Recessive Inheritance & Genetic Screening of Egg and Sperm Donors
We all have two copies of our genetic material – one we inherit from our mother’s egg and the other we inherit from our father’s sperm. Genetic screening searches for mutations, which are errors or changes in the DNA within a gene that may cause the gene not to function correctly. Most genetic diseases result from mutations in both the gene stemming from the egg and that from the sperm. If an individual has only one copy containing a mutation, they are termed a carrier of that mutation. In almost all cases, there is no disease or other issue resulting from being a carrier of a mutation.
A carrier of a mutation is called autosomal recessive. For these recessive diseases traits, having one mutation is not enough to cause symptoms, as only one working copy of the gene is required to be healthy. Most of the population, including donors, are carriers for between 2-5 autosomal recessive diseases, depending on the number of genes tested. For a majority of autosomal recessive diseases, being a carrier is not associated with any health problems.
If both genetic parents are a carrier of the same disease, there is an increased risk of 25% for a child to be born affected with the disease. There is also a 50% chance for a child to be a carrier of the disease who would not be expected to be symptomatic and a 25% chance for a child to inherit two normal copies of the gene and be healthy as well.
What if my egg or and/sperm donor is a carrier of a disease?
It is relatively common for individuals who undergo carrier screening for recessive diseases to find out they are a carrier of one or more conditions because many carrier screens look for many different diseases. When an egg and/or sperm donor is identified to be a carrier of a recessive genetic disease, it does not mean there is anything wrong with the donor, and is typically not a reason to deny a particular donor. Since there are typically no problems caused by carrying only one mutation, egg and/or sperm donors who are carriers are typically healthy and show no signs of the condition. Every person, whether an egg and/or sperm donor or not, has an average of 2.3 recessive mutations.
If you select an egg and/or sperm donor who is a carrier of a recessive genetic disease, each partner should undergo carrier screening for that condition as well. While testing negative for a condition cannot eliminate the possibility that someone could be a carrier, it dramatically reduces the risk. Testing utilizing a method called sequencing is very accurate and can often reduce the risk of someone being a carrier to less than 1%. Generally, if your egg and/or sperm donor is a carrier of a recessive disease and each partner has been tested and is negative, the risk is significantly reduced to have an affected child.
Using an egg and/or sperm donor who is a carrier of a recessive genetic disease does mean that there is a 50% risk of having a child who is a carrier–even if the one partner is negative for that condition and there is a low risk of having an affected child. A child who is a carrier would, like the egg and/or sperm donor, typically be healthy with no symptoms. This is not something that would be concerning for the child’s health. It would be information to share with them as an adult, allowing them to have genetic screening to find out if they may be at risk of having an affected child with their own partner someday.
Are there other types of genetic diseases egg and/or sperm donors are screened for?
Some genetic diseases egg and/or sperm donors are screened for are inherited in another pattern called X-linked. When we consider our DNA on a larger scale, we find that our chromosomes are made up of many genes. Most individuals have 46 chromosomes, and they are found in 23 pairs. We inherit one copy of each chromosome from the egg and one copy from the sperm. The first 22 pairs of chromosomes are the same in males and females (numbered pairs 1-22), while the last pair indicates our biological sex. Females have two copies of an X chromosome, while males have one copy of an X chromosome and one copy of a Y chromosome.
X-linked diseases are diseases resulting from genes found on the X chromosome. For these diseases, males are more likely to be affected because they only have one copy of those genes; females, having two X chromosomes, have one mutated and one normal gene. Some X-linked diseases can affect males and females, but these typically affect males more severely. If egg donors are carriers of X-linked diseases, they would be at 50% risk for an affected male offspring and 50% risk for a female offspring to be a carrier. Egg and Sperm donors that test positive as carriers for X-linked diseases are excluded from being donors at The World Egg and Sperm Bank because of this.
Genetic testing is evolving rapidly. Consult with your physician on specific genetic testing questions or concerns.
