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X-linked recessive inheritance

In this video, we will learn about x-linked recessive inheritance, where affected individuals has a mutation in one or more genes on the X chromosome.
You will remember from previous tutorials that women have two X chromosomes and men have an X and a Y chromosome. X-linked recessive inheritance occurs when there is a mutation or deletion affecting one or more genes on the X chromosome. Because a woman has two X chromosomes, even if there is a gene mutation in one copy, the other, normal copy, means that she is not usually severely affected with an X-linked condition. However, the situation is slightly complicated by X inactivation. Normally in cells, only one of the two X chromosomes is expressed. This is usually a random process with a 50-50 expression.
However, if they’re skewing towards the X chromosome with the gene mutation, it is possible that a female may show features of the associated condition or disease, although not usually as severely as in the males. As males only have one X chromosome, unlike the females, they will usually manifest the features of an X-linked condition. An X-linked inheritance where the mother is a carrier and the father does not have an X linked condition, there are four possible outcomes, each with a 25% or a one in four chance. If the child inherits her father’s X chromosome, she will be a girl.
If she also inherits from her mother the X chromosome with a normal copy of the gene, she will not be a carrier. However, if she inherits the X chromosome with the mutated gene, she will be a carrier, just like her mother. Therefore, the daughter of a female carrier has a 1-in-2 chance of either being a carrier or not. If the child inherits the father’s Y chromosome, he will be a boy.
He also has a 1-in-2 chance of either inheriting the X chromosome with the normal gene and a 1 in 2 chance of inheriting the mutated copy of the gene and developing the X-linked condition Where a father has an X-linked condition, remembering that he will pass on his X chromosome to his daughters and his Y chromosome to his sons, all his daughters will be carriers for the X-linked condition, and none of his sons will be affected. It is therefore not surprising that one of the defining features of an X-lined recessive pedigree is the lack of male to male transmission. In addition, affected individuals are usually male and related through unaffected females.
In the next step, you will be introduced to an X-linked recessive condition. Duchenne muscular dystrophy. Duchenne muscular dystrophy is a devastating condition that causes progressive muscular weakness, and is usually diagnosed in early childhood, when motor milestones are delayed.

In this video, we learnt about X-linked recessive inheritance.

X-linked recessive conditions are caused by a gene alteration on the X chromosome. As males have only one X chromosome, if they have a gene alteration on their X chromosome they will develop the condition.

Females rarely show signs of X-linked recessive conditions as they usually have a second unaltered copy of the gene on their other X chromosome to compensate for an altered gene.

A female who has a gene alteration on one of her X chromosomes is said to be a carrier for the X-linked recessive condition.

Features of X-linked recessive inheritance include:

  • Males are affected almost exclusively.
  • The gene mutation can be transmitted from female carriers to sons.
  • Affected males cannot transmit the condition to their sons.

A pedigree depicting x-linked recessive inheritance An example family pedigree depicting X-linked recessive inheritance Click to expand
© St George’s, University of London

In the next step you will be introduced to an X-linked recessive condition, Duchenne Muscular Dystrophy, caused by a mutation/deletion affecting the dystrophin gene.

Image ©“Nick van Steendelaar” by Partij van de Arbeid. Licensed by CC BY 2.0

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