Is Down Syndrome Inherited from a Parent?
Before getting into whether or not you can inherit Down Syndrome from a parent, let’s first talk about what Down Syndrome actually is. In the United States alone, one in 700 babies will be diagnosed with Down Syndrome, making it one of the most common chromosomal conditions. This also means that there are about 6,000 Down Syndrome births per year in the United States.
Let’s get it out there: your genome is a happy mix of part mom and part dad, both good and bad things. This means that, along with all of the good things that you inherit from your parents, you are also a mix of all of the genetic diseases, conditions, and ailments that your parents have and as such you are at risk of having those as well. However, there are genetic diseases that affect some individuals without being inherited from a mother or a father. So, yes, you are absolutely right to derive the thought that genetic may not always mean inherited. (Inherited traits are genetic because they happen by virtue of being inherited based on the genes that are passed down. But the reverse is not true, namely, genetic diseases are not always inherited.) There are genetic diseases that are marked by spontaneous mutations, making them genetic.
So, what is Down Syndrome, and in which category can it be classified?
Down Syndrome is caused by the number of chromosomes. But what does this mean exactly?
Each one of us typically inherits genetic material that is packaged in 23 chromosome pairs. The reason we are saying pairs is because each one of the 23 pairs is genetic material that is inherited from each of the parents. In other words, each one of us carries a total of 46 chromosomes or 23 chromosome pairs. However, this is not always the case. Some people end up having an extra chromosome, such as those with Down Syndrome. Their 21st chromosome has an extra copy, so there is additional genetic material. This extra chromosomeis also called trisomy 21 and it is caused by a cellular process called nondisjunction. As you can probably deduce from the word itself, this process simply means that the two parts of the pair of chromosome 21 in either the egg or the sperm have not separated. This extra genetic material then goes on to be copied in every cell of the body during an embryo’s development. So, rather than being a specific illness, Down Syndrome is a term used to cover a broad category of all of the symptoms that are a result of the aforementioned nondisjunction that results in surplus genetic material. This extra DNA manifests itself via several characteristics, namely physical characteristics, mental ability, a person’s development in general as well as greater chances for other health problems. Trisomy 21 accounts for approximately 95% of all cases of Down Syndrome.
Another way by which Down Syndrome can manifest itself is by a process called translocation, which is just another way of saying genetic material between two different genes gets reshuffled or mixed. In the case of Down Syndrome, this happens with the genetic material of chromosome 21 and another chromosome (typically chromosome 14). If the rearrangement is balanced, meaning that no genetic material is either gained or lost, no side effects are visible. However, if a parent is a carrier of chromosomal abnormalities or translocation (meaning their genetic material has been reshuffled), and they pass that on to their offspring, that can lead to a mis balance which may result in Down Syndrome. Approximately 4% of all cases of Down Syndrome are said to be due to translocation.
Similar to nondisjunction, but also very different, is another type of Down Syndrome, namely mosaic Down Syndrome. This type of Down Syndrome marked by a jumble of cells that either have 46 or 47 chromosomes (rather than all of the cells having 47 chromosomes in Down due to nondisjunction). Individuals who have mosaic Down have been found to have fewer of the characteristics that are associated with Down Syndrome. Also, approximately only 1% of all cases of Down Syndrome are attributed to mosaicism.
So, what does all of this mean in terms of inheritance?
The actual cause of the additional genetic material that is associated with the cause of Down Syndrome is still unknown. The only sure aspect that has been associated with increased chances of having Down is maternal age that contributes to nondisjunction or mosaicism. (The reason that the majority of children who have Down Syndrome are born to younger mothers is simply because that age group is marked by higher birth rates and as such more children are born to mothers who are younger than 35. This does not, however, take away from the fact that maternal age plays an important factor in the development of Down Syndrome.)
Look at things this way. There is no genetic mutation or anything else within a gene pool of an individual with Down Syndrome that can be inherited by subsequent generations. (Not even the genetic shuffle associated with trisomy 21 is considered a genetic mutation that is heritable. It is simply that a mixing of genetic material rather than a genetic marker.)
So, while Down is not a classically inheritable disease, it is still a risk factor as it can be passed on from a parent by virtue of a translocation. This can happen via either the mother or the father, whereby 5% of the cases have been linked to the paternal side. Furthermore, the risk of a translocation happening again is 3% if coming from the father’s side and 10–15% if coming from the mother’s side.
In terms of the mother’s age being a factor—a more advanced age of achieving pregnancy has an increased risk of miscarriage, as well as the child having Down Syndrome, congenital heart defects,hearing loss, or learning disabilities. Mothers who are aged 35 or over are at a higher risk (1 in 350) and those over the age of 40 at an even higher risk (1 in 100) and may wish to have screening tests and prenatal testing. Women who conceive at over the age of 45 have a 1 in 30 chance of having a child with Down Syndrome. It should be noted, however, that the age of the mother has not been linked to the risk of translocation.