HOLLANDIC INHERITANCE: CHARACTERISTICS, GENE FUNCTIONS, DEGENERATION - BIOLOGY - 2025

The holandric heritage is the transfer of genes linked to the sex chromosome from parents to children. These genes are transferred or inherited intact, that is, they do not undergo recombination, so they can be considered as a single haplotype.

The Y chromosome is one of the two sex chromosomes that determines the biological sex of the embryo in humans and other animals. Females have two X chromosomes, while males have one X and one Y chromosome.

Schematic of the hollandic inheritance pattern (Source: Madibc68 via Wikimedia Commons)

The female gamete always transmits an X chromosome, while the male gametes can transmit an X chromosome or a Y chromosome, which is why it is said that they "determine sex."

If the father transmits an X chromosome the embryo will be genetically female, but if the father transmits a Y chromosome the embryo will be genetically male.

In the process of sexual reproduction, the two sex chromosomes recombine (exchange genetic information with each other) combining the traits transmitted by both parents. This combination helps eliminate possible faulty traits in the progeny.

However, 95% of the Y chromosome is exclusive to male organisms. This region is commonly known as the "male Y-specific region," and it does not sexually recombine with the X chromosome during reproduction.

Furthermore, most of the genes on the Y chromosome do not recombine with any other chromosome during sexual reproduction, since they are linked together, so most are the same in parents and offspring.

Y chromosome characteristics

The Y chromosome is the smallest of all chromosomes. In mammals, it is made up of about 60 mega bases and has only a few genes. The region available to be transcribed (euchromatin) is 178 triplets, and the rest are pseudogenes or repeat genes.

Repeated genes are found in multiple copies and in palindromic form, which means that they are read in the same way in both senses, for example, the word “swim”; a DNA palindrome sequence would be something like: ATAATA.

Human chromosomes (Source: National Center for Biotechnology Information, US National Library of Medicine via Wikimedia Commons)

Of the 178 units or triplets exposed for transcription, 45 unique proteins are obtained from this chromosome. Some of these proteins are associated with the sex and fertility of the individual and other non-reproductive ones are ribosomal proteins, transcription factors, etc.

The architecture of the Y chromosome is divided into two different regions, a short arm (p) and a long arm (q). The short arm contains 10 to 20 different genes, comprises about 5% of the entire chromosome, and can recombine with the X chromosome during meiosis.

Y chromosome from humans. The small arm (p) and the large arm (q) are identified (Source: John W. Kimball via Wikimedia Commons)

The long arm constitutes about 95% of the remaining Y chromosome. This region is known as the “non-recombinant region” (NRY), although some researchers suggest that recombination does occur in this region and the region should be called the “male specific region” (RMS).

The genes belonging to the non-recombinant region of Y (95%) have holandric inheritance, since they are located exclusively on said chromosome and are linked or linked between them. There is no recombination in this region and the mutation rate is very low.

Functions of genes with hollandic inheritance

In 1905, Nettie Stevens and Edmund Wilson observed for the first time that the cells of men and women had a different chromosome structure.

The cells of the women had two copies of the large X chromosome, meanwhile the men only had one copy of this X chromosome and, associated with it, they had a much smaller chromosome, the Y chromosome.

In the first 6 weeks of gestation, all embryos, whether genetically female or male, develop in the same way. In fact, if they continued to do so until delivery they would result in a physically female newborn.

All this changes in male embryos by the action of the gene called "region of sexual determination Y" located on the Y chromosome. It derives its name from the English "sex-determining region Y" and is abbreviated in the literature as SRY.

The SRY gene was discovered in 1990 by Robin Lovell-Badge and Peter Goodfellow. All embryos that have an active copy of this gene develop a penis, testicles, and beard (in adulthood).

This gene works like a switch. When it is "on" it activates masculinity and when it is "off" it gives rise to female individuals. It is the most studied gene on the Y chromosome and regulates many other genes related to the sex of individuals.

The Sox9 gene encodes a transcription factor that is key in the formation of the testes and is expressed in conjunction with the SRY gene. The SRY gene activates the expression of Sox9 to initiate the development of male gonads in many animals.

Degeneration of genes with hollandic inheritance

All genes found on the Y chromosome, including those passed down through hollandic inheritance, are found on a dwarf chromosome. While the X chromosome has more than 1,000 genes, the Y chromosome has fewer than 100.

The Y chromosome was once identical in size to the X chromosome. However, for nearly 300 million years it has progressively decreased in size, to the point where it has less genetic information than any other chromosome.

In addition, the X chromosome has a homologous pair, since in women it appears in pairs (XX) but the Y chromosome is only found in men and does not have a para homologous. The absence of a pair prevents the Y chromosome from recombining all its parts with a match.

This absence of a pair prevents the genes with hollandic inheritance, unique to the Y chromosome, from being able to protect themselves from mutations and normal genetic deterioration of nucleic acids.

The absence of recombination means that every mutation that occurs in genes linked to the Y chromosome or with hollandic inheritance is transmitted intact to male descendants, which can mean a great disadvantage.

Despite the fact that the Y chromosome and its genes are degenerate and vulnerable to mutations, scientists believe that it is far from being completely damaged or disappearing, since some genes on this chromosome are important for the production of sperm.

Being involved in the production of sperm, spontaneous mutations that damage or inactivate them are “self-selected”, reducing the fertility of the parent with said mutation, preventing this from passing its genes to offspring.

References

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