Your 60 second guide to ‘three-parent’ babies…

The Westminster parliament will discuss ‘mitochondrial transfer’ or ‘three-parent’ babies today. Concerns about the ethics and safety of this have been raised. If parliament agrees, such procedures will become legal in the UK. There has already been considerable discussion and debate about this, as Polly Toynbee describes.

The problems seem to centre around the mitochondrial DNA (mtDNA), and confusion of this with the DNA which we inherit from both our parents.

A chicken’s egg is a cell; the yolk is the nucleus, which is contained by a membrane. The white of the egg is the cytoplasm, again contained by a membrane. The nucleus holds the genetic material which makes individuals unique. Within the cytoplasm are a number of ‘machines’. One such ‘machine’ is the mitochondrion. All our cells, except red blood cells, contain varying numbers of mitochondria.

Mitochondria are the ‘power houses’ of the cell; technically, they ‘respire’, that is use oxygen in a series of chemical reactions to produce energy. Mitochondria have their own DNA, quite separate from the DNA in the nucleus.

Mitochondria have DNA similarities to bacteria, and it’s thought that they represent bacteria which were captured, during evolution, by primitive cells—a process called symbiogenesis. (Symbiosis is the name given to two organisms which live together, and ‘support’ each other. Lichens are an example of symbiosis.)

Occasionally, mitochondria are damaged, for example by mutation, and do not function properly. Infants born with such defective mitochondria then have a range of diseases.

The concept behind mitochondrial transfer is to replace the diseased mitochondria with healthy ones.

Our mitochondria all came from our mothers, and their mothers before them. The sperm does not contribute to mitochondria in the fertilised ovum. (The mitochondria in the sperm are destroyed or ‘lysed’ at fertilisation.)

So, mitochondria are in the ovum. It’s not technically feasible to remove diseased ones from a prospective parent’s ova. Instead, this is what is proposed.

A donor ovum is necessary, one with healthy mitochondria. The nucleus of this cell is removed, and replaced with the nucleus from the ovum of a prospective mother. The ovum is then fertilised as in a conventional ‘test-tube’ baby, and then implanted into the womb.

Alternatively, the donor ovum may already be fertilised, and this fertilised nucleus is removed, and replaced with a fertilised ovum. This BBC link has a couple of diagrams to explain the process; it includes some of the surrounding discussion.

Although we are told that this will be a world first, the BBC reports that there were 50 or more such mitochondrial transfers some years ago in the US.

Our inherited characteristics come from out parents, with their DNA, their chromosomes ‘combining’ to form us. mtDNA pays no part in our characteristics (unless the mitochondria are diseased). Any suggestion that mitochondrial transfer or replacement is the start of something that will produce ‘designer babies’ is totally mistaken and totally without foundation.

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