Sex linkage

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Sex linkage means a gene’s location on a sex chromosome (X or Y) changes how a trait is inherited and expressed in males and females.

The basics
- Humans have 23 chromosome pairs. The last pair are the sex chromosomes: females usually have two X’s (XX) and males have one X and one Y (XY).
- X-linked genes are on the X chromosome and can affect both sexes. Y-linked genes are on the Y chromosome and can only affect males.
- Females have two X chromosomes, so one X can be inactivated in each cell. This X-inactivation helps explain why some X-linked traits show different patterns in females.

Three main patterns of X- and Y-linked inheritance
- X-linked recessive
- Males are more often affected because they have only one X. Females are usually carriers.
- Examples: Duchenne muscular dystrophy, red-green color blindness, hemophilia A, G6PD deficiency, Wiskott-Aldrich syndrome.
- Sons get the trait from their mother; daughters can be carriers or affected if both parents pass the gene.
- X-linked dominant
- A single mutated copy on the X can cause the trait in both sexes.
- Fathers pass the condition to all daughters but to no sons; mothers pass it to about half of their children.
- Some conditions are more severe in males or are lethal in males, so affected females are more commonly seen.
- Examples: Rett syndrome (often lethal in affected males), Fragile X syndrome (a form of intellectual disability with strong effects in males and females).
- Y-linked
- Only affects males and is passed from father to son.
- Examples include certain rare traits and some aspects of male fertility. A father with a Y-linked trait passes it to all sons.

Common X- and Y-linked conditions (short examples)
- X-linked recessive: Duchenne muscular dystrophy, red-green color blindness, hemophilia A, G6PD deficiency.
- X-linked dominant: Fragile X syndrome, Rett syndrome.
- Y-linked: some rare male-specific traits and Y-linked infertility.

Sex chromosome differences and disorders
- Turner syndrome (XO) and other X-chromosome variations can cause developmental and health issues.
- Klinefelter syndrome (XXY) and related variants can affect male development and fertility.
- These conditions arise from having an abnormal number of sex chromosomes and are not inherited in the usual way.

In other species
- Some animals use different sex-determination systems (for example, birds use ZW, where females are ZW and males are ZZ).
- In fruit flies (Drosophila), sex is determined by the ratio of X chromosomes to autosomes, not just presence of a Y chromosome. This changes how dosage compensation works.

How scientists study and use this knowledge
- Carrier screening checks if people carry X-linked recessive mutations.
- Prenatal screening tests for certain X-linked or other conditions in a fetus.
- Newborn screening tests babies soon after birth to catch some genetic disorders early.

Key takeaway
- Sex-linked traits depend on which chromosome carries the gene and on the child’s sex. This is why some disorders are more common in males or females and why inheritance patterns can look very different from autosomal (non-sex chromosome) traits.