Skip to 0 minutes and 13 secondsYou 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 mutational deletion affecting one or more genes on the X chromosome. Because a woman has two X chromosomes, even if there was a gene mutation on 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 50-50 expression.

Skip to 0 minutes and 47 secondsHowever, if there's skewing towards the X chromosome with the gene mutation, it is possible that a female may show features of the associated conditional 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. In 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 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 the normal copy of the gene, she will not be a carrier.

Skip to 1 minute and 25 secondsHowever, 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 one-in-two 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 one-in-two chance of either inheriting the X chromosome with the normal gene and a one-in-two chance for inheriting the mutated copy of the gene and developing the X-linked condition.

Skip to 1 minute and 52 secondsWhere 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-linked 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'll 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.

X-linked Recessive Inheritance

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

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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The Genomics Era: the Future of Genetics in Medicine

St George's, University of London

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