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Genetics 101: The Basics of Your Genetic Library

A blog by Kara Nguyen, MD, MPH, RMA of Central Pennsylvania at PinnacleHealth, October 14, 2015

We have all heard about “genes”, “DNA”, and “chromosomes”. These are terms your doctor may use when explaining tests that may be ordered, problems that may explain a patient’s difficulty getting pregnant or why some patients have miscarriages. Without any background knowledge genetics can be confusing! Understanding genetics can be simplified into a library analogy…

We each are made up of a library of bookshelves, books and words that determine our individual characteristics. In general, we all got 23 bookshelves from our mother and 23 bookshelves from our father so that each of us has a total of 46 bookshelves. Bookshelves are like chromosomes. On each bookshelf are a unique set of books, or genes. These books are made of words, or the genetic code. Our library is neatly organized in the cells of our body. When cells grow and divide, this entire library has to be duplicated by a sophisticated typewriter.

When there is a mutation, it means there is a typo, missing sentence, or extra sentence in one of the books. Sometimes those mutations go unnoticed and cause no problems. Sometimes the mutation causes a critical change and the gene or book doesn’t make sense. These mutations cause diseases. In conditions such as cystic fibrosis, a person can simply have a typo in one book and it causes no problem but if a person inherited 2 typos in the same book from both their mother and father it causes the disease. Mutations in the cystic fibrosis gene are so common (1 in 24 Caucasians) that it is recommended to offer genetic testing to see if a patient is a carrier of cystic fibrosis mutations before conceiving to determine their risk of having an affected child.

The most common genetic disorders are when there are extra or missing bookshelves. This is the cause of the majority of miscarriages. Some however are viable such as Trisomy 21, or Down syndrome. In Down syndrome, the baby inherited an extra copy of the entire “bookshelf 21”. Missing bookshelves are much more severe and don’t generally survive.

If a patient has experienced recurrent pregnancy loss, it is common to do genetic testing to see if the pregnancy was chromosomally imbalanced. The genetic testing usually done is a karyotype which is a spread of all the chromosomes to make sure there is the right number and right arrangement. Chromosomal imbalances may mean missing bookshelves, extra bookshelves, or individual shelves that have been rearranged onto the wrong bookshelf. Karyotypes do not determine if there has been a typo in a book for example. Some patients have all the books needed in their library but they are arranged on different bookshelves. It causes few or no problems but when these rearranged bookshelves are shared to a pregnancy, the pregnancy inherits an unequal number of books and becomes imbalanced.

We all have typos! It is normal to have typos and most don’t cause any problems. There is no genetic test that identifies all the typos a person may have. However, your doctor can determine the genetic tests that are most appropriate given the clinical situation and which ones are most likely to cause a problem.


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