MONOSOMIES: CHARACTERISTICS AND EXAMPLES - BIOLOGY - 2024

The monosomy refer to the chromosomal constitution of individuals with one chromosome instead of the normal pair in diploid organisms. That is, if there are 23 pairs of chromosomes, there is monosomy for one of them if only one of the chromosomes is present. An individual with a monosomy in this case will present 45 chromosomes instead of 46.

Monosomies can be total or partial. In the first case, the entire chromosome is missing. In the second, a deletion of only a portion of the chromosome determines the partial lack of information of the affected chromosome.

Karyotype of a female with only one X chromosome, instead of two. No machine-readable author provided. The cat ~ commonswiki assumed (based on copyright claims)., via Wikimedia Commons

Since a monosomy affects only one pair of chromosomes of a diploid species, for example, it is considered aneuploidy. The true ploidy changes or euploidies, affect, on the contrary, the complete number of chromosomes that define a species.

Characteristics of monosomies

Monosomies can affect somatic chromosomes or sex chromosomes. The only monosomy of the sex chromosomes in humans is that of the X chromosome.

These individuals are XO women and have what is called Turner syndrome. There are no YO monosomics because every human being requires an X chromosome to exist.

Women are XX and men XY. In cases of aneuplodia, women can also be XXX (trisomy of X) or XO (monosomy of X). Aneuploid men can be XXY (Kleinefelter syndrome) or XYY. These last two are also trisomies.

Total autosomal monosomies are often fatal, as they lead to severe developmental defects. Furthermore, any (and all) mutation may manifest itself since the individual will be hemizygous for all the genes of the solitary chromosome.

Aneuploid organisms generally arise by fusion of gametes, one of which has a numerical chromosomal aberration. Aneuploidies can also arise from somatic tissue, and appear to play an important role in the appearance and development of certain types of cancer.

Partial monosomy of chromosome 5 in humans: the crying cat syndrome

A partial (or total) deletion in the short arm of chromosome 5 is the cause of the so-called cri-du-chat syndrome. It is also known as Lejeune syndrome, in honor of its discoverer, the French researcher Jèrôme Lejeune. In French, cri-du-chat means “crying cat”.

80% of the gametes where the deletion that characterizes this syndrome occurs are of paternal origin. Most deletions are spontaneous and occur de novo during gametogenesis. In minority cases, the aberrant gamete arises from other types of events, such as translocations or unequal chromosomal segregations.

Characteristics of the disease

Due to laryngeal and nervous system problems derived from the condition, affected children cry similar to that of small cats. This type of crying disappears when the child gets a little older.

On a physical level, they may have a head, small jaws and drool a lot. The most relevant physical sign of this syndrome, however, is not visible to the naked eye. It is about cerebellar hypoplasia, a congenital disorder of brain morphogenesis.

For the rest of their lives, affected people will have problems feeding (sucking and swallowing difficulties), gaining weight, and growing. They will also have severe motor, intellectual and speech retardation.

At the behavioral level, people with this syndrome usually present some disorders that include hyperactivity, aggressiveness and "outbursts". They also tend to have repetitive movements. In very rare cases, the individual may exhibit normal appearance and behavior, except for learning difficulties.

Treatment of the disease

Affected individuals require constant medical attention, particularly for therapies related to motor and speech impairments. If heart problems occur, surgery will most likely be required.

Some genes that contribute to the manifestation of the disease

The genes in the missing fragment, including the entire short arm of chromosome 5, are in the hemizygous state. That is, in only one copy from the other complete chromosome of the pair.

The genetic makeup of this chromosome, therefore, will determine some of the causes of the disease. Some may be explained by the deficient expression of a mutated gene. Others, on the contrary, due to the effect of the gene dosage derived from the existence of a single copy of the gene instead of two.

Some of the genes that contribute to the development of the disease through genetic dosing include TERT (for accelerated telomere shortenig). People affected by the syndrome have deficiencies in telomere maintenance. Telomere shortening is linked to the appearance of various diseases and early aging.

On the other hand, the SEMA5A gene in a hemizygous state interrupts normal brain development in individuals with deletions on chromosome 5. For its part, the hemizygous state of the MARCH6 gene seems to explain the characteristic cat cry of those affected with trisomy.

Total monosomy of the X chromosome: Turner syndrome (45, X)

Autosomal monosomies, as a rule, are always lethal. Interestingly, however, the X chromosome monosomy is not, as many XO embryos manage to survive.

The reason seems to lie in the role of the X chromosome in sexual determination in mammals. As the females of the species are XX and the males XY, it is an indispensable chromosome. The Y chromosome is only essential for the sexual determination of males, not for their survival.

The X chromosome carries almost 10% of the genetic information in humans. Obviously, their presence is not an alternative; it is mandatory. In addition, it is always partially present. That is, in males there is only one copy of X.

But in females, functionally speaking, too. According to the Lyon hypothesis (already corroborated) in females only one of the X chromosomes is expressed. The other is inactivated by genetic and epigenetic mechanisms.

In this sense, all mammals, male and female, are hemizygous for X. XO women too, but in a different condition not without problems.

Syndrome overview

There is no proven cause for the syndrome in 45, X karyotype females. Turner syndrome affects 1 in 2,500 living women.

Therefore, it is a rare aneuploidy when compared, for example, with XXY or XXX trisomies. In general, XO pregnancies are not viable. An estimated 99% of XO pregnancies end in abortion.

Associated physical and somatic characteristics

The hallmark physical feature of Turner syndrome is short stature. XO females are small at birth, do not experience the explosive growth associated with puberty, and as adults reach a maximum of 144 cm in height.

Other somatic features associated with the syndrome include congenital heart disease, as well as kidney abnormalities. Women affected with Turner syndrome are at increased risk for otitis media, hypertension, diabetes mellitus, thyroid disorders, and obesity.

Mental development and skills

The IQ of XO women is equivalent to that of their XX peers. However, it is possible that deficits can be observed in spatial orientation, in handwriting and in solving mathematical problems. They do not present problems, for example, in arithmetic calculations, but they do in counting.

Speech is normal, but problems can occur when otitis media is not treated. Many of these deficiencies are believed to be the result of decreased estrogen production. Motor skills may also show some delay.

Treatment of symptoms of the syndrome

In terms of short stature, women with Turner syndrome can receive injections with recombinant growth hormone during childhood. They can expect to reach at least 150 cm in height.

Hormone replacement therapy should be started between the ages of 12 and 15 to ensure a proper transition to adolescence and adulthood. This therapy, in most cases, must be prolonged in order to prevent premature coronary heart disease and osteoporosis.

With the rest of the conditions, medical advice and follow-up are essential during the development and adult state of XO women. Counseling is also important, as physical deficiencies can affect your emotional development.

Monosomies in other organisms

Monosomies were first discovered and reported by Barbara McClintock in 1929 from her work on corn. As in corn, monosomies in other diploid plants have a greater effect than in polyploid plants.

Bárbara MClintock in 1947. Smithsonian Institution / Science Service; Restored by Adam Cuerden, via Wikimedia Commons.

The loss of a chromosome of the pair in a diploid plant gives rise to genetic imbalances that consequently alter enzyme levels. Thus, all the metabolic pathways where they participate can be affected.

As a consequence, the normal phenotypes of the individual are altered. On the other hand, monosomics are easy to study, since their hemizygous condition makes genetic analysis of mutants easier.

These plants are very useful in basic science, for example, to study meiosis and chromosome segregation events. It has been observed, for example, that not all chromosomes in different monosomes behave in the same way.

All this will depend on the existence of homologous regions in chromosomes that are not necessarily those of the proper pair. In applied science, a specific monosomal plant is easier to manipulate than a disomic. Then, you can proceed to conventional crosses to generate new varieties (without the monosomy).

References

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