Trisomy 21 look like in a karyotype as B. extra chromosome 21
In the first part of the twentieth century, there was much speculation of the cause of Down syndrome. In 1959 Jerome Lejeune and Patricia Jacobs, who working independently, first determined the cause to be trisomy (triplication) of the 21st chromosome. Down syndrome (DS / DNS), also known as trisomy 21 is a genetic disorder caused by the presence of all or part of a third copy of chromosome 21. Trisomy 21 is the most frequent viable chromosome disease. In Down syndrome, 95% of all cases are caused by the condition in which one cell has two 21st chromosomes instead of one, so the resulting fertilized egg has three 21st chromosomes. Hence it named as the scientific name trisomy 21.
Three to four percent of all cases of trisomy 21 are due to Robertsonian Translocation in which two breaks occur in separate chromosomes, usually the 14th and 21st chromosomes. There is rearrangement of the genetic material so that some of the 14th chromosome is replaced by extra 21st chromosome. Picture below showed the the karyotype of a male with trisomy 21.
Grade: 9
Subject: Biology
Chapter: Karyotypes
Keywords: Karyotypes
