Table of Contents
- What is Aneuploidy?
- Different Conditions of Aneuploidy
- Aneuploidy Disorders
- Lesson Summary
There are normally two copies of each chromosome, or long piece of DNA, in each cell. In abnormal instances there may be more or less than two copies of each chromosome in the cell. This is referred to as aneuploidy. Aneuploidy occurs when there are errors during the process of meiosis. This is the process to create sex cells in the body. Normally, each sex cell should have one copy of each chromosome.
In aneuploidy, however, there are either 2 chromosomes where there should be one or the chromosome is missing altogether. During meiosis, one parent cell divides into two daughter sex cells. Each daughter sex cell should get one of each chromosome from the parent cell as the cell divides into two.
What? causes these errors in meiosis to occur that lead to aneuploidy? Some are caused by substances known as aneugens. Aneugens are agents that are capable of interfering with the normal function of cell division by causing the spindle fibers to not separate chromosomes properly. Some of the most common aneugens are pesticides, tobacco smoke, perfluorinated carbons, benzene and insecticides. X-rays are also a known aneugen. Aneugens are also referred to as genotoxicants.
In normal body cells where there are two copies of each chromosome, it is notated as 2n where n refers to the chromosome. This notation comes from the fact that when two sex cells come together, they each bring their one copy of each chromosome into the new body cell. This gives two copies of each chromosome in every body cell. The total number of chromosomes should be 46. The notation is different in the different conditions of aneuploidy because they do not have exactly two copies of each chromosome.
The first aneuploidy condition is nullisomy. Nullisomy is when a pair of chromosomes is missing. This occurs due to what is known as nondisjunction during the process of meiosis. Nondisjunction is when both of the chromosomes that should separate and end up in different daughter sex cells both end up in one daughter sex cell. This leaves the other daughter sex cell without a copy of the chromosome at all. When two sex cells without the chromosome are joined together during fertilization then the chromosome pair is missing and this is known as nullisomy. Nullisomy is notated as 2n -2. meaning there are two copies of the chromosome that are missing in the body cell. This would cause the total number of chromosomes in the cell to be 44. This particular aneuploidy is fatal and does not produce a viable baby.
Monosomy is when one chromosome is missing from the set of chromosomes. Instead of there being two copies of the chromosome, there is just one copy of the chromosome. This occurs when one sex cell without one copy of one chromosome joins during fertilization with another sex cell that has the normal one copy of each chromosome. The resulting number of chromosomes in the body cell will be 45 instead of the normal 46. This condition can produce a viable baby depending on which chromosome is missing. The notation for this condition is 2n-1 indicating that there are two copies of each chromosome minus 1 chromosome.
The next aneuploidy is trisomy which is when there is one extra copy of one of the chromosomes. This occurs because of two sex cells coming together during fertilization where one sex cell has an abnormal two copies of a chromosome and the other sex cell has one copy of the chromosome. This results in the body cell having three copies of the chromosome rather than the normal two copies. The total number of chromosomes in this cell will be 47 instead of the normal 46. The notation for this condition is 2n + 1 meaning there are two copies of each chromosome plus an additional chromosome. This condition can produce a viable baby depending on which chromosome is affected.
Tetrasomy is the last of the aneuploidies. In the case of tetrasomy, a body cell has four copies of one of the chromosomes. This occurs when two sex cells that each have two copies of the chromosome come together during fertilization. Since each sex cell has two copies, this gives the body cell four copies rather than the normal two. The total number of chromosomes in the body cell ends up being 48 instead of the normal 46. The notation for this condition is 2n + 2 meaning there are two copies of each chromosome plus 2 additional chromosomes. This condition can result in a viable baby depending on which chromosomes are involved. The most common chromosomes involved in this condition are the sex chromosomes.
There are several different aneuploidy disorders of most of the different conditions of aneuploidy. As mentioned earlier, nullisomy is fatal and doesn't produce a viable baby so there are no nullisomy disorders.
Aneuploidies are genetic disorders so the diagnosis of them comes from looking at the genes of a fetus or baby. They are often diagnosed prior to birth. An amniocentesis has been the most common method for diagnosing aneuploidies. A sample of the amniotic fluid is removed from around the fetus. The amniotic fluid contains DNA from the developing fetus. The chromosomes are viewed through a process called karyotyping. This is when all of the chromosomes in the cell are paired up to view the full set of DNA. Viewing the chromosomes allows the doctor to tell if there are extra or missing chromosomes.
Karyotyping can also be done by doing chorionic villus sampling which is a technique where a sample of placental tissue is removed from a pregnant woman. This is usually done when testing is needed very early on during the pregnancy. There are newer techniques that are less invasive such as cell-free fetal DNA. Cell-free fetal DNA is easily performed by simply drawing blood from the pregnant woman. The fetal DNA is present in the maternal blood, so karyotyping can be performed on the blood.
A chromosome is a long piece of DNA and an aneuploidy is when there is an abnormal number of chromosomes present in the body cells. Meiosis is the process of creating sex cells. Aneuploidy occurs due to errors during meiosis. Aneugens are agents that can cause aneuploidies to occur. Aneuploidy conditions include:
Aneuploidy disorders include:
Diagnosis of aneuploidies is done through karyotyping which is viewing the full set of chromosomes. This is done through removal of amniotic fluid called amniocentesis, chorionic villus sampling, which is the removal of placental tissue, and cell-free fetal DNA, which is getting DNA from maternal blood.
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The four types of aneuploidy are nullisomy, where both copies of a chromosome are missing, monosomy, where one copy of a chromosome is missing, trisomy, where there is one extra chromosome, and tetrasomy, where there is an extra set of one of the chromosomes.
Some examples of aneuploidies include Trisomy 21 also called Down Syndrome, Klinefelter's Syndrome, cri du chat syndrome also called cat cry syndrome, Turner Syndrome, Trisomy 18 also called Edward's Syndrome, Tetrasomy X,
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