Reproductive Genetics

Humans grow and develop based on the instructions contained in our genetic material. A healthy human cell has 46 chromosomes, which are matched into 23 pairs.

At conception, an egg containing 23 chromosomes combines with a sperm containing 23 chromosomes. The two sets of genetic information combine so that the growing embryo has 23 pairs, or 46 total, chromosomes and is a mixture of genes from both biological parents.

Sometimes when an egg or sperm develops, it may have extra or missing chromosomes. When an abnormal egg or sperm is involved in conception, there will not be exactly 46 chromosomes, and the resulting pregnancy will have a chromosome abnormality.

A common example of this is Down syndrome. In Down syndrome, there is an extra copy of chromosome 21, creating a total of 47 chromosomes.

Although familial inherited genetic traits may play a part in how chromosomal abnormalities occur, the most important factor is the age of the female patient. Chances for chromosomal abnormality in a fetus increase with the age of the patient, which is why specialists often recommend prenatal testing for patients 35 years of age or older.

Genetic traits can be passed through families in several distinct patterns. The most common patterns are:

Dominant Genetic Diseases

If one parent has a dominant genetic disease, then each of that person’s children has a 50% chance of inheriting the disease. Dominant diseases can also occur spontaneously; this happens when a random mutation in one gene occurs at conception. Examples of dominant genetic diseases include achondroplasia and Huntington’s disease.

Recessive Genetic Diseases

When both parents are carriers of the same recessive disease, there is a 25% chance that the pregnancy will inherit the trait from both parents and have the disease, a 50% chance that the pregnancy will inherit the trait from only one parent and be a carrier, or a 25% chance that the pregnancy will not inherit the trait from either parent and will not be a carrier or have the disease.

Sex-Linked Genetic Diseases

Sex-linked genetic diseases affect males and females differently. This is a smaller set of diseases related to gene mutations on the X or Y chromosome. Some common examples include hemophilia and color-blindness.

Multifactorial Traits

Multifactorial traits occur due to many complex interactions and combinations of maternal, environmental, and genetic influences. The most common congenital abnormalities are members of this category, including congenital heart defects, cleft lip, and spina bifida.

Chromosome Abnormalities

Chromosome abnormalities can occur in any pregnancy. Sometimes these risks are related to the parent’s age and involve extra or missing chromosomes. A second type of chromosome abnormality is a change in the structure or organization of the chromosomes. These changes include deletions (a small missing piece), inversions (a piece flipped upside down), insertions (an added piece), or translocations (exchange of pieces involving two or more chromosomes). Structural changes are not associated with maternal age. They can be passed from parent to child in many complex patterns and can be the cause of multiple unexplained miscarriages, congenital abnormalities, or apparent infertility.