It seems impossible, right? We have always learned that babies are created by joining a sperm with an egg, and that the DNA of these two cells is combined to create a unique individual with half the mother's DNA and half the father's DNA. As we can involve a third person in this process?
To understand the concept of a baby with three parents, it is necessary to talk about DNA. We are familiar with the Double helix DNA It contains 23 pairs of chromosomes that can be found in the nucleus of each cell of our body and that provides the instructions to create a complete organism and the proteins that shape our existence from conception to death. However the Cell nucleus DNA It is not the only type of DNA we need to live, since there is also another type of DNA in the mitochondria of each cell in our body.
Cross section of an animal cell showing the location of mitochondria, brown bean-shaped structures. The 23 chromosomes are found in the innermost compartment of the cell: the nucleus. 
In Babies and more A woman with infertility gives birth to her baby in Greece with the 'technique of the three fathers'Do you remember the mitochondria? As we were told in the institute's biology class, mitochondria are those bean-shaped organelles that are often drawn a meandering line and are presented as the engine of the cell. Every cell in the body, including sperm and ovules, needs energy to carry out all its functions and cells that do not have mitochondrial DNA (mtDNA) are like cars without gasoline.
Cross section of a mitochondria. Many times they are called the cell engine. They contain their own DNA and produce energy for the cell Unlike the cell's core DNA, mtDNA is not created from the combination of male and female DNA. The mitochondrial DNA it only comes from the mother, which means that the mitochondria present in the fertilized egg are the ones that will be replicated in each cell of your body during development and for the rest of your life.
However, like the core DNA, mtDNA can have mutations that can cause very serious and debilitating diseases and even infertility in the case of women with defective mitochondria. This is where the third person comes into play.
The third "father"
In 2016, a child was born from a couple who had problems because the mtDNA contained the mutations responsible for leigh syndrome: a progressive neurometabolic disease. When the woman's defective mitochondria were replaced by mitochondria from a donor who did not carry the mutation, the resulting baby carried DNA from three people: the mother who provided the nucleus DNA in the ovule, the man who provided the DNA of the ovum sperm and the donor of mitochondria. This is how the first baby was born using this technique.
How to make a baby with three parents: 1) The mother's egg contains the DNA (yellow circle) and the defective mitochondria (red ovals). 2) The DNA is extracted from the mother's egg with a very small pipette. 3) DNA is extracted from the donor egg leaving healthy mitochondria (green ovals). 4) The mother's DNA is passed to the ovum with healthy mitochondria. 5) The result is an ovule that has the DNA of the mother's nucleus and the DNA of the mitochondria of the donor ovum and that can be fertilized with the father's sperm. 6) As the cells multiply during the development of the embryo, each cell will have a combination of the father and mother's DNA in the nucleus of the cells and the mitochondria of the donor egg and its corresponding mtDNA. Note: Fertilization can occur before or after transferring the DNA to the donor egg. If previously produced, both the mother's and father's DNA will pass into the donor egg after the donor's DNA has been removed. If it occurs later, the egg will be fertilized after the mother's DNA has been transferred to the donor egg, as described here. 
In XatakaA baby, two genomes and three parents: the Chinese doctor who explores the limits of human reproductionThis technique, known as mitochondrial replacement, is similar to organ transplantation, or organelle transplantation in this case. However, there are important differences that have created concerns among politicians and have caused mitochondrial replacement to be banned in the US.
Unlike organ transplantation, the effects of mitochondrial replacement will persist in the following generations if the baby is a girl and decides to have children (men do not transmit their mitochondria to their children). It should also be borne in mind that the replacement of mitochondria will affect all body tissues and not just an organ of the body, such as the case of the cardiovascular system after a heart transplant.
However, these are real mitochondria that already exist in people and they are not genetically modified or altered cells. As long as they are healthy mitochondria, it has not been proven that there is any risk to the baby beyond occasional cell mutations, even if it is a controversial point.
Since 2016 it is difficult to know how many times this technique has been carried out by the three parents and in how many cases the pregnancy has been successful. However, the successful birth of a baby in Ukraine with three parents has led many countries to learn how to use this technique and its ban in the United States has not slowed its acceptance in other countries, such as the United Kingdom.
Is the donor of the mitochondria also a mother?
To what extent is a mother a woman who donates her mitochondria?
The short answer is little. More than 99 percent of your body's proteins are encoded by the DNA of the cell nucleus. Features such as hair color, eye color and height, for example, are in the DNA of the cell nucleus, while the genes established in mtDNA are primarily related to energy production and metabolism.
While the similarity of babies with three parents will be with respect to the father and mother whose sperm and ovules have been combined to produce the 23 chromosomes in the nucleus of that first cell. The important thing here is to understand these differences because in the future there will be many headlines about babies born with three parents and we can fall into hasty conclusions if we do not understand the scientific basis.
One thing is certain: for those women with fertility problems caused by mutations in the DNA of their mitochondria or who have the possibility of transferring mitochondrial genetic defects to the baby, this new technique gives them the hope that one day they may be able to have a Healthy baby that is a genetic representation of the biological partner with a small help from a third person.
Author: Jennifer Barfield, Assistant Professor of Assisted Reproduction Techniques, Colorado State University
This article has originally been published in The Conversation. You can read the original article here.
Photos | iStock
Translation | Wild Urbón
