You might know, or have seen somebody with Down syndrome. But in this video, you're going to learn what it is and why it happens. Down syndrome is caused by a change in the DNA. You probably remember that any change in the structure or the amount of DNA of an organism is called a mutation. Dutations are caused by mutagenic agents like some chemicals or ultraviolet light, for example. If a mutation happens in a somatic or body cell, only this cell and its daughter cells will be affected. If a mutation happens in a sex cell or gamete and fertilization happens, then all of the cells of the embryo or fetus and a baby will carry this mutation. This is what happens in Down syndrome. Humans usually have 23 pairs of chromosomes, 46 in total, 44 autosomes and 2 sex chromosomes. Sex cells are produced in the type of cell division called myosis or reduction division. In humans, myosis produces 4 daughter cells that each have 23 chromosomes, half the number of a somatic cell. However, sometimes the chromosomes are not separated or divided up how they should be. And this is called non-disjunction. One example of non-disjunction in humans is the condition known as Down syndrome. In this case, the 21st chromosome fails to segregate and the gamete produced possesses 24 chromosomes. 23 plus an extra copy of chromosome 21. The fusion of this gamete with a normal one results in the offspring having 47 chromosomes. So what are the consequences of having an extra chromosome for people with Down syndrome? Well, the difficulties vary in severity. Facial characteristic can include a flattened, broadened face and narrowed eyes. But people with Down syndrome don't all look the same. They also look like their parents and family. People with Down syndrome may have a lower than average IQ and are shorter than average life expectancy. Some conditions more common in people with Down syndrome include problems with the heart and bowel, difficulties with hearing and vision and a higher risk of infections. However, everybody who has Down syndrome may be affected in different ways. Pablo Pineda, for example, a Spanish actor who won a Silver Sheller Award in the San Sebastian Film Festival, was also the first person with Down syndrome in Europe to get a university degree. He now has two. Non-disjunction does occur with other chromosomes, but these usually result in miscarriage. This is less likely to happen with a 21st chromosome because it is relatively small. The Down syndrome mutation appears to occur in the production of over rather than sperm, and its incidence increases with the age of the mother. The chance increases from approximately 1 in several thousand in a teenage mother to 1 in 100 in a 40-year-old mother. In some countries, people who are pregnant are offered diagnostic genetic testing for Down syndrome. This can be done by blood tests or amniocentosis, which involves taking a sample of the amniotic fluid surrounding the fetus in the uterus and looking at the number of chromosomes in those cells to see if any have the extra chromosome 21 for Down syndrome. Having the tests may be the choice of the parents and will allow them to be more prepared for a baby with Down syndrome. Or choose to have an abortion. In Iceland, this type of genetic screening is widespread, and people are counseled when they have the test. Most choose abortion. This means that almost nobody is born with Down syndrome in Iceland. What do you think about this? The estimated incidence of Down syndrome is about 1 in 1000 births worldwide. There are about 15,000 births per hour in the world. So how many people are born with Down syndrome each day in the world? Pause the video while you work it out. So now you know a little more about what Down syndrome is, what causes it and just how common it is.