What are chromosomes?
Chromosomes are structures made up of our genes. We have about 20,000 genes that need to fit inside every one of our cells. Our genes package themselves into chromosomes in order to fit inside our cells. I like to think of a chromosome as a string with beads on it. The beads represent our genes. If you hold one end of the string and twist the other end, it will coil - this is similar to how genes become chromosomes. If we unravel a chromosome, we get an extremely long piece or string of our genes.
Typically, we have a total of 46 chromosomes. Our chromosomes come in pairs, so we have 23 pairs in total. We inherit one chromosome from our mom/egg donor and the other from our dad/sperm donor. We label the first 22 pairs based on their size. If you look at the picture below, you can see that chromosome pair labeled one is the largest sized chromosomes and chromosome pair 22 are the smallest. The last pair our sex chromosomes. Two X chromosomes typically determine female reproductive organs and an X and Y chromosome determines male. Genes on the Y chromosome are what tell the developing human to create male genitalia; otherwise, the default is female reproductive organs.
What happens if there is not the typical number of chromosomes?
On each chromosome, there are roughly 500-1000 genes. Genes are what tell our bodies how to grow and develop. Genes are considered the instruction manual of our body. If someone is missing a chromosome or has an extra one, then they are missing or have extra genes. With extra or not enough genes, a person may not grow and develop as is typically expected. They may have physical or intellectual differences, depending on which chromosomes are extra or missing.
One common example of having an extra chromosome is Down syndrome. Most of the time, individuals with Down syndrome have an extra chromosome, so they have a total of 47 chromosomes. The extra chromosome is the reason for the unique features you see for someone with Down syndrome.
How does a pregnancy get an extra or missing chromosome?
An extra chromosome can come from the egg or sperm. Our eggs and sperm have 23 chromosomes in them because when we go on to have children, we give half of our genetic information, not all of it. By chance, an egg or sperm cell could have 24 or 21 chromosomes which would lead to a pregnancy with too many or too few chromosomes. Because of the chromosomal difference, the pregnancy may lead to miscarriage. Chromosomal abnormalities are a common cause of first trimester miscarriage. The pregnancy may also lead to a baby with a chromosomal condition. It is usually by chance that a pregnancy has an extra or missing chromosome and it isn’t anything that the person did to cause it to happen.
Chromosomes can also have small pieces of them missing or extra. These are called deletions or duplications, and usually only mean a few genes are too few or too many. Even having a few genes extra or missing genes can be enough to cause certain features like intellectual disability or a birth defect. Although, there are times that a person has no symptoms related to having a deletion or duplication.
How can you know if someone or a pregnancy has extra or missing chromosomes?
Since it is mostly by chance that a chromosomal condition occurs for a pregnancy, screening and diagnostic tests are offered for every pregnancy. These include: multiple marker screening, cell-free DNA screening, chorionic villi sampling, and amniocentesis. These tests all focus on the pregnancy’s chromosomes.
Most people after being born do not have their chromosomes evaluated and are assumed to have the 46 chromosomes with all of the pieces. However, someone may have a chromosomal analysis if they have a personal history of developmental delays, autism, a birth defect, mental illness, a history of recurring miscarriages, or a pregnancy that had a chromosomal abnormality.
When someone has their chromosomes looked at, it is done through a blood draw and either a karyotype or microarray is ordered. The above picture is called a karyotype which is a snapshot of our chromosomes. As you can see, there are the 23 pairs of chromosomes lined up. This would be a for someone considered genetically male since there is one X and one Y chromosome.
If you are curious about the testing available in pregnancy, check out this informational page on Genetic Testing Available in Pregnancy.
* This blog constitutes general information about genetic testing and medical screening. This blog does not offer or provide medical advice or diagnosis, and nothing in this blog should be construed as medical advice or diagnosis. Do not rely on the information in this blog/article to make medical management decisions. Please consult with a medical professional before making those decisions. Do not delay in seeking professional medical advice if you think you have a medical concern. Do not disregard professional medical advice based on any information received in this blog.
