Genetics 101

Genetics 101: What Causes Genetic Defects?

In cells are string-like structures called chromosomes, which contain all of our genetic material, called genes. Each cell has approximately 30,000 genes. In turn, genes are made up of sequences of DNA (deoxyribonucleic acid), the strands of molecules that give each of us our unique characteristics.

• Too many or too few chromosomes (aneuploidy)
• Chromosome pieces attached to the wrong chromosome (translocation)
• A missing piece of a chromosome (deletion)
• Part of a chromosome is upside down (inversion)
• A change in the gene's DNA sequence (mutation)

Chromosome Abnormalities

Abnormal genes/mutations are the primary cause of genetic disease. Most cells contain 23 pairs or 46 chromosomes. These include chromosomes 1 to 22 (the autosomes) and chromosomes X and Y (the sex chromosomes). Sperm and eggs only contain 23 single chromosomes (one from each chromosome pair).

During fertilization, the embryo receives one chromosome of each pair from each parent resulting in a normal male (46,XY) or a normal female (46,XX). If the sperm or egg harbors a gene mutation the embryo might carry a genetic disorder.

In the IVF setting and in natural conception, chromosomally abnormal embryos have a low implantation rate. If they do implant, the pregnancy often results in miscarriage or the birth of a baby with physical problems, developmental delay, or mental retardation. Chromosome disorders occur in well over half of all first trimester pregnancy losses.

Down syndrome is usually associated with advanced maternal age and is a common example of aneuploidy. Down syndrome is caused by having an extra number-21 chromosome (three instead of two). It is also referred to as trisomy 21.

Single-Gene Abnormalities

Single gene abnormalities (mutations) are caused by changes in the DNA sequence of a gene. Genes produce proteins that allow cells to work properly. Single gene disorders usually show a characteristic family history of a specific genetic disease.

Gene mutations alter the cells’ normal function due to a lack of a required protein. For example, cystic fibrosis (CF) is a common genetic disorder that primarily affects the lungs of CF patients. The CF mutation affects a protein within the cell that reduces the cell’s ability to function properly. This results in a buildup of mucus within the lungs, lung dysfunction, and possible death.