47 Chromosomes?

In cells of a typical human being, there are 46 chromosomes.  There are 24 different chromosomes.  In females, there are two nearly identical copies of 23 different chromosomes.  In males, there are two nearly identical copies of 22 different chromosomes, plus a copy of the X chromosome and a copy of the Y chromosome.  The exception is in sex cells such as sperm and ovum where there are only one copy of each:  22 plus one X or Y in the sperm, and 22 plus the X in the ovum.

When cells carrying 46 chromosome divide, each of the two daughter cells end up with 46 chromosomes again after mitosis is completed.  These types of cells are also known as somatic cells, and since they carry two copies of each chromosome, they are also called diploids.

The sperm and ovum sex cells are derived from meiosis.  They started with 46 chromosomes but have undergone two rounds of cell division, generating four haploid cells, each with 23 chromosomes.  When the sperm fertilizes ovum, the 23 chromosomes they each bring are combined.  The beginning of another human being starts with 46 chromosomes.

So, why do individuals with Down Syndrome end up with 47 chromosomes?  During meiosis that generated the sperm and ova, the chromosomes line up but do not segregate equally into the daughter cells.  One cell (either the sperm or the ovum) ended up both copies of chromosome #21 and with 24 chromosomes total.  The fertilized egg now has 47 chromosomes.  The hallmark of Down Syndrome is having 47 chromosomes, a third copy of chromosome #21, in the cells of the individual.

When I was pregnant, people often asked me if I knew the sex of the baby.  Like most people, I would respond that I didn't really care.  I would then add that as long as the baby had 10 fingers and 10 toes, it didn't really matter.

Then, Olivia was born.  She was born with 10 fingers, 10 toes, but each cell in her little body had 47 chromosomes.  That extra 21st chromosome led to her to have one of the most common genetic disorders.