Medical Genetics So while the number of chromosomes remain normal there is a triplication of the 21st chromosome material some of these children may only have triplication of part of the 21st chromosome instead of the whole chromosome, which is called a partial trisomy 21Medical Genetics So while the number of chromosomes remain normal, there is a triplication of the 21st chromosome material. Some of these children may only have triplication of part of the 21st chromosome instead of the whole chromosome, which is called a partial trisomy 21