Sex-Link Characteristics in Humans - SS3 Biology Lesson Note

Sex-linked characteristics, also known as sex-linked traits or X-linked traits, are genetic traits that are controlled by genes located on the sex chromosomes, primarily the X chromosome. In humans, sex-linked characteristics are more commonly observed in males due to their unique sex chromosome composition (XY), as compared to females (XX).

Since males have only one X chromosome, any genetic variation or mutation on that chromosome will be expressed, regardless of whether the allele is dominant or recessive. In contrast, females have two X chromosomes, so they have a second copy that can potentially mask or compensate for the effects of certain genetic variations.

Key features of sex-linked characteristics in humans include:

1.       X-Linked Dominant: Inheritance of an X-linked dominant trait requires only one copy of the mutated allele to be present on the X chromosome for the trait to be expressed. Both males and females can exhibit the trait, but it is more commonly observed in females due to the presence of two X chromosomes.

2.       X-Linked Recessive: Inheritance of an X-linked recessive trait requires the presence of two copies of the mutated allele on the X chromosome, as females have two X chromosomes. Males, on the other hand, have only one X chromosome, so they are more likely to express X-linked recessive traits if they inherit the mutated allele.

Examples of sex-linked characteristics in humans include:

1.       Haemophilia: Haemophilia is an X-linked recessive disorder that affects the blood's ability to clot properly. It is more commonly observed in males, as they inherit a single X chromosome from their mother, which, if carrying the mutated allele, will result in the expression of the disorder.

2.       Colour blindness: Various types of colour blindness, such as red-green colour blindness, are also X-linked recessive traits. Males are more frequently affected by colour blindness because they have a single X chromosome carrying the mutated allele.

3.       Duchenne muscular dystrophy: Duchenne muscular dystrophy is an X-linked recessive disorder that primarily affects males. It is characterized by progressive muscle degeneration and weakness.

It's important to note that while sex-linked characteristics are more commonly observed in males, females can also be carriers of these traits and pass them on to their offspring. However, due to the presence of a second X chromosome in females, the expression of the trait is often milder or masked altogether.

(bio.libretexts.com)