Sex linkage facts for kids

Sex linkage is a term in genetics. It describes traits or conditions that are passed down through genes found on the sex chromosomes.

In mammals, like humans, there are two main sex chromosomes: X and Y. Females usually have two X chromosomes (XX), and males usually have one X and one Y chromosome (XY). The X chromosome is much larger and carries many more genes than the Y chromosome.

This difference explains why some conditions, like color blindness and haemophilia, are more common in males. These conditions are caused by recessive alleles on the X chromosome.

If a female has one X chromosome with a recessive allele for a condition and another X chromosome with a normal allele, she usually won't show the condition. This is because the normal allele on her other X chromosome can make up for the faulty one. She is a "carrier" and can pass the gene on.

However, if a male has an X chromosome with a recessive allele for one of these conditions, he will develop the condition. This is because he only has one X chromosome, and his Y chromosome does not have a matching gene to cover for the faulty one on his X.

How Sex-Linked Traits Are Passed On

Sex-linked traits are passed from parents to children in a special way because they are on the X chromosome.

From Mother to Son

A mother passes one of her X chromosomes to each of her children. If a mother is a carrier for a sex-linked condition (meaning she has one normal X and one X with the recessive gene), there's a 50% chance that her son will inherit the X chromosome with the recessive gene. Since sons only get one X chromosome (from their mother), they will show the condition if they inherit the affected X.

From Father to Daughter

A father passes his X chromosome to all of his daughters. If a father has a sex-linked condition, all of his daughters will inherit his affected X chromosome. However, since daughters also get an X chromosome from their mother, they will usually only be carriers (not show the condition) unless their mother also passes on an affected X.

Examples of Sex-Linked Conditions

Many conditions are sex-linked, meaning they are connected to genes on the X chromosome.

Color Blindness

Color blindness is a common example. People with color blindness have trouble telling certain colors apart, often red and green. It's much more common in males because the genes for normal color vision are on the X chromosome.

Haemophilia

Haemophilia is another well-known sex-linked condition. It affects the blood's ability to clot. People with haemophilia can bleed for a long time, even from small cuts or bruises. Like color blindness, it's more common in males.

See also

In Spanish: Herencia ligada al sexo para niños