There are different types of Trisomy matches, the number that follows indicates which chromosome has a set of three chromosomes instead of just a pair of chromosomes. In the case of Down Syndrome, the chromosome that is tripled instead of just duplicated is chromosome 21, hence Trisomy 21. (http://www.stanfordchildrens.org/en/topic/default?id=down-syndrome-trisomy-21-90-P02356)
This post will be the first of a set of 3 blogs based on Down Syndrome so that we can learn more about it together. Please ask questions so I can hopefully address it in one of the later blog posts.
People with Down Syndrome generally have similar physical characteristics. The following diagram demonstrates the physical (inside and outside the body) characteristics associated with Trisomy 21. It also shows the chromosomal pairs that we have as humans while highlighting the set of chromosomes that differentiate people who have Down Syndrome from those who do not. Down Syndrome children typically develop slower than their peers who are without the syndrome.
Chromosomes become tripled during reproduction. Usually the father provides 23 single chromosomes and the mother provides the other half to total 23 pairs of chromosomes, 46 individual chromosomes. When Trisomy 21 occurs, the mother or father supply 2 chromosome 21 to pair with the individual chromosome 21 of their partners, thus resulting in 3 copies of chromosome 21.
The likelihood of parents having a child with down syndrome increases as the mother ages. As demonstrated in the following chart, the frequency of live births of babies with Down Syndrome get significantly more likely each year a mother has a baby past 30 years old.
| Age (years) | Frequency of Fetuses with Down Syndrome to Normal Fetuses at 16 weeks of pregnancy | Frequency of Live Births of Babies with Down Syndrome to Normal Births |
|---|---|---|
| 15 - 19 | ---- | 1 / 1250 |
| 20 - 24 | ---- | 1 / 1400 |
| 25 - 29 | ---- | 1 / 1100 |
| 30 - 31 | ---- | 1 / 900 |
| 32 | ---- | 1 / 750 |
| 33 | 1 / 420 | 1 / 625 |
| 34 | 1 / 325 | 1 / 500 |
| 35 | 1 / 250 | 1 / 350 |
| 36 | 1 / 200 | 1 / 275 |
| 37 | 1 / 150 | 1 / 225 |
| 38 | 1 / 120 | 1 / 175 |
| 39 | 1 / 100 | 1 / 140 |
| 40 | 1 / 75 | 1 / 100 |
| 41 | 1 / 60 | 1 / 85 |
| 42 | 1 / 45 | 1 / 65 |
| 43 | 1 / 35 | 1 / 50 |
| 44 | 1 / 30 | 1 / 40 |
| 45 and older | 1 / 20 | 1 / 25 |
This chart makes me question how many parents terminate their pregnancy, or have still-births, as they gain the knowledge about their Down Syndrome baby. I wonder this because the fetus' with Down Syndrome are more frequent than that of live births of babies with the syndrome. If we look at age 40, the likelihood of having a fetus with Down Syndrome is 1/75, but the births of baby's with Down Syndrome is 1/100. That shows that there are more fetus' with the syndrome than babies. Due to the personal decision of terminating pregnancies it is difficult to find out whether my opinion is correct or incorrect.
Other very informative sites for further information can be found by clicking the following links:
Keep your eyes open for the next post which will be focusing on the tests that can be done in regards to Down Syndrome. For instance, there is a test that can be done during pregnancy that determines whether their baby may or may not have Trisomy 21.
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