Dominance and zygosity

An allele is a version of a gene. For a gene with two copies, an individual has two alleles, which may be the same or different. The pair of alleles is the individual’s genotype for that gene.

• Homozygous means that the two alleles are the same.
• Heterozygous means that the two alleles are different.

The observable form of a characteristic is its phenotype. When two different alleles are present, their relationship determines how they affect the phenotype. A dominant allele is expressed when at least one copy is present, including in a heterozygous individual. In this two-copy model, a recessive allele is expressed when both alleles are recessive.

Dominance and zygosity in an allele pair

Suppose $$A$$ represents a dominant allele and $$a$$ represents a recessive allele. The possible genotypes are:

• $$AA$$: homozygous, with the dominant phenotype.
• $$Aa$$: heterozygous, with the dominant phenotype.
• $$aa$$: homozygous, with the recessive phenotype.

In $$Aa$$, the recessive allele is still present; the dominant allele does not remove it. Dominance describes the effect of the alleles on the characteristic, not how common either allele is in a population.

Inheritance of polydactyly and cystic fibrosis

Some inherited disorders result from particular versions of a gene, called alleles. A person normally inherits one allele of a gene from each parent. Whether one or two copies of an allele are needed for a disorder to be expressed depends on whether the allele is dominant or recessive.

Polydactyly: a dominant allele

Polydactyly is the presence of extra fingers or toes. An inherited form is caused by a dominant allele. One copy of this allele is enough for a person to have the characteristic, even if their other allele is different.

Cystic fibrosis: recessive alleles

Cystic fibrosis is caused by recessive alleles that affect cell membrane function. They disrupt the transport of chloride across cell membranes, leading to thick, sticky mucus, particularly in the lungs and digestive system.

A person with two cystic fibrosis alleles has the disorder. A person with one cystic fibrosis allele and one unaffected allele does not have the disorder but is a carrier. A carrier can pass the recessive allele to their offspring.

The distinction is that one copy of a dominant polydactyly allele can produce the characteristic, whereas two copies of a recessive cystic fibrosis allele are needed for a person to have cystic fibrosis.

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.