirm the accuracy of the information, Alberta Health Services does not make any representation or warranty, express, implied or statutory, as to the accuracy, reliability, completeness, applicability or fitness for a particular purpose of such information. Alberta Health Services expressly disclaims all liability for the use of these materials, and for any claims, actions, demands or suits arising from such use.