Down syndrome is a genetic condition caused by the presence of an extra copy of chromosome 21, which causes morphological, biochemical and functional alterations on different organs (especially the brain) during different stages of life.
The molecular composition of the extra-chromosome 21 determines the structure and function of the brain and nervous system, in addition to influencing the learning capacity and conduct of people with Down syndrome.
The genes contained in the extra-chromosome 21 are also responsible for the features (phenotype) present in trisomic people, who require specialised medical care and preventive medicine programmes designed by experts.
MYTHS AND FACTS ABOUT DOWN SYNDROME
MYTH – It is a disease.
FACT – Down syndrome is a genetic variation caused by the presence of an extra copy of chromosome 21.
MYTH – People with Down syndrome do not live for very long.
FACT – Their current life expectancy is around 65 years of age.
MYTH – They will always be child-like and cannot achieve normal goals in life.
FACT – With proper education and support, the majority learn to read, attend public schools, and live full lives as adults.
MYTH – People with Down syndrome are always happy and affectionate.
FACT – Each individual has their own personality and moods just like anyone else.
TYPES OF DOWN SYNDROME
The most common type of Down syndrome is trisomy 21, which results from a genetic mutation that takes place early in the cell reproduction process.
The pair 21 of the egg or sperm fails to separate correctly and, as a result, some of the gametes contain 24 chromosomes instead of 23.
The combination of one of these gametes carrying an extra chromosome with other of the oppposite sex results in a zygote containing 47 chromosomes. In order for the fetus to develop, the zygote undergoes mitosis, which generates identical cells, that is, cells with 47 chromosomes. Thus, a baby with Down syndrome comes into being. Trisomy 21 is known as regular or free trisomy.
Less frequently, during the process of meiosis, a chromosome 21 breaks off and some of its fragments, or a whole chromosome, attach to another pair, generally the 14.
This means that, apart from the chromosome pair 21, the pair 14 contains extra genetic material from the latter: an entire chromosome 21 or a fragment generated during the process of meiosis.
The new rearranged chromosomes are known as translocated chromosomes, from which the name of this type of Down syndrome derives. It will not be necessary for the chromosome 21 to be completely triplicated in order for these people to manifest the typical features of trisomy 21, as the latter depend on the translocated gene fragment.
Trisomy 21 Mosaicism
Once an egg has been fertilized, the rest of the cells are generated through a process of mitotic cell division. If during said process the genetic material is not correctly separated, it would be possible for one of the daughter cells to have three chromosomes on pair 21 and only one chromosome on the other daughter cell. In that case, the result will be a percentage of trisomic cells (three chromosomes), whereas the rest of them will have the usual genetic material.
Those people with Down syndrome that exhibit this genetic structure are known as “genetic mosaics”, because their bodies possess a mixture of cells of different genetic makeup (genotype).
The physical traits (phenotype) of a person with mosaicism and his or her potential development will depend on the percentage of trisomic cells their organism has, even though they generally have a lower level of cognitive disability.