Sex-linked Inheritance: Characteristics, Disorders, and Examples Explained
MahrukhOctober 15, 2025
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The information provided is intended solely for academic, study, and educational purposes. Some of the terms and keywords mentioned may be considered offensive or sensitive, but they are standard genetic terminology used strictly for educational and scientific understanding.
In Mendel’s experiments, gender (sex) had no effect on the inheritance patterns observed in his crosses.
However, Mendel’s laws do not apply to genes that are exclusively located on either the X or Y chromosome.
Sex-linked inheritance refers to the inheritance of a trait (phenotype) that is determined by a gene located on one of the sex chromosomes.
Genes found only on the X chromosome or on the analogous Z chromosome (as seen in birds and some other species) are referred to as X-linked or Z-linked genes.
Genes that occur solely on the Y chromosome are known as holandric genes.
The inheritance pattern of these X-linked, Z-linked, or holandric genes is collectively termed sex-linked inheritance.
Characteristics of Sex-linked Inheritance
Genes that are located exclusively on the X chromosome are present twice in females (since females have two X chromosomes) and only once in males (as males possess one X and one Y chromosome).
The differential region of each sex chromosome, particularly the X chromosome, contains genes that do not have corresponding alleles on the other type of sex chromosome.
These genes, whether dominant or recessive, are expressed in the male phenotype, since there is no corresponding allele on the Y chromosome to mask their effect.
In males, such genes located in the differential region of the X chromosome are termed hemizygous (“half zygous”), as they possess only one copy of the gene instead of a pair.
Inheritance of X-Linked Recessive Genes
X-linked recessive genes display a criss-cross pattern of inheritance, where the trait passes from a male parent (P₁) to his grandsons (F₂ males) through heterozygous carrier daughters (F₁ females).
In criss-cross inheritance, the X-linked recessive gene is transmitted from the father (P₁) to his daughters (F₁ carriers) and then to his grandsons (F₂ males). Reciprocal crosses produce different F₁ and F₂ results (ratios).
The X-linked recessive phenotype is more frequently observed in males than in females.
An affected female arises only when both parents carry the X-linked recessive allele (e.g., Xá´¬Xᵃ × XᵃY), while an affected male results when only the mother carries the gene.
Offspring of an affected male are generally not affected; however, all his daughters become carriers (heterozygous for the recessive allele).
These carrier daughters can pass the gene to half of their sons, making 50% of grandsons affected.
Sons of an affected male do not inherit the X-linked recessive gene, as males receive the Y chromosome from their fathers; hence, they are unaffected and do not transmit the gene to their offspring.
Example in Drosophila:
The gene for white eye color is X-linked and recessive to the red-eye dominant gene.
When a white-eyed male is crossed with a red-eyed female, all F₁ offspring are red-eyed.
In the F₂ generation, the ratio of red- to white-eyed flies is 3:1, but all white-eyed flies are males.
When an F₁ red-eyed female is crossed with a normal red-eyed male, 50% of the males are white-eyed and 50% are red-eyed, showing that the recessive allele expresses in males only.
Common human X-linked recessive disorders include Color-blindness and Haemophilia, both of which are more prevalent in males.
Disorders in Human
Colour-blindness
It is a genetic defect in which a person cannot distinguish between certain colors, most commonly red and green, or in some cases, both.
The condition arises due to mutations in X-linked genes that are responsible for producing color-detecting pigments in the retina.
Since the genes are located on the X chromosome, males are more frequently affected, while females are usually carriers.
Haemophilia (Bleeder’s Disease)
Haemophilia is often referred to as the “royal disease” because it was historically present in several members of European royal families.
It is considered one of the most serious inherited disorders, where the blood fails to clot normally, leading to prolonged bleeding even from minor injuries.
The disorder results from a deficiency of specific blood-clotting proteins, mainly clotting factor VIII (Haemophilia A) and clotting factor IX (Haemophilia B).
As it is an X-linked recessive disorder, males are predominantly affected, while females act as carriers, transmitting the defective gene to their offspring.
Inheritance of X-Linked Dominant Genes
Dominant X-linked genes are more frequently found in females than in males, as females possess two X chromosomes, increasing the likelihood of carrying and expressing the dominant allele.
Affected males transmit the X-linked dominant trait to all of their daughters (since daughters inherit their father’s X chromosome) but to none of their sons (as sons inherit the Y chromosome from the father).
Affected females usually transmit the condition (defective phenotype) to half of their sons and half of their daughters, depending on which X chromosome (normal or mutant) is passed on.
An X-linked dominant gene is not transmitted to any son from a mother who does not exhibit the trait, since the mother must carry and express the gene for it to be inherited.
Human Disorders:
X-linked dominant disorders are relatively rare in humans.
One notable example is hypophosphatemia (vitamin D-resistant rickets), a condition in which the body cannot properly absorb phosphate, leading to bone weakness and deformities.
Another example is hereditary enamel hypoplasia (hypoplastic amelogenesis imperfecta), where the tooth enamel is abnormally thin, causing small, weak teeth that wear down to the gums.
Inheritance of Y-Linked Genes
Y-linked genes are located in the non-homologous region of the Y chromosome, which is present only in males.
These genes are transmitted directly from father to son, with no female involvement, as females lack a Y chromosome.
In humans, Y-linked (holandric) inheritance ensures that the trait passes strictly through the male line, meaning that all sons of an affected father will inherit the gene, while none of the daughters will receive it.
Examples of Y-linked genes and traits include:
Ichthyosis hystrix gravis hypertrichosis: a condition characterized by excessive hair growth on the pinna (outer ear).
Genes responsible for H-Y antigen and histocompatibility antigen.
Genes involved in spermatogenesis (formation of sperm cells).
Genes influencing height (stature) and slower maturation rate in individuals.
References
Verma, P. S., & Agrawal, V. K. (2006). Cell Biology, Genetics, Molecular Biology, Evolution & Ecology (1st ed.). S. Chand & Company Ltd.
Gardner, E. J., Simmons, M. J., & Snustad, D. P. (1991). Principles of Genetics. New York: John Wiley & Sons.
University of Camerino. (n.d.). Sex-Linked Inheritance. Retrieved from http://docenti.unicam.it/tmp/229.pdf
Kullabs. (n.d.). Sex-Linked Inheritance – Notes and Study Material. Retrieved from https://www.kullabs.com/classes/subjects/units/lessons/notes/note-detail/8353
Palero, J. M. (n.d.). Sex-Linked Inheritance. Retrieved from https://www.slideshare.net/jmpalero/sex-linked-inheritance-65260691
Your Article Library. (n.d.). Useful Notes on Sex-Linked Inheritance (Biology). Retrieved from http://www.yourarticlelibrary.com/notes/useful-notes-on-sex-linked-inheritance-biology-853-words/6327
Notes for Pakistan. (2009, October). Sex-Linked Inheritance – Short Note. Retrieved from http://notesforpakistan.blogspot.com/2009/10/sex-linked-inheritance-short-note.html
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