Basic Genetics provides a brief overview of genetics including DNA, cells, what a genetic mutation is and inheritance patterns. The genes we inherit from our biological parents explain many things about our bodies, from our hair and eye color to the length of our toes. In the case of Lynch Syndrome, inherited genetic changes, called mutations, increase our risk of certain types of cancer. But what is a gene? And how are genetic traits passed down from one generation to the next?
Every cell in our body contains hereditary (inherited/passed down from parent to child) material known as deoxyribonucleic acid (DNA). This DNA comes in packages called chromosomes.
Our cells typically contain 23 pairs of chromosomes. For each pair, we inherit one from our father and one from our mother.
A long piece of DNA is called a gene.
Genes contain the instructions necessary to build and maintain our bodies. Based on the underlying DNA “code,” genes tell our bodies what proteins to produce. Proteins, in turn, provide the structure of our bodies and play key roles in regulating bodily function.
Each chromosome consists of many genes. And since we have two copies of each chromosome—one from each parent—we also have two copies of each gene. An exception to this involves what are known as the “sex chromosomes” (the X and Y chromosomes). Women normally have two X chromosomes and men normally have one X chromosome and one Y chromosome. This means that for genes on the X chromosome, women have two copies but men have only one.
A mutation in a gene refers to a change in the underlying DNA code that makes the gene not function properly. If you think of DNA as spelling out a word, a mutation involves a change in the letters of the word. This change may involve a substitution of letters, or an addition or loss of letters.
When the underlying DNA code changes, protein production can also change. A gene may create a different protein than it’s supposed to, it may create the wrong amount of protein, or it may not make a protein at all. Any of these changes can affect how the body functions.
Gene mutations are either germline or somatic. Germline mutations are present in the egg or sperm cells that make us. These mutations are often inherited from our parents, but can also occur for the first time in us. Germline mutations are present in every cell in our body, and we can pass them on to our children. Somatic mutations, in contrast, develop in body cells over the course of life, but do not involve the egg or sperm cells. These mutations may cause us to develop health problems such as cancer, but we will not pass the mutation on to our children.
Lynch Syndrome involves inherited (germline) mutations.
Cancer is caused by DNA damage (i.e., mutations) in genes that regulate cell growth and division.
Some mutations are inherited, while others are caused by exposure to radiation or to mutation-inducing chemicals such as those found in cigarette smoke. Mutations also can occur spontaneously as a result of mistakes that are made when a cell duplicates its DNA molecules prior to cell division.
When cells acquire mutations in specific genes, like those associated with Lynch syndrome, that control proliferation, such as proto-oncogenes or tumor suppressor genes, these changes are copied with each new generation of cells. Later, more mutations in these altered cells can lead to uncontrolled cell replication and the onset of cancer.
For inherited conditions that can be traced to a single gene, the patterns of inheritance are fairly straightforward. Depending on the gene, common patterns of inheritance are autosomal dominant, autosomal recessive, X-linked and mitochondrial.
Autosomal Dominant Inheritance
With autosomal dominant inheritance, a person only needs to inherit one copy of the abnormal gene in order to develop the condition. Lynch Syndrome is an example of an autosomal dominant condition. A parent with an autosomal dominant disorder has 50/50 chance of passing the disorder to each of his or her children.
Autosomal Recessive Inheritance
With autosomal recessive conditions, a person needs to inherit abnormal copies of the gene from both parents in order to develop the condition. If the person inherits the abnormal gene from only one parent, the person is a carrier of the mutation (and can pass it on to their children), but does not develop the health problems associated with the mutation. If two carriers of the same autosomal recessive condition have a child together, there’s a 25 percent chance that their child will have the condition, a 50 percent chance that the child will be a carrier, and a 25 percent chance that the child will not inherit any copies of the abnormal gene. This risk applies to each pregnancy. Examples of autosomal recessive conditions include cystic fibrosis and sickle cell disease.
In X-linked inherited conditions, the abnormal gene is located on the X chromosome. Because women have two copies of the X chromosome and men have one X and one Y chromosome, mutations on the X chromosome can affect men and women differently. Duchenne muscular dystrophy, which is the most common form of muscular dystrophy, is an example of an X-linked condition.
For some inherited genetic conditions, the condition has been well defined, tests are available, and there are options to manage disease risk. For many other conditions that appear to have a hereditary basis, however, the underlying genetic causes are still poorly understood. Ongoing research in this area will allow individuals and families to better understand and manage their health.
Reference: Genetics Home Reference. Handbook: Help Me Understand Genetics. Published by the Lister Hill National Center for Biomedical Communications, US National Library of Medicine, National Institutes of Health, Department of Health & Human Services. June 4, 2012.