Sex Linked Inheritance

​Sex Linked Inheritance refers to the inheritance of traits or characters determined by the genes on the sex chromosomes. Genes on the X and Y chromosome are called sex-linked genes. In humans, a female carries the XX chromosome (homogametic), while a male carries the XY chromosome (heterogametic). Genes present on the X chromosome form X-linkage formation, and those present on the Y chromosome form Y-linkage. Y-linked traits are transferred only through males, and females are carriers of X-linked diseases. ​

The concept of sex lsex-linkednked inheritance was discovered by Thomas H. Morgan in 1910. Sex-linked genes follow crisscross inheritance. Criss-cross inheritance is the inheritance of a character from the father to his grandson through his daughter.

Types of Sex-Linked Inheritance

Sex-linked inheritance is of two types:

1. X linked inheritance.
2. Y linked inheritance

X linked inheritance

When genes are located on the X chromosome, it is known as X-linked inheritance. It can be either dominant or recessive.

X linked dominant inheritance

A gene on the X chromosome is dominant, so the trait is expressed in both males and females. Example: Rett syndrome

X-Linked Recessive Inheritance​

For X-linked recessive traits or disorders, males are more frequently affected because they have only one copy of the X chromosome. In females, to express the trait or disorder, two copies of the genes are required. However, females carrying a single copy of the genes are carriers, and they pass it on to their offspring. Example: Haemophilia a and Haemophilia b.

Y linked inheritance

When genes are located on the Y chromosome, it is known as Y-linked inheritance. Since the Y chromosome is passed from father to son, Y-linked traits are expressed only in males. Y-linked inheritance is relatively rare compared to X-linked inheritance. Example: Hypertrichosis of the ears, webbed toes, and porcupine man.

Characteristics of Sex-Linked Inheritance.

• Sex-linked inheritance refers to the gene located on the X chromosome, as the Y chromosome is smaller and has fewer genes.
• Males are more affected by the X-linked disorder, as they have a single copy of the X chromosome.
• ​Females act as a carrier of sex-linked disorders. It passes the trait or disorder to both son and daughter.
• ​Sex-linked inheritance is crisscross inheritance. It is an example of non-Mendelian inheritance.

Sex-linked Disorders

What is Haemophilia?

Haemophilia is X linked recessive inherited genetic disorder characterized by an inability of the blood to form a clot. It is also known as bleeder’s disease. There are two types of haemophilia:

• Haemop​hilia a: Also known as classical haemophilia. It is caused by a mutation in the factor VIII gene on the X chromosome.
• Haemophilia b: Also known as Christmas disease. It is caused by a mutation in the factor IX on the X chromosome.

Signs and symptoms of haemophilia include uncontrolled bleeding, joint pain, and swelling. As it is x linked recessive disorder, it is more commonly seen in males. Females act as a carrier of the disease. She will transmit the disease to both the son and the daughter.

What is Colour Blindness?

Colour blindness is x linked recessive genetic disorder, caused due to a defect in either the red or green cone of an eye, where the individual is unable to distinguish between red, blue, and green colour. It is also called Daltonism.

​Symptoms include difficulty to differentiating between colour shades and sensitivity toward a bright light. It is of two types, monochromacy (complete colour blindness) and dichromacy(​partial colour blindness). Diagnosis of colour blindness is made by the ​Ishihara plate test. Males are more affected than females as they have only one copy of the chromosome. The son of a female who is a carrier of the disease will have a 50% chance of being colour-blind. The daughter will be normal unless the mother is a carrier and the father is colour-blind.