What is down syndrome?

What Is Down Syndrome?
Down Syndrome is the world’s most common chromosomal disorder and a cause of intellectual disability. It occurs in one of every 700 to 900 births worldwide and affects people of all ethnic and social backgrounds.
Three types of genetic variation that cause Down Syndrome:
1)Trisomy 21
• Presence of extra chromosome 21 in all cells of an individual
• Extra chromosome originates in the development of either the egg or sperm
• When egg and sperm unite to form fertilized egg, 3 chromosome 21 are present
• As embryo develops, extra chromosome is repeated in every cell
• A condition where 3 copies of chromosome 21 are present in all cells of an individual is known as trisomy 21
2) Mosaic trisomy 21
• Similar to trisomy 21 but here the extra chromosome 21 is present in some but not all the cells of an individual
• An individual with Down Syndrome due to mosaic trisomy 21 will have 46 chromosomes in some cells but will have 47 chromosomes(including an extra chromosome 21) in others
3)Translocation Trisomy 21
• In the earlier two conditions, Down Syndrome occurs because some or all the cells have 47 chromosomes
• However there are some individuals with Down Syndrome having cells containing only 46 chromosomes
• This is because material from one chromosome 21 gets stuck or is traslocated onto another chromosome
• In such situations, cells from individuals with Down Syndrome have 2 normal chromosome 21 but also have additional chromosome 21 material on the traslocated chromosome
• Since there is too much material from chromosome 21 this results in features associated with Down Syndrome

Down Syndrome affects but does not determine development. People with Down Syndrome are unique, with their own talents, abilities, thoughts and interest. Everyone with Down Syndrome will experience some delay in all areas of development and some degree of learning disability. This however varies significantly from individual to individual.