Evaluating embryo screening

Embryo screening tests cells from early embryos for particular genetic variants before an embryo is placed in the uterus. It is usually carried out during in vitro fertilisation (IVF), when embryos are created outside the body. The results can help people decide which embryos to transfer.

Screening can reduce the risk of a child having a particular inherited disorder by identifying embryos with a genetic variant associated with it. However, testing is not guaranteed to give a correct result, and transferring an embryo does not guarantee a pregnancy or a particular outcome.

Weighing different considerations

An informed judgement about embryo screening weighs evidence about its benefits and limitations alongside different viewpoints. The same outcome may be valued differently by different people.

• Economic: Testing and fertility treatment cost money. If healthcare services fund screening, those resources are less available for other forms of care.
• Social: Screening may help families reduce the risk of a particular inherited disorder, but access may be unequal. Selecting against embryos with certain variants may also contribute to stigma towards people living with those genetic conditions.
• Ethical: Screening can give prospective parents more reproductive choice. However, people hold different beliefs about creating and selecting embryos, and about storing or disposing of embryos that are not transferred.

A judgement depends partly on how much importance a person gives to each consideration. For example, someone may place greater weight on reducing the risk of an inherited disorder, while someone else may give greater weight to concerns about selecting embryos.

Polydactyly

Polydactyly is the presence of extra fingers or toes. It can be caused by a dominant allele. In a simple single-gene model, a person needs only one copy of this allele to have the condition.

A person inherits one allele for the gene from each parent. Let $$P$$ represent the dominant allele that causes polydactyly and $$p$$ the recessive allele. A person with the alleles $$Pp$$ has polydactyly, but can pass either $$P$$ or $$p$$ to a child.

An affected parent therefore does not necessarily have an affected child. For example, if the affected parent passes on $$p$$ and the other parent also passes on $$p$$, the child will have $$pp$$ and will not have polydactyly in this model.

Cystic fibrosis

Cystic fibrosis is an inherited disorder in which thick, sticky mucus builds up, particularly in the lungs and digestive system. It is caused by disease-causing alleles of the CFTR gene. An allele is a version of a gene.

Inheritance of cystic fibrosis

The disease-causing CFTR alleles are recessive. A person has cystic fibrosis when they inherit a disease-causing allele from each parent. A person with one disease-causing allele and one functioning allele is usually an unaffected carrier. A carrier does not usually have the disorder but can pass the disease-causing allele to a child.

CFTR and mucus

The CFTR gene provides instructions for making a protein involved in transporting chloride ions, $$\ce{Cl-}$$, across cell membranes. In cystic fibrosis, this chloride transport is impaired. This disrupts the movement of water and contributes to the formation of thick, sticky mucus.

In the lungs, the mucus increases the risk of infection. In the digestive system, it interferes with digestion.