Sex-linked traits are special features that are decided by genes on the sex chromosomes, mostly the X chromosome. In humans, girls have two X chromosomes (XX), while boys have one X and one Y chromosome (XY). This difference makes how traits are passed down from parents to kids quite different.

Inheritance Patterns

1. X-Linked Recessive Traits:

   • These traits are more common in boys because they only have one X chromosome.
   • For example, hemophilia happens in about 1 out of 5,000 boys.

2. X-Linked Dominant Traits:

   • Both boys and girls can be affected, but how serious it is can vary.
   • A good example is fragile X syndrome, which is the most common inherited cause of learning disabilities. It happens in about 1 in 4,000 boys and 1 in 8,000 girls.

3. Y-Linked Traits:

   • These traits are passed down only from father to son.
   • For instance, Y chromosome infertility occurs in about 1 out of 1,000 boys.

Knowing these patterns helps us understand how traits linked to sex are passed down in families.