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Genes are inherited from our biological parents in specific ways. One of the basic patterns of inheritance of our genes is called X-linked inheritance.
X-linked inheritance means that the gene causing the trait or the disorder is located on the X chromosome. Remember, females have two X chromosomes, while males have one X and one Y. Genes on the X can be recessive or dominant, and their expression in females and males is not the same because the genes on the Y chromosome do not exactly pair up with the genes on the X. X-linked recessive genes are expressed in females only if there are two copies of the gene (one on each X chromosome). However, for males, there only needs to be one copy of an X-linked recessive gene in order for the trait or disorder to be expressed. For example, a woman can carry a recessive gene on one of the X chromosomes unknowingly, and pass it on to a son, who will express the trait:
Therefore, half of the daughters have the gene and can pass it to the next generation. The other half do not have the gene and, therefore, cannot pass it on. Half of the sons do not have the gene and cannot pass it on. The other half of the sons have inherited the gene and will express the trait or disorder.
Examples of X-linked recessive conditions include red-green color blindness and hemophilia A:
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