Chromosomal Analysis
This page will dive into what is a chromosomal analysis, who might be offered one, and how the information can be of use.
What are chromosomes?
Let’s first discuss what chromosomes are before diving into how they can be analyzed.
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 pictures, 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.
genetic male karyotype - note the Y chromosome
genetic female karyotype - note the two X chromosomes and no Y chromosome
How are chromosomes assessed?
There are a few ways chromosomes can be assessed, here we will discuss the two more common assessments for adults.
Karyotype
A karyotype is performed via a blood test which counts the number of chromosomes someone has as well as checks for any structural differences or rearrangements. The above pictures are karyotypes. It is a snapshot of what the chromosomes look like under a microscope after they have been lined up and organized.
Microarray
A microarray is also performed via a blood test. It can also count the number of chromosomes but is better able to detect small pieces of the chromosome that may be extra or missing, called microduplications and microdeletions, respectively. Unlike the karyotype, it is not able to detect structural differences or rearrangements of the chromosomes.
Why would someone have a karyotype done?
A karyotype can be performed when someone has had multiple miscarriages, a previous pregnancy with a chromosomal abnormality, or unexplained infertility.
The karyotype may provide better understanding as to why someone has had the above history. The karyotype may reveal a chromosomal rearrangement or translocation. This means the person has the typical amount of chromosomes, but they are arranged in a unique way. Sometimes, a piece of one chromosome is stuck onto another chromosome and vice versa which is called a translocation shown in the pictures below. Therefore, when that person goes to conceive a pregnancy, they have an increased chance to “pass on” chromosomal abnormalities. If a pregnancy is conceived with a chromosomal abnormality, then a miscarriage can result and may explain why someone has a history of recurrent miscarriages.
Why would someone have a microarray done?
A microarray is not intended to discover if someone has a chromosomal rearrangement like a translocation. It is intended to identify both if full chromosomes are extra or missing as well as small pieces of the chromosome that may be missing or extra, called microdeletions and microduplications, respectively. The karyotype can detect large deletions and duplications but will miss small ones. The microarray may be performed if someone is suspected to have a deletion or duplication either because of symptoms such as autism, mental health concerns, multiple birth defects, or if other testing suggested there is the possibility.
Typical karyotype with chromosome pairs 9 and 11 emphasized. Chromosome 9 DNA is represented by the dark pink and chromosome 11 is represented by the blue.
Karyotype with translocation of chromosome 9 and 11. Notice the blue on chromosome 9 that came from chromosome 11 and the dark pink on chromosome 11 that came from chromosome 9. None of the chromosome material is lost, it is just now in a new location.
What would someone do if they have a chromosomal rearrangement?
If someone is found to have a chromosomal rearrangement, or translocation, then they may have a higher chance to conceive a pregnancy with a chromosomal abnormality. They should meet with a healthcare provider to discuss the next best steps for them. Some of the possible options that can be discussed include:
Conceive spontaneously, knowing there is a chance of a miscarriage or a baby with the unbalanced version of the translocation. There is also the chance the baby has the typical amount of chromosomes.
During pregnancy, diagnostic testing such as amniocentesis or CVS can be considered to evaluate the pregnancy’s chromosomes. Testing after birth can be considered as well.
Conceive with in vitro fertilization and have the embryos tested with preimplantation genetic testing for structural rearrangements (PGT-SR) to reduce the chance of transferring an embryo with a chromosomal abnormality
Conceive with in vitro fertilization using egg or sperm donor
Consider adoption
Consider being childfree
What other considerations are there for chromosomal analysis?
This page primarily focused on the karyotype and when someone can consider this assessment and what to do if they are identified to have a rearrangement. There are other types of chromosomal assessments such as the fluorescence in situ hybridization test or FISH test as well as the Y micro deletion assessment for males. These topics are covered in handouts and in future blog posts. The intention of this page is to bring to light the more common chromosome analysis of an adult in their reproductive years. If you have questions, please contact your healthcare provider and feel free to reach out to genetics@modernreproduction.org.
Resources:
Rull, Kristiina et al. “Genetics of recurrent miscarriage: challenges, current knowledge, future directions.” Frontiers in genetics vol. 3 34. 19 Mar. 2012, doi:10.3389/fgene.2012.00034