A surprising fact about genetics is that there are many syndromes that are termed "genetic," but have not been inherited from a parent. When DNA is passed from one generation to the next, it is copied. Generally, DNA is copied faithfully, however mistakes do occasionally occur. These mistakes are sporadic and can sometimes lead to disease. A defining characteristic of these syndromes is that no family history is present before the affected child is born. If a family has a child with a sporadic genetic syndrome, the family's chance to have another affected child is low.
Down Syndrome
Down syndrome, also called Trisomy 21, is one of the most common, non-inherited genetic syndromes. It occurs in about 1 of every 100 to 150 births. In Down syndrome, a mistake in how the chromosomes segregate causes a child to inherit an extra chromosome 21. Chromosomes are structures of DNA and proteins where the genes are located. Normally there are 46 chromosomes total and they are paired--one set inherited from the mother and the other set from the father. Sometimes mistakes occur, resulting in a child having too many or too few chromosomes. Individuals with Down syndrome generally have characteristic facial features, lower than average cognitive abilities, developmental delays, birth defects and other features.
Turner Syndrome
Of the 23 pairs of chromosomes, only one pair is different in men and women--the sex chromosomes X and Y. Women have two X chromosomes, written as 46,XX, while men have an X and a Y chromosome, written as 46,XY. Individuals with Turner syndrome have only one X chromosome, written as 45,X. This occurs in 1 of every 2,000 to 2,500 births. All individuals with Turner syndrome are female, since they do not have the Y chromosome that makes individuals male. About half of girls with Turner syndrome are diagnosed at birth, while the rest are diagnosed later in life. Common features of Turner syndrome include heart defects; swelling, especially in the hands and feet due to excess lymphatic fluid; short stature; characteristic physical features and lack of sexual development. Girls with Turner syndrome have intelligence in the normal range. They are, however, more likely than other children to have nonverbal learning disabilities.
22q11.2 Deletion Syndrome
The disease 22q11.2 deletion syndrome is caused by a loss of DNA, called a deletion, at a specific place on chromosome 22. There are many other names for 22q11.2 deletion syndrome: DiGeorge syndrome, velocardiofacial syndrome, Shprintzen syndrome and others. It is estimated that 22q11.2 deletion syndrome occurs in 1 of every 4,000 to 7,000 births. Individuals with 22q11.2 deletion syndrome have a wide range of features, including cardiac birth defects, cleft lip and/or palate, immune dysfunction and varying degrees of intellectual disability. They are also more likely than the general population to have psychiatric disorders. Most cases of 22q11.2 deletion syndrome occur sporadically, though it is inherited from a parent in less than 10 percent of cases. In these instances, the parent is often mildly affected and may be unaware he or she has a genetic syndrome. The chance of a second affected child is much higher in these families.
Williams Syndrome
Williams syndrome is due to a deletion, or loss, of DNA at a specific place in chromosome 7. Williams syndrome occurs in about 1 of every 7,500 births and affects both males and females. A wide range of features are seen in Williams syndrome and some children are born with heart defects. They also have characteristic facial features as well as intellectual disability. Individuals with Williams syndrome have a friendly, outgoing personality and are quite empathetic. For this reason, Williams syndrome is sometimes described as being the "opposite" of autism.
Duchenne Muscular Dystrophy
Duchenne muscular dystrophy (DMD) is characterized by progressive muscle weakness and mostly affects males. It is an X-linked disorder, frequently inherited from the child's mother, who is an unaffected carrier. Nearly one-third of cases, however, are due to a sporadic genetic change, or mutation, in the DMD gene. In these cases, there is no family history of DMD and the chance to have another affected child is very low. Genetic testing can be done when a male is diagnosed with DMD to determine if the non-working gene has been inherited or is due to a sporadic change.
Other Syndromes
There are many other genetic syndromes that occur without being inherited. Most of these syndromes are chromosomal in nature--that is, a gain or loss of chromosomal material or a structural chromosomal change that causes a genetic disease. There are other syndromes that are due to smaller changes in a specific disease gene. Genetic counselors and other genetic medical providers can analyze a family's medical history to determine if a specific disease or syndrome has been inherited or occurred sporadically.
References
- "Thompson & Thompson Genetics in Medicine, 7th edition"; Robert Nessbaum, M.D.; 2007
- Turner Syndrome Society of the United States: Turner Syndrome--A Guide for Families
- National Center for Biotechnology Information: 22q11.2 Deletion Syndrome
- National Center for Biotechnology Information: Williams Syndrome
- National Center for Biotechnology Information: Dystrophinopathies
