Too many parents spoil the child?

When does a genetic donor become a biological parent and how many biological parents are too many? In this post, our guest blogger, REBECCA BOLLARD, looks at the recent technological developments around the possibility of three-parent babies.

In human fertilisation an egg cell is fertilised by a sperm cell, and each has 23 chromosomes. The fertilised cell has 46 chromosomes (the standard number for a human) and develops into an embryo. Chromosomes are made of DNA and exist in an area of the cell called the nucleus, and contain genes. This is what most people think of when talking about DNA – half comes from each biological parent and combines to form a new person. Every person has two biological parents no matter how complicated the legal or social arrangements may be.

This is, however, not quite the full story. Human cells also contain mitochondria, tiny organelles responsible for making a specific chemical that the cells need to function. Human mitochondria contain a small amount of DNA (often called mitochondrial DNA or mtDNA) in the form of a single chromosome. This chromosome is inherited exclusively from the egg cell and therefore the biological mother. mtDNA consists mostly of genes devoted to the maintenance and running of the mitochondria. Defects in these genes can lead to a range of serious diseases, including a range of incurable heart and muscle problems.

Several technologies may overcome mitochondrial disease. Pronuclear transfer (PNT) and maternal spindle transfer (MST) both allow the nucleus of the mother’s egg to be placed inside the egg from a woman with no mitochondrial issues. Thus, the resulting embryo would have three genetic donors – the man who supplied the sperm, the woman who supplied the egg nucleus, and the woman who supplied the egg and therefore the mtDNA. The first two of these would have supplied most, but certainly not all, of the DNA in the embryo (and resulting person). So, perhaps that person would have three genetic parents?

These techniques could also be used in other circumstances, such as where an intending mother has no eggs or has gone through menopause. Scientists believe they can take a cell from elsewhere in a woman’s body and halve the chromosomes using electricity. This cell could then be used with the egg cell (and mtDNA) of another woman.

The UK Human Fertilisation Embryology Authority (HFEA) recently began a consultation on whether such techniques could be used to help the 1 in 200 children born every year in the UK with mitochondrial diseases. This has led to much discussion in the press, including in the Daily Mail and in The Guardian. This follows earlier works by the Nuffield Council on Bioethics that such techniques would be ethical once proved safe and efficient.

Bioethics professor John Harris argues that mtDNA donors would not be genetic parents as “DNA contained in the donated mitochondria comprises much less than 1% of the total genetic contribution and does not transmit any of the traits that confer the usual family resemblances and distinctive personal features... No identity-conferring features are transmitted by the mitochondria”. He then goes on to argue that he would be more upset at having a mitochondrial disease than “having some third-party DNA in my genome”.

So, when does a genetic donor become a biological parent? The two people donating nuclear DNA are obvious candidates, but what about mtDNA donors? What about gestational surrogates, particularly as the evidence builds that gestation is not a simple relationship (see e.g. the research article by Chen et al in Plos One)? Does it matter, when the biological, social, and legal aspects of parenthood are split more than ever?

Should the number of parents in technology-driven reproduction matter? Or is it irrelevant when we’re talking about serious and otherwise incurable disease?