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Genetic Screening and Recessive Inheritance

Genes produce the instructions for our bodies’ growth and function. Our genes come in pairs; one of each pair is inherited from each of our biological parents. Recessive genetic diseases are those that result when both copies of a gene that a person has inherited have been altered or changed in a way to make the genes non-working. Everyone carries a few of these altered genes, and each ethnic group has certain genes that are more likely to be non-working.

A child who inherits an altered or non-working gene from only one biological parent will be an unaffected carrier for the condition. Carriers function normally, usually exhibiting no symptoms of the disease, but they can pass the carrier gene on to their own offspring.

When both biological parents are carriers for the same recessive condition, each pregnancy has exactly the same risks.

  • There is a 25% chance of inheriting no non-working genes (unaffected and non-carrier).
  • There is a 50% chance of inheriting one non-working gene and being an unaffected carrier.
  • There is a 25% chance of inheriting the non-working gene from both biological parents and being affected with the disease.


When only one biological parent is a carrier, the child has (virtually) no chance of being affected with the disease, but there is a 50% chance of being a (unaffected) carrier like the parent.

Effective June 2021, TSBC program donors are screened for a large panel of recessive conditions called Expanded Carrier Screening. Program donors receive genetic counseling prior to testing to review the risks, benefits and limitations of Expanded Carrier Screening. If a TSBC program donor is found to be a carrier for a recessive disease, he will receive additional genetic counseling. Non-identifying Expanded Carrier Screening results are shared with recipients so they can make an informed decision when selecting a donor. If a program donor is found to be a carrier, we recommend that his recipient receive genetic counseling and consider genetic testing to determine her own carrier status.