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 to the right 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, including things such as hair color, eye color, and height. 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 from 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 that 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 instructions (genes) 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 other family members.

Many of the genetic conditions that are discussed in prenatal genetics are due to a baby having an extra (called a trisomy) or missing (called a monosomy) chromosome. An important thing to remember is that we have no control over how many chromosomes are packaged in our egg or sperm cells. If a baby has an extra or missing chromosome, there is nothing that was done to cause it and nothing that could have been done to prevent it.

Other types of genetic conditions involve changes in the instructions (genes) that are on the chromosomes. Some of the more common genetic conditions involving a single gene can be categorized into the three 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 to understand how genetic conditions may run in a family.

Because each gene does a specific job in the body, if someone inherits a non-working gene(s), what job that gene(s) does in the body generally determines what the genetic condition will be. For example, if someone inherits a non-working copy of a gene that is responsible for helping to build bone, then that person may have a genetic condition that causes differences with the bone development, while someone who inherits a non-working copy of a gene that helps to form structures in the ears may be hard of hearing.