WHAT IS SEX-LINKED INHERITANCE? (WITH EXAMPLES) - BIOLOGY - 2025

The heritage linked to sex is defined as determined by sex chromosomes. That is, the existence and possession of sex chromosomes determine the inheritance pattern of the genes they carry, as well as their manifestation.

It should not be confused with the manifestation of the characters influenced by biological sex. There, a series of factors determine how certain genes manifest differentially depending on the sex of the individual.

Sexual dimorphism. Taken from pixabay.com

Sex-linked inheritance is also not inherited from sex in living organisms where biological sex is determined by specific chromosomes. That is, by sex chromosomes.

Is biological sex inherited?

Sex is not inherited: it is determined in post-fertilization events depending on the chromosomal constitution of the individual. For example, females of the human species are chromosomally XX.

However, there is no gene on the X chromosome for "being female." In fact, males of the same species are XY. What Y carries is a testicular development factor that obviously makes a big difference.

Genetically, we could suggest that all human beings are women. But some are so "peculiar" or "different" because they develop testicles.

In other organisms, for example many plants, sex is genetically determined. That is, by one or a few genes on autosomal chromosomes.

Therefore, since a whole set of genes linked on the same sex chromosome is not involved, there is no sex-linked inheritance. At best, there could be sex-linked inheritance for some genes closely linked to sex-determination genes.

Sex determination

The word sex derives from the Latin sexus, which means section or separation. That is, the biological distinction between males and females of a species.

Sex determination can be chromosomal: that is, dictated by the presence of sex chromosomes, generally heteromorphic. Sex determination can also be genetic: that is, dictated by the expression of few specific genes.

In other cases, in haplodiploid sex determination, haploid and diploid individuals represent different sexes of the same species.

Finally, in some species, certain environmental conditions during embryonic development determine the sex of the individual. This is the so-called environmental sex determination.

Chromosomal sex

Sex-linked inheritance is a genetic phenomenon that is only observed in organisms with a chromosomal sex determination system. In these cases, there is a pair of chromosomes, generally non-homologous, that determine the sex of the individual.

In this way, they also determine the type of gametes that each one produces. One of the sexes is homogeneous because it only produces one type of gametes.

Female mammals (XX), for example, only produce X gametes. The other sex, which produces both types of gametes X and Y, is the heterogametic sex. In the case of male mammals, they are chromosomally XY.

Gamete production

The most important characteristic associated with sex is the production of specific gametes: ovules in females, and sperm in males.

In flowering plants (angiosperms) it is common to find individuals that produce both types of gametes. These are hermaphroditic plants.

In animals this situation is possible (starfish, for example) but not frequent, and in humans, impossible. Intersex individuals (XXY or XYY) have been reported, but none with the ability to simultaneously produce eggs and sperm.

Chromosomal sex determination

X-linked inheritance was first observed by Lilian Vaughan Morgan in Drosophila Melanogaster. She showed before anyone else that there were characters that were specifically inherited from females to males. These characters were determined by genes that resided on the X chromosome.

For any gene on the X chromosome, females can be homozygous or heterozygous. But males are hemizygous for all genes on this same chromosome.

That is, all the dominant or recessive genes on the X chromosome in a male are in a single copy. All are expressed because there is no homologous pair with which to establish allelic dominance / recessiveness relationships.

Hemophilia

To illustrate the above, let's turn to an example of X-linked inheritance: hemophilia. There are different types of hemophilia that depend on the gene that is mutated for the production of clotting factor.

In hemophilia A and B, the affected individual is unable to produce coagulation factor VIII or IX, respectively. This is due to recessive mutations in different genes on the X chromosome. Hemophilia C is caused by a gene on an autosomal chromosome and is therefore not sex-linked.

A female heterozygous (X ^h X) for a mutation that determines hemophilia A or B is not hemophiliac. However, it is said to be a carrier - but of the recessive mutant gene, not of the disease.

You will produce gametes X ^h and X. Regardless of who you have offspring with, your male children will have a 50% chance of being healthy (XY) or a 50% chance of being hemophiliac (X ^h Y).

Their female offspring will be healthy (XX) or healthy carriers (X ^h X) if the father is not hemophiliac (XY). In contrast, a hemophilic father (X ^h Y) will always donate an X ^h chromosome with the mutation to his daughters. You will donate a Y chromosome to your children.

A female can be hemophilic, but only if she is homozygous for the mutation (X ^h X ^h).

Inheritance of X-linked hemophilia. Taken from commons.wikimedia.org

Genes and inheritance linked to Y

The inheritance of Y, and its genes, is patrilineal. That is, the Y genes are inherited exclusively from father to son.

The Y chromosome has been found to be much smaller than the X chromosome. Therefore, it has fewer genes than this, and they do not match on the X chromosome.

Therefore, males are also hemizygous for genes on the Y chromosome. Females obviously do not have any inheritance for traits carried by a chromosome that they do not possess.

The Y chromosome has information that codes for the generation of testes and for the production of sperm. That is, for the manifestation of primary sexual characteristics, and therefore secondary, and for the fertility of man.

Other functions have to do with the functioning of the immune system and for different characters that do not depend exclusively on the Y chromosome.

In general, the genetic makeup of the Y chromosomes strongly influences men's health. These traits, however, are fundamentally of complex inheritance, involve other autosomal genes and are also associated with particular lifestyles.

References

1. Brooker, RJ (2017). Genetics: Analysis and Principles. McGraw-Hill Higher Education, New York, NY, USA.
2. Dobyns, WB, Filauro, A., Tomson, BN, Chan, AS, Ho, AW, Ting, NT, Oosterwijk, JC, Ober, C. (2004) Inheritance of most X-linked traits is not dominant or recessive, just X-linked. American journal of medical genetics, Part A, 129a: 136-143.
3. Goodenough, UW (1984) Genetics. WB Saunders Co. Ltd, Pkiladelphia, PA, USA.
4. Griffiths, AJF, Wessler, R., Carroll, SB, Doebley, J. (2015). An Introduction to Genetic Analysis (11th ed.). New York: WH Freeman, New York, NY, USA.
5. Maan, AA, Eales, J., Akbarov, A., Rowland, J., Xu, X., Jobling, MA, Charchar, FJ, Tomaszewski, M. (2017) The Y chromosome: a blueprint for men's health? European journal of human genetics, 25: 1181-1188.
6. Peyvandi, F., Garagiola, I., Young, G. (2016) The past and future of haemophilia: diagnosis, treatments, and its complications. Lancet, 388: 187-197.