A normal human cell contains 46 chromosomes. If a human cell contains 45 or 47, it is considered abnormal and is referred to as aneuploidy. Having an extra chromosome or missing a chromosome is the most common cause for genetic disorders including some birth defects. Aneuploidy only applies to a single extra or missing chromosome and does not apply to a difference of complete sets (chromosomes typically function in pairs). Cancer cells may have an abnormal quantity of chromosomes. Aneuploidy occurs during cell division when a chromosome does not properly separate.
Chromosome abnormalities occur is as many as 1 in 160 live births of humans. However, in many cases, the aneuploidy goes undiagnosed. Often the woman experiences a miscarriage before the pregnancy is discovered or before aneuploidy can be tested for.
Sex cells and reproduction
As stated above, most of the cells in a human body have 46 chromosomes, or 23 pairs. Sperm and egg cells, on the other hand, only have 23 unpaired chromosomes each. During sexual reproduction in humans, a copy of each pair is genetically inherited from both parents to make a total of 46 chromosomes in the fertilized egg. Chromosomes are numbered 1 through 22, and the 23rd pair, the sex chromosome, is known as X/Y. The mother always provides an X chromosome as her genetic make up for the 23rd chromosome is X/X. The father will provide a number of X chromosomes and a number of Y chromosomes. If a sperm with an X chromosome fertilizes the egg, the gender will be a girl (X/X). If the sperm with a Y chromosome fertilizes the egg, the gender will be a boy (X/Y).
The extra or missing chromosome occurs during meiosis, when germ cells divide to create gametes (sperm or eggs). Ideally, each half should have the same number of chromosomes. Sometimes, however, a whole pair will end up on one side (with an extra chromosome), leaving the other gamete short a chromosome. The most frequent chromosomes for this to happen with while still resulting in a live birth are numbers 21, 18, and 13.
Down syndrome, or Trisomy 21, is the result of an extra chromosome, specifically number 21. Down syndrome affects 1 in 800 births. Trisomy 21 is the result of a developing egg or sperm cell taking an entire pair of the number 21 chromosome. 88% of the cases of nonseparation are from the mother, 8% are from the father, and 3% occur after the sperm and egg have merged. Also, in 2-4% of cases, the extra chromosome material may occur as a result of the Robertsonian translocation.
Edwards syndrome, or Trisomy 18, is a genetic disorder that is formed from a 3rd copy, or partial 3rd copy, of the 18th chromosome. Common results are low birth weight and heart defects. Other symptoms of the condition may include small jaw, small head, severe intellectual disability, or clenched fists with overlapping fingers. The chances of the child being born with Edwards Syndrome increase with the age of the mother.
Patau syndrome, or Trisomy 13, is caused by an extra 13th chromosome, in all or part. The presence of this extra material disrupts progressive development and causes complex organ defects. Babies born with Patau syndrome are likely to have a host of congenital defects. The child will likely have an intellectual disability or motor disorder as well as a musculoskeletal physical abnormality. Some common abnormalities include extra fingers and toes, improperly-set ears, and cleft palate. It can be expected that a child born with Patau syndrome will not likely survive their first year of life as 80% of babies born with this condition do not survive to be older than one year.
A Few Other Examples
Trisomy, especially the examples listed above, is the most common form of aneuploidy, but some other, less common examples are listed below.
- Monosomy – This describes the lack of a chromosome. This can be complete or partial. Monosomy can occur in the sex chromosomes (X/Y), causing Turner syndrome.
- Disomy – Disomy describes the being of two copies of a chromosome. For humans, a pair of chromosomes, or disomy, is normal. For other organisms that have three or more copies (or more) a pair of chromosomes would be considered an aneuploidy.
- Tetrasomy/Pentasomy – Tetrasomy is the presence of 4 copies of a chromosome, while a pentasomy is the presence of 5 copies of a chromosome. This is often seen with the 23rd chromosome – the sex chromosome.
Aneuploidy occurs during during gamete production if a gamete is missing a chromosome or has an extra one. In most cases, a fertilized egg that suffer aneuploidy aborts in the early stages, resulting in a miscarriage. However, that's not always the case. In some specific cases, such as Down Syndrome, Patae Syndrome, and Edwards Syndrome, a fetus can develop enough to survive and be born. While it is possible for children with these disorders to have few neurological or physical disorders, the expected outcome is both neurological and physical defects. Typically, children born with aneuploidy require extra care and attention, often for the duration of their lives.