Genetic Carrier Screening
Genetic screening is the process where a population is tested for changes in DNA (the sequence of genes that comprise who we are) that can potentially be passed on to offspring and result in a genetic disorder. Genetic screening is typically accomplished through a blood draw or cheek swab. The sample is then screened for specific changes in the DNA sequence (DNA mutations) that have been associated with a particular genetic disease. Genetic testing is typically performed on both the female and male partners that will be contributing to the resulting pregnancy.
There are over 30,000 genes that comprise the human genome. Individuals carry two copies of the vast majority of genes that comprise the genome. In some cases, a DNA mutation in single copy of a gene in either partner can be associated with a genetic disorder. More commonly, both partners are carriers (where they have one unaffected gene and one gene affected by a clinically significant mutation). In either situation, there is a risk to transmit a genetic disorder to the resulting offspring.
Genetic Carrier Screening is offered to patients before they reproduce. This testing will evaluate whether they are a carrier for a condition. With regard to autosomal recessive conditions, both the egg and the sperm have to be carriers of a condition to have a 25% risk of having a child affected with this autosomal recessive disorder. An example of an autosomal recessive disorder is cystic fibrosis.
Pre-Genetic Testing of the Embryo
When there is a recognized risk of having an affected child through genetic screening, it may be possible to prevent transmission through testing of blastocysts (a process known as pre-implantation genetic testing for monogenic disorders or PGT-M). This involves biopsy of day 5-6 embryos from IVF and testing the embryos for their unaffected, carrier, and fully affected status for the genetic disorder in question. Unaffected or carrier embryos can then be preferentially transferred to avoid having an affected child or to eliminate future transmission of the disorder in question.
Pre-Genetic Testing (PGT) is when 5-10 cells of the day 5-7 embryo (blastocyst stage) are biopsied and evaluated to determine if the embryo is 23 chromosome competent. The cells that are biopsied are the cells that make up the cells for the placenta, not the cells that contribute to the fetus. It is not regarded at detrimental to the embryo.