The National Institutes of Health explains that having more or fewer chromosomes than the typical number – 46 – can cause birth defects or miscarriage. It can also be a factor in conditions that include Down Turner Syndrome.
The NIH's National Human Genome Research Institute describes chromosomes as the biological structures that hold genes. Genes are responsible for telling the body how to develop and function. Genes, in conjunction with environmental factors like upbringing, dictate what that person looks like, his personality, and so on.
The typical number of chromosomes is 46. A person typically inherits 23 chromosomes from his biological father and 23 from his biological mother, according to the March of Dimes. The NIH describes that sometimes something goes wrong when the chromosomes come together. Usually, this is a result in an error of cell division. If this happens, it is possible for the developing fetus to be missing a chromosome from a pair (which is called monosomy) or to have more than two chromosomes of a pair (known as trisomy). This abnormality disrupts the typical development of the fetus and causes birth defects or other conditions.
One common trisomy is trisomy 21, also known as Down syndrome. Trisomy 21 means that an individual has three copies of the chromosome 21. The March of Dimes explains that those with Down syndrome have varying degrees of mental and psychological development, often have hearth defects and can usually be identified by a similar set of facial features. Down syndrome is one of the less severe types of trisomies; children with trisomy 13 or Patau syndrome usually die before their first birthday.
On the other hand, an individual can be born with a monosomy. The NIH lists Turner syndrome, which is a monosomy in which the individual is born with only one sex chromosome. Everyone usually has two sex chromosomes. Females have two X chromosomes, while males have one X and one Y. An individual with Turner syndrome has only one X chromosome. The female, as March of Dimes explains, is usually infertile, and does not undergo puberty. They are typically short, and have health problems such as heart or kidney defects.
Many other chromosomal defects are deadly, and the fetus does not even survive long enough to be born, according to the March of Dimes. The health effects of having more or less chromosomes than the typical 46 are often very severe.