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Maternal Age: Women are born with all the eggs they will ever have. Other factors that can increase the risk of chromosome abnormalities are: Errors can also occur when the chromosomes are being duplicated. However, errors in cell division can result in cells with too few or too many copies of a chromosome. In both processes, the correct number of chromosomes is supposed to end up in the resulting cells. This is the type of cell division that occurs in the reproductive organs, resulting in the eggs and sperm. Meiosis results in cells with half the number of chromosomes, 23, instead of the normal 46. This is the way most of the cells that make up our body are made and replaced. This kind of cell division occurs throughout the body, except in the reproductive organs. One cell with 46 chromosomes divides and becomes two cells with 46 chromosomes each. Mitosis results in two cells that are duplicates of the original cell. There are two kinds of cell division, mitosis and meiosis. This is why, when a child is found to have an abnormality, chromosome studies are often performed on the parents.Ĭhromosome abnormalities usually occur when there is an error in cell division. Some abnormalities, however, happen after conception then some cells have the abnormality and some do not.Ĭhromosome abnormalities can be inherited from a parent (such as a translocation) or be " de novo" (new to the individual). In these cases, the abnormality is present in every cell of the body. Most chromosome abnormalities occur as an accident in the egg or sperm. This can happen with or without loss of genetic material. Rings: A portion of a chromosome has broken off and formed a circle or ring. As a result, the genetic material is inverted. Inversions: A portion of the chromosome has broken off, turned upside down, and reattached. In a Robertsonian translocation, an entire chromosome has attached to another at the centromere. In a reciprocal translocation, segments from two different chromosomes have been exchanged. There are two main types of translocation. Translocations: A portion of one chromosome is transferred to another chromosome. Structural Abnormalities: A chromosome's structure can be altered in several ways.ĭeletions: A portion of the chromosome is missing or deleted.ĭuplications: A portion of the chromosome is duplicated, resulting in extra genetic material. In Turner syndrome, a female is born with only one sex chromosome, an X, and is usually shorter than average and unable to have children, among other difficulties. An example of monosomy, in which an individual lacks a chromosome, is Turner syndrome. An individual with Down syndrome has three copies of chromosome 21 rather than two for that reason, the condition is also known as Trisomy 21. When an individual has more than two chromosomes instead of a pair, the condition is called trisomy.Īn example of a condition caused by numerical abnormalities is Down syndrome, which is marked by mental retardation, learning difficulties, a characteristic facial appearance and poor muscle tone (hypotonia) in infancy. Numerical Abnormalities: When an individual is missing one of the chromosomes from a pair, the condition is called monosomy. However, they can be organized into two basic groups: numerical abnormalities and structural abnormalities. There are many types of chromosome abnormalities. The test examines the baby's DNA in the mother's blood. This is a test to screen a pregnancy to determine whether a baby has an increased chance of having specific chromosome disorders. One such method is called noninvasive prenatal testing. The comparison can be used to find chromosomal abnormalities where the two samples differ. These newer methods compare the patient's DNA to a normal DNA sample. In the past decade, newer techniques have been developed that allow scientists and doctors to screen for chromosomal abnormalities without using a microscope. To help identify chromosomes, the pairs have been numbered from 1 to 22, with the 23rd pair labeled "X" and "Y." In addition, the bands that appear after staining are numbered the higher the number, the farther that area is from the centromere. The karyotype can help identify abnormalities in the structure or the number of chromosomes. A picture, or chromosome map, of all 46 chromosomes is called a karyotype. Once stained, the chromosomes look like strings with light and dark "bands," and their picture can be taken. In order for chromosomes to be seen this way, they need to be stained. For a century, scientists studied chromosomes by looking at them under a microscope.