Genetically, a foal takes its main DNA half from the sire and half from the dam. But the dam passes on things the sire never does: all of the foal's mitochondria — the energy factories in every muscle cell — and, to a colt, his only X chromosome. She carries the foal, feeds it and raises it.
Most importantly for a buyer, her record is evidence. A stallion has hundreds of foals, so his average tells you little about this one mating. A mare has a handful — and every one of them has run for the same family you're about to buy into.
That's why it pays to read the female line closely: what the dam, her foals and her family actually won, how far, and at what level.
Research into whether a mare's mitochondrial type affects racing ability is still mixed, so it's best not to read too much into it. Her family's proven racing record is the evidence to look at.
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