Genetics 101

To the Point– Our bodies are made up of billions of cells. In our cells there are structures called chromosomes that are made up of individual instructions called genes.  Genes tell our bodies how to grow and function. We have 46 total chromosomes or 23 pairs; half from mom and half from dad. To grow and function as usual, it is important to have all 46 chromosomes and the corresponding genes working as usual or genetic conditions can happen. 

The birds and the bees and how we pass on our genes….

As you know, our bodies are made up of billions of cells.  We have skin cells, heart cells, brain cells, etc.  These cells all have special functions in our bodies, but one thing they have in common is that each cell has a set of 46 chromosomes, or 23 pairs. The first 22 pairs of chromosomes are the same in men and women and the 23rd pair is different.  Women have two ‘X’ chromosomes for their 23rd pair, and men have one ‘X’ chromosome and one ‘Y’ chromosome. (See picture below of a set of male/female chromosomes, also called a karyotype.)


Along the chromosomes are the individual instructions, or genes, that tell our bodies how to grow and function; instructions for things such as hair color, eye color, height, etc.  Since we have two copies of every chromosome, we also have two copies of every gene.  All in all it is estimated that we each have about 20,000 pairs of genes, one set form our mother and one set from our father.   Males only have one X chromosome though, so they only have one copy of all of the genes on the X chromosome, and one copy of the genes on the Y chromosome. Genes are specific instructions which contain our genetic code or DNA (deoxyribonucleic acid).  DNA is made up of four similar chemicals, called bases: adenine (A), thymine (T), cytosine (C ), and guanine (G).  The specific order of these letters (bases) is what makes the genes (instructions) work properly.  If one or more of these letters is changed, deleted, or duplicated and this change causes the gene to not work as usual, this is called a mutation.  Mutations in our genes may lead to genetic conditions in ourselves or our children.

In order for our bodies to grow and function as usual, it is important that we have the typical 46 chromosomes.  It is also critical that we have only two of each.  Many of the genetic conditions that are discussed in prenatal genetics are due to a baby having an extra (trisomy) or missing (monosomy) chromosome.  An important thing to remember is that we have no control over how many chromosomes are packaged in our egg or sperm. If a baby has an extra or missing chromosome, there is nothing that could have done to cause or prevent it.

Other types of genetic conditions involve changes at the gene level, not the chromosome level.  Some of the more common genetic conditions involving a single gene can be categorized into 3 main ways that they can be passed down in a family: autosomal recessive, autosomal dominant and X-linked.  These three terms are also referred to as “patterns of inheritance” and understanding the basics behind these terms will help you learn about the chances of your baby having certain genetic conditions. Click on the links above for more information.