When I hear the word "karyotype", I think about a picture of chromosomes. When somebody has their blood studied to look at how many chromosomes they have and whether the chromosomes are complete, we come up with a picture in which we can line up all the chromosomes and count them. That way we can tell whether or not somebody has all the proper number of chromosomes, which is 46, and that way we can look at the X and the Y chromosomes and determine if it's a female or male. Somebody might order a chromosome study and look at a karyotype if they were worried that a child might have an extra or missing bit of chromosome material. So one of the most common things we can see on karyotyping is an extra Chromosome 21, which is associated with Down syndrome. We also get karyotypes when pregnant women choose to have testing on their unborn fetus, and the karyotype allows the providers to look at and count the chromosomes to determine whether or not the child is affected by having an extra chromosome.Barbara Bowles Biesecker, Ph.D.