The mechanism of sex-linked inheritance has always been a puzzle before genetics. The chromosomal mechanism of sex inheritance can be traced back to some of the experiments carried out in insects. A human female has 23 pairs of chromosomes, and a human male has 22 similar pairs and one pair consisting of two chromosomes that are dissimilar in size and structure. The 23rd pair in both the sexes is called sex chromosomes.

The inheritance of a trait determined by a gene located on one of the sex chromosomes is called sex-linked inheritance.

What is sex-linked inheritance?

Sex-linked inheritance is the transmission of characters and their determining genes and sex-determining genes on the sex chromosomes, therefore, are inherited together from one generation to the next.

Most of the sex-linked genes are present on the X chromosome, resulting in X-linkage formation. A gene that occurs on the Y chromosome forms  Y-linkage. The Y-linked traits are transmitted only through the male. Females are usually carriers of X-linked diseases. As they are ‘X’ linked, fathers never transfer hemophilia or color blindness to their sons. Examples of sex-linked human

diseases are hemophilia and color blindness. Besides sex-linked inheritance, sex-limited genes and sex-influenced traits have also been observed.

Sex-limited traits: Traits controlled by autosomal genes, whose expression is limited to one of the sexes because of sex hormones, e.g., secondary sex characters, milk production in mammalian females, egg production in chicken, premature baldness in human males.

Sex-influenced traits: Traits controlled by autosomal genes, which are expressed as dominant in one sex and as recessive in the other because of sex hormones, e.g., white forelock, harelip, gouts, certain types of pattern baldness are all dominant in men, spina bifida is dominant in women.

Some examples of sex-linked inheritance are hemophilia, red-green color blindness, congenital night blindness, some high blood pressure genes, and fragile X syndrome.

Types of sex-linked Inheritance 

Sex-linked inheritance is of two types.

1. X-linked inheritance 

When certain sex-linked genes are located only on X-linked chromosomes, their alleles are absent from Y-chromosome. 

Dominant X-linked inheritance:

Whey Single x-chromosome has the gene of a character that expresses itself in the absence of another alternate gene known as dominant X-linked inheritance. Example-Rett syndrome.

Recessive  X-linked inheritance:

Sometimes only one x-chromosome bears the gene of characters, and characters do not express themselves in the absence of another alternate gene. But when both chromosomes bear the gene of a character, then the character expresses itself; such a condition is called recessive X-linked inheritance. Example-Hemophilia.

2. Y-linked inheritance 

When Y-chromosome bears the gene of a character and expresses itself only in males it is known as Y-linked inheritance. Example-hypertrichosis of the ears

Characteristic of sex-linked Inheritance 

•  As we all know that sex-linked inheritance is a criss-cross inheritance in which the father does not pass the sex-linked allele of a trait to his son. The same is passed to the daughter, from where it reaches the grandson, which is digenic inheritance.

• In sex-linked inheritance, the mother who is in a homozygous state passes the alleles of a sex-linked trait to both sons and daughters.

• The majority of the sex-linked traits are present on the X chromosome, which is a recessive gene. Hence we conclude that the majority of sex-linked traits are recessive.

•  In males, sex-linked traits show more continually in comparison to that of females.

• Males suffer more from sex-linked disorders because they are heterozygous, and traits frequently show in a heterozygous state.

• As we all know, recessive genes can only express themselves when they are in a homozygous state, and female contains homozygous gene and generally function as a carrier of sex-linked disorders.
• Sex-linked non- criss-cross inheritance is holandric (if it passes directly from father to son) and Hologic (if it passes directly from mother to daughter).

Disorders caused by sex-linked Inheritance 

 Haemophilia

• It is a sex-linked (recessive) disease known as bleeder’s disease. It is marked in heterozygous condition.
• A female becomes hemophilia only when both its chromosomes carry the gene (XhXh).

• As it is a recessive character, a lady may carry the disease and would transmit the disease to 50% of her sons, even if the father is normal. It can indicate the origin of hemophilia traits present in the ancestors. These types of traits may cause harm to the marriage taking place between close relatives.

• Hemophilia A is characterized by antihemophilic globulin (factor – VIII). About 4/5th of the hemophiliac cases are of this type.

• Hemophilia B is also called ‘Christmas disease’. It results from a defect in the plasma thromboplastin component (factor IX).

• Hemophilia disease is also known as a royal disease because this disease spread through the children of Queen Victoria to the royal families living in Europe. 

Color blindness

Color blindness is a recessive sex-linked trait in which the eye fails to distinguish red and green color. The gene for normal vision is dominant. The normal gene and its recessive allele are carried by the X (XCXC) chromosome, and therefore men are more likely to show the defect, although women may be carriers.

Conclusion 

Some genes that are one the same chromosome are inherited together and called linked genes, and when these linked genes are present on the sex chromosome, these genes are known as sex-linked genes, and trait transfer by these genes are known as sex-linked traits. Certain traits which are inherited by the organisms are linked to biological sex. Males are more likely to show sex-linked traits, and they cannot be carriers of sex-linked traits because they have only one X chromosome. A carrier has the recessive genes in its genotype, but this is not expressed in the phenotype. In sex-linked inheritance male to the male transfer of sex-linked traits is not observed. Homozygous men and women become bald. The trait is therefore dominant in men and recessive in females.