Index
It is a form of aneuploidy with the presence of only one chromosome from a pair.
Partial monosomy occurs when only one portion of the chromosome has one copy, while the rest have two copies.
Monosomy is an example of aneuploidy, an imbalance in the number of chromosomes. This means that we can find extra or missing chromosomes in cells due to errors that occur during cell division.
In monosomy, a chromosome from a homologous pair is missing. In the case of humans, we usually have two copies of each chromosome, one from each parent, resulting in 46 total chromosomes found in the nucleus of cells.
When a pair has a missing chromosome, the resulting number becomes 45 chromosomes instead of 46, leading to abnormalities.
Note that monosomy is very rare in humans, as embryos lacking a chromosome die in most cases. Interestingly, many cases of monosomy only have a part of the missing chromosome, as in cri du chat syndrome.
human monosomy
Human conditions due to monosomy:
- Turner syndrome: People with Turner syndrome usually have one X chromosome instead of two sex chromosomes.
- Turner syndrome is the only complete monosomy seen in humans; all other cases of complete monosomy are fatal, and the individual will not survive development.
- Cri du chat syndrome : (French for “cat cry” after the distinctive noise from the deformed larynx of affected people) partial monosomy caused by a deletion of the end of the short p (from the word petit, French for “small ») Arm of chromosome 5.
- 1p36 deletion syndrome: partial monosomy caused by a deletion at the end of the short p arm of chromosome 1.
Types
Complete monosomy
Meiosis is a process that results in four reproductive cells (eggs or sperm), each ready for fertilization and containing half the number of chromosomes found in a normal cell.
Complete monosomy occurs due to the unsuccessful separation of a pair of chromosomes during the meiosis process, called nondisjunction. Once a cell with an extra or missing chromosome goes through fertilization, the embryo will have the wrong number of chromosomes.
Partial monosomy
Partial monosomy is where only part of the chromosome is missing. This can happen when a portion of the chromosome is removed at meiosis or sometime during the early stages of the embryo’s development.
Like complete monosomy, this case of partial monosomy cannot be inherited. On the other hand, partial monosomy caused by translocation is often inherited from the parents.
Translocation is a type of mutation in which a section detaches from one chromosome and joins a different, non-homologous chromosome. This means that the first chromosome has lost crucial genetic information.
When this occurs in people, they will still have the genetic information within their cells and, therefore, generally do not have abnormalities.
However, if they are going to have offspring, there is a chance that they will pass on a chromosome that has missing information, leading to partial monosomy.
Of course, the father can also pass on the chromosome that carries the additional information, leading to a partial trisomy. Trisomy is the presence of an extra third copy of a chromosome, as in the case of Down syndrome.
Therefore, partial monosomy refers to an extra copy of a part of a chromosome.
Related Biology Terms
Down syndrome is a genetic disorder characterized by mental retardation caused by trisomy 21.
Mitosis: the division of the nucleus that leads to two identical daughter cells in living organisms.
Mosaicism: cells within the same organism that have different genetic information.
Polyploidy: the presence of more than two sets of each chromosome in the cell.
Monosomy Example
Turner syndrome
Also called monosomy X, a genetic disorder in which an individual ends up with only one X chromosome and nothing else on their last pair of chromosomes. Remember that women have two X chromosomes as sex chromosomes, and men have one X and one Y.
Therefore, people with Turner syndrome turn out to be women with only one X chromosome. Having only one pair of chromosomes, these females do not develop the same way as average women but show some physical and hormonal abnormalities.
All girls with Turner syndrome are infertile, but some have more symptoms than others. For example, they may not experience puberty, develop breasts, or have a menstrual cycle.
Also, almost all girls are shorter than average, and they often have wide necks or swollen hands and feet. Other problems can also occur, including hearing or heart problems.
Turner syndrome occurs in about 1 in 2,500 girls born and is not inherited from parents. Interestingly, Turner syndrome is the only case of monosomy that someone can live their life with, although most fetuses with this syndrome die before birth.
