Making Heads or Tails of Genetic Screening (Part 1)

Choosing whether to perform chromosome abnormality screening during early pregnancy can be a challenging decision for some parents. The question of how the tests will be done is also another concern.

There can also be anxiety associated with what to do if an abnormality is identified and also some uncertainty about what is being tested and the extent to which the test results can be trusted.

I want to shed some light on the options for genetic testing that we offer at Frisco Women’s Health.  In order to make the information a little easier to digest, I’m going to divide the blog into two parts.  This first part will cover some background information about genetics to better understand genetic testing. It will be followed by the second part that will go over the screening tests themselves. My apologies in advance for the density of the material.  For many of you with insomnia, this might be useful as a non-pharmaceutical sleep aid.

While screening options for chromosome abnormalities in your baby is not one of the simplest topics for a blog, I do feel strongly that it could use some additional attention.

There is a barrage of information given to parents with a new pregnancy at their initial visit.  Often, it makes patients feel like they’re trying to drink from a fire hydrant struggling to take in, understand, and remember everything that is being discussed.  After confirming the viability of the pregnancy and the due date, allowing the excitement of the sonogram to settle in, and addressing the questions that patients have, we, as obstetricians, also try to provide some guidance about what to expect from early pregnancy, outline how the prenatal visits will be scheduled, make recommendations about vaccinations, review the lab testing that will be performed, and give advice on common early pregnancy concerns.

Amongst all the counseling that goes on at that first visit, we also ask new parents whether they want to test their pregnancy for certain chromosome abnormalities.  We offer this testing to all patients regardless of age or other risk factors, but the counseling about this particular test can get glossed over when so many different topics are addressed concurrently.

I want to elaborate on what chromosome abnormality screening is and why we care about it.  In the next installment, I’d like to review chromosome abnormality screening options and what we offer at our office and why. The subject of chromosome abnormality screening is challenging to talk about both thoroughly and quickly.  Depending on an individual’s familiarity with genetics, more detail can often be more confusing than less.  Our goal as your providers when counseling about different tests or procedures is to give you, our patients, the information you need to make the decision that you feel is best for you and for your baby.

Chromosome abnormality screening is testing meant to determine if a pregnancy is at high risk or low risk for the baby having an abnormal number of chromosomes specifically having an extra copy of a chromosome or missing one copy a chromosome.

Before we get into more details about the testing, we must first establish a common understanding of basic genetics in order to know what problems we are trying to detect with these screening tests and why these tests are relevant.  Here is my version of genetics in a nutshell…

We begin our journey of life as a single cell when the egg from our mother unites with one of the sperm from our father.  That first cell contains all the instructions needed to make a complex human being that will eventually be composed of tens of trillions of cells.

Chromosomes are the physical structures in each of our cells that carry our genes. Genes are our genetic coding information.  A single gene provides the instructions to make a specific protein or product used in a cell.  Each chromosome contains hundreds of genes.  Think of an individual’s entire set of chromosomes as a library that holds the information needed to make all the components or parts of that person.  In this analogy, each chromosome would be a book filled with hundreds of chapters.  Each of these chapters represents a gene.  A single chapter contains the coding instructions for one specific protein or element of a cell.  Each book in the library contains distinct information from all the others and has a specific number of chapters that cover specific portions of the coding information.  The number of books in the library and the order of the chapters contained in those books is the same for all human beings but the specific coding instructions in the individual chapters vary from person to person.

Every cell in a person’s body contains the entire library but not all the books are used.  For example, cells that make the muscle in the heart may use coding instructions from selected chapters in books 5,8, 17, 25, and cells that make hair may use coding instructions from chapters in books 11 and 14.  In humans, there are 46 books or chromosomes that are the blueprint for making a person.  Half of the books in an individual’s genetic library come from that person’s father’s library and half the books come from a person’s mother’s library.  So, in every genetic library, there are two copies of book one, two copies of book two, two copies of book three, et cetera.

While a person’s two copies of book one have the same number of chapters that cover coding information for the same types of proteins, in the same order, the contents of the chapters themselves are different.  For example, Book Seven, Chapter Three might contain instructions for making a protein that determines a person’s hair color, and a person might inherit instructions for brown pigment protein from one parent and red pigment-protein from the other parent and end up with the instructions for auburn hair.  Clearly, this is an oversimplification, but it illustrates in a general sense of how traits are inherited and why chromosomes are important.

The last point in chromosome abnormality screening is that we, as humans, need exactly two copies of each chromosome.  No more, no less.  This is so important that most pregnancies that are missing a chromosome or have an extra copy of a chromosome will end in early pregnancy failure.  Of pregnancies with an extra chromosome that survive past the first few weeks, the most common types of chromosome problems are an extra copy of chromosome 13, 18, 21, or missing or having an extra chromosome that determines gender.

If the number of copies of chromosomes is so important, how do they get messed up in the first place? Eggs and sperm are made in the ovaries of women and the testes of men respectively. Each egg or sperm only has one copy of books from the person’s genetic library instead of both.  If that weren’t the case, the number of sets of chromosomes in the genetic library would double every generation.  In the ovary of a woman, where eggs are being developed, there are cells dedicated to making eggs.  The genetic library of these precursor cells is split evenly between two eggs.  Before packing up the set of books (chromosomes) that are being given to an egg, the corresponding pair of each book (the one inherited from the woman’s mother and the one inherited from the woman’s father) are laid next to each other.  Random chapters from maternal book one are swapped with the corresponding chapters in a paternal book one.  This occurs with each of the 2 copies of the remaining 22 chromosomes.  Then one copy of the newly rearranged books 1-23 are boxed up and given to the first egg and the other copy of books 1-23 are given to the second egg.  Each one has one complete set of 23 chromosomes but the genes on those chromosomes represent some of what was inherited from the mother’s side and some from the father’s. This process occurs in an identical fashion in a man when his body is making sperm.   The benefit of the swap is to add variety to the gene pool of the population.  The downside is that sometimes one of the boxes ends up with both copies of a book when they are being packed up and the other box is missing that book.  When this occurs, neither of the eggs (or sperm), the one with the extra chromosome or the one missing a chromosome, will result in a pregnancy that will have the right number of chromosomes.  Any pregnancy has the potential to be affected by a random abnormality of chromosome number, but the risk goes up with a mother’s age.

The next installment will cover the types of tests we have that detect random abnormalities in chromosome number and inherited abnormalities in specific genes.

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