), The gale encyclopedia of science (4th ed.). Retrieved from Gale Student Resources in Context database. (Accession No. CV2644030714) Down syndrome facts. (n.d.).
Instead of 46 chromosomes in each cell it was determined that an extra 21st chromosome affected Down syndrome, this occurs 95% of the time. There are three copies of this chromosome. It is causes by a cell error called non-disjunction. Two other types are mosaicism and translocation. Non-disjunction causes faulty cell division and produces three copies of the 21st chromosome instead of two.
The type of mutation that occurs in Down syndrome is aneuploidy that is the irregular number of chromosomes in a cell. The most common of the three is the trisomy 21 that occurs in about 90% of people with the disorder. In this factor the human is given three copies of the chromosome 21 instead of the common two copies. This occurs due to the complications of the cell division in the process of the egg or sperm. The next case is mosaic which happen when there are inequality of cells with three copies of chromosome 21 and others with the original two copies.
Some investigators reported that older fathers may also be at an increased risk of having a child with Down syndrome. It is well known that the extra chromosome in trisomy 21 could either originate in the mother or the father. Most often, however, the extra chromosome is coming from the mother. Description People with Down syndrome are first and foremost human beings who have recognizable physical characteristics and limited intellectual endowment which are due to the presence of an extra chromosome 21. The estimated incidence of Down syndrome is between 1 in 1,000 to 1 in 1,100 live births.
Rarely is it inherited, Down syndrome is caused by three types of chromosomal abnormalities: Trisomy 21, Mosaic Down syndrome, and Translocation Down syndrome. The most common variation, Trisomy 21, occurs in about 95% of cases. According to “Down Syndrome”, this common variation is caused by “an error during the formation of the egg or sperm” and results in “either one having an extra chromosome.” After the sperm and egg mate, the cells produced will have three copies of chromosome 21. In addition, Mosaic Down syndrome, which accounts for 3% of cases, leads to some cells being affected with three copies of chromosome 21, with others unaffected. Finally, Translocation Down syndrome results when an “extra part or a whole extra chromosome 21 is present, but it is attached or “trans-located” to a different chromosome rather than being a separate chromosome 21” (“Facts about Down Syndrome”).
Mosaicism is a rare occurrence, happening “in approximately one to two percent of Down syndrome cases” (Johnson, 2013). Initially, the correct number of chromosomes is passed on from the parents, but during the phase following fertilization, when the cells are dividing quickly, one cell divides irregularly, creating a group of cells with an extra copy of chromosome 21. A child with this type of Down syndrome has two kinds of cells, those with 46 chromosomes and those with 47, and usually has less severe signs and symptoms of Down syndrome. Translocation also occurs quite rarely, and happens during cell division when chromosome 21 breaks and attaches to another. All cells continue to have 46 chromosomes, but the broken part of chromosome 21 results in signs and symptoms of Down syndrome.
Numerical abnormalities are caused due to any deviation from the human diploid number of 46 chromosomes which is called chromosomal aneuploidy. This leads to multiple birth defects as it disturbs the action of multiple genes. Usually, numerical chromosomal abnormalities are resu... ... middle of paper ... ...Synthetic lethality does indicate that when two genes have the same role and mutation occurs in them both making them non-functional only then it results in lethality. Moreover when both the genes have the same function in the pathway, each mutation in the genes diminishes the function of the pathway.
A child with trisomy 21 has three copies of chromosome 21 — instead of the usual two copies — in all of his or her cells.”(Para.3) so from this we can conclude that this is the most common type of Down syndrome. Next there is Mosaic Down syndrome according to the Mayo Clinic Staff (2011) “In this rare form of Down syndrome, children have some cells with an extra copy of chromosome 21. This mosaic of normal and abnormal cells is caused by abnormal cell division after fertilization.” (Para.4) we can conclude from this that this is a rare type of Down syndrome and is a mixture of normal cells and the cells that have more than two chromosomes so it’s like a mosaic because the cells are mixed. Lastly there is Translocation Down syndrome. According to Mayo Clinic Staff (2011) “Down syndrome can also occur when part of chromosome 21 becomes attached (translocated) onto another chromosome, before or at conception.
There are three possible mutations to cause this inactivity. The most common cause is the deletion of the part of chromosome 15 that is inherited from the father, occurring in 70% of cases. In 25% of cases uniparental disomy (UPD) occurs and two copies of the chromosome are received from the father and none from the mother. 5% of patients diagnosed with Prader-Willi syndrome have an ‘imprinting mutation’. This is a form of translocation and means that the sequences of the genes on chromosome 15 are altered.
Down Syndrome in Children Down syndrome is one of the most common and easily recognized genetic disorders. It is caused by abnormal cell division that involves the 21st chromosome pair of the 23 pairs of chromosome found in all normal human cells. There are three types of these abnormalities, which are non-disjunction, translocation, and mosaicism. Ninety five percent of children born with Down syndrome are a non-disjunction type which is three rather than two chromosome 21s. About three to four percent of cases are because of translocation which is chromosome 21 detaches and attaches to another chromosome.