Sex linked inheritance

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 Sex-linked inheritance refers to the transmission of traits or disorders that are determined by genes located on an organism's sex chromosomes. In human beings, the sex chromosomes are X and Y. Females typically have two X chromosomes (XX), while males have one X and one Y chromosome (XY).


Here's a breakdown of sex-linked inheritance in humans:


### 1. X-Linked Inheritance


Genes located on the X chromosome are called X-linked genes. Since males have only one X chromosome, they are more frequently affected by X-linked disorders, particularly recessive ones.


* **X-Linked Recessive Inheritance:**

    * **Males (XY):** Males have only one X chromosome inherited from their mother. If this X chromosome carries a recessive gene for a disorder, the male will express the disorder because there is no second X chromosome to mask the recessive allele (they are hemizygous).

    * **Females (XX):** Females have two X chromosomes.

        * If a female inherits one X chromosome with the recessive gene and one normal X, she will typically be a **carrier** and not express the disorder herself. However, she can pass the gene to her children.

        * A female will only express an X-linked recessive disorder if she inherits the recessive gene on both of her X chromosomes (one from each parent), which is rare.

    * **Transmission Pattern:**

        * Affected fathers cannot pass X-linked traits to their sons (since sons receive the Y chromosome from the father).

        * Affected fathers will pass the X-linked gene to all of their daughters, making them carriers or, rarely, affected.

        * Affected mothers or carrier mothers have a 50% chance of passing the gene to each child. Sons who inherit the gene will be affected; daughters who inherit the gene will be carriers (or affected if they receive another affected X from their father).

    * **Examples:** Red-green color blindness, Hemophilia A and B, Duchenne muscular dystrophy, Fragile X syndrome.


* **X-Linked Dominant Inheritance:**

    * These are less common than X-linked recessive disorders.

    * **Males (XY):** If a male inherits a dominant X-linked gene, he will express the trait.

    * **Females (XX):** Females will also express the trait if they inherit just one copy of the dominant X-linked gene. They are generally less severely affected than males, or may have variable expressivity, due to having a second X chromosome and X-inactivation.

    * **Transmission Pattern:**

        * Affected fathers pass the trait to all of their daughters but none of their sons.

        * Affected mothers have a 50% chance of passing the trait to each child, regardless of sex.

    * **Examples:** Hypophosphatemic rickets (Vitamin D-resistant rickets), Rett syndrome (often lethal in males).


### 2. Y-Linked Inheritance


Genes located on the Y chromosome are called Y-linked genes.


* **Characteristics:**

    * Only males are affected, as only males possess a Y chromosome.

    * The trait is always passed directly from father to all of his sons.

    * Females are never affected and cannot transmit the trait.

    * There are very few functional genes on the Y chromosome, most of which are involved in male sex determination (e.g., SRY gene) or spermatogenesis.

    * **Examples:** Certain forms of male infertility, hypertrichosis pinnae auris (hairy ear rims) although this is disputed and rare.


In summary, X-linked inheritance shows distinct patterns in males and females due to the difference in their sex chromosome complement, with males often being more susceptible to X-linked recessive conditions. Y-linked inheritance is exclusively male-to-male transmission.

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