Why a mule’s sterility was no barrier to cloning
A mule is usually sterile because it inherits one set of chromosomes from a horse and another from a donkey. Those sets do not pair up neatly when the animal tries to make eggs or sperm, so producing a viable reproductive cell is generally the obstacle. Cloning sidesteps that step: it uses a body cell, not a reproductive one.
In the process, called somatic cell nuclear transfer, scientists put the nucleus—the compartment containing most of a cell’s DNA—from a mule’s body cell into an egg whose own nucleus had been removed. The egg’s contents can reset that mature nucleus and begin the coordinated development of an embryo. A surrogate then carries the pregnancy.
A genetic copy without sexual reproduction
The embryo does not form through the usual meeting of sperm and egg, so the mule never needs to produce either. Its body-cell DNA supplies the instructions for development. The egg still contributes cellular material, including mitochondrial DNA, so a clone is not an exact copy in every genetic detail.
Cloning also does not repair the chromosome mismatch that makes mules sterile. It creates another mule by taking a different route to development; it does not make ordinary mule reproduction possible. That is the important distinction: sterility blocks making offspring through sex, not making an embryo with laboratory help. A surrogate can carry that embryo to term even when the animal whose cells supplied its nucleus could not produce viable eggs or sperm.
