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Humans possess 46 chromosomes consisting of 23 pairs in each cell. One copy of the 22nd chromosome from each parent is joined together and inherited. It is considered as one of the smallest human chromosomes that represents 1.5 to 2 percent of the total DNA content in each cell. When the human genome project started in 1999, scientists sequenced the entire 22nd chromosome. Scientists have also reported that the chromosome 22 contains approximately 500 to 800 genes that are responsible for performing various body functions.
There are several genetic conditions associated with changes in existing genes on chromosome 22. The change in the number of copies or structure can cause health and development related problems. There are several chromosomal conditions which are associated with chromosome 22.
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22q11.2 Deletion Syndrome
It has been found that there are about three million base pairs (bp) missing from one of the copies of chromosome 22 in each cell. Deletion occurs in the middle and at a specific site, which is designated as q11.2. This particular region consists of approximately 30 to 40 genes, but they are not well differentiated. However, the loss of a gene called TBX1 is considered to be responsible for the syndrome. These defects include heart problems, a cleft palate, low levels of calcium, and characteristic facial features, etc. In addition, the loss of one more gene called COMT in the similar region is accountable for mental illness and behavioral related problems.
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As the name suggests, this disorder occurs due to the addition of an extra copy of some genetic material at position q11.2 on chromosome 22. The additional genetic material consists of about three million base pairs. The duplication has an effect on on one of the copies of chromosome 22 in each cell. This syndrome may cause developmental delay.
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22q13.3 Deletion Syndrome
This syndrome occurs due to the deletion of the (q) arm end of the chromosome 22. In addition, a ring chromosome also causes 22.13.3q deletion syndrome. It is a circular structure that takes place when a chromosome breaks into two pieces. The end of the chromosome is lost and the broken ends join together. The loss of a gene called SHANK3 is thought to be responsible for many of the characteristic features of 2213.3q deletion syndrome. The defects caused of the gene include developmental delay, delayed speech and intellectual disability, etc.
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This kind of genetic disorder is caused due to the presence of additional genetic material from chromosome 11 plus chromosome 22 in each cell. People with Emanuel syndrome possess a supplementary chromosome that consists of a piece of chromosome 11 attached to chromosome 22. This additional chromosome is called the der(22) chromosome. As a consequence, people with this genetic syndrome carry three copies of similar genes in each cell in place of the normal two copies. The additional genetic material upsets the normal course of development.
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This is a rare disorder, which is caused due to a chromosomal change called an inverted-duplicated 22. People with cat-eye syndrome possess at least 1 additional small chromosome made up of genetic-material form chromosome 22 that has been unusually copied. People with this syndrome have an eye abnormality, small pits in the ear, heart problems, kidney defects, and delayed development, etc.