First things first. What is an embryo? An embryo is the beginning stage of an organism or life form that is multicellular. We human beings are multicellular. And we have different organs performing different functions: the brain thinks, the heart pumps blood, the liver is necessary for metabolism. So each organ has cells which are special and modified to serve the function of the organ to which it belongs. But right at the beginning of a human life, the cells are all identical.
In human beings, the embryo is the term reserved for the organism till 7 weeks. After that, it is called a fetus. As stated earlier, in the initial stages of development, the embryo is composed of identical cells, which are totipotent. By totipotent, we mean that each cell is perfectly capable of developing into any kind of cell: heart, brain, liver.
In an embryo biopsy, the embryologist extracts just one or two cells from the embryo. The embryologist uses a laser to drill a hole in the embryo wall and aspirates a cell. Genetic studies can be performed on this extracted cell. The embryo does not miss this extracted cell and can develop into a full human being using the remaining cells within it, which further divide and later differentiate into various organs. Each of these cells has an identical genetic make-up or constitution. Thus, if the individual has some genetic defect it can be picked up right at the beginning by doing an embryo biopsy.
