Strong chins don't grow on trees. Genes are constant and once a boxer has been struck down - he is always liable to go down again. But the spectrum of 'chin strength' is vast and while some fighters are painfully fragile to a big punch, others can be put down by a flush shot and still recover their senses, swiftly putting the incident behind them.

Joe Calzaghe, undefeated throughout his 15-year career, was on the canvas four times, yet never looked in trouble. He always had a response, usually by fighting fire with fire,  swiftly snuffing out the danger. Byron Mitchell dropped Calzaghe so heavily that it the Welshman did a transition from standing upright to being a crumpled heap on the floor in a split second. But such was his instinct, he jolted back to his feet, realised he was in deep waters and finished Mitchell in that very same round - the second.

Calzaghe was unique and his recovery powers are part of what made him unbeatable. Contrary to Calzaghe, some fighters have seemingly iron plated chins and just don't go down. Shane Mosely has to be in this category because he has one of the most awe-inspiring resumes in boxing, and yet has never been put down, to my knowledge. Even after fighting animals like De La Hoya, Cotto, Forrest, Vargas, Mayorga, Margarito, all of whom landed big shots on 'Sugar', but could not make enough impression to drop him.

And then there's Amir Khan. The 22-year old enigma has been dropped three times, one in a devastating knock out defeat to Columbian slugger Breidis Prescott. Khan's first knock down, against power-puff puncher Willie Limond, rang alarm bells to the expectant British public. Khan survived a rocky patch after being clubbed down in a rather ugly attack from the Scot, who threw everything he had to finish the fight. No-one knew what how to react; this wasn't meant to happen. Limond was a natural super-featherweight who had eight stoppage wins from 29 fights. Khan bounced back to finish Limond spectacularly, but when lightening struck twice, against Michael Gomez - another super featherweight - Frank Warren must have been trembling with fear. His brightest young 'superstar' had a fundamental chink, which would almost certainly hinder his entire career.

Briedis Prescott, a young puncher who had bypassed the radar of much of the general boxing public, obliterated Khan and exposed his frail chin in front of the world on Khan's first box office appearance. For the Khan camp, it had been a catastrophe waiting to happen. Khan has made habitual mistakes in routine fights which, for someone of his potential, should not have happened. And now he had faced an opponent equal to his size (Prescott had campaigned at light-welterweight) his chin had wilted under a couple of wild swings from the Columbian.

Still, on the positive side, Khan has an exhilerating edge to his style and can finish fights ruthlessly, with precise flurries of heavy punches. For he is heavy-handed, and has lightening fast hand-speed. But that won't carve him a legacy. Neither will his defeat over the half-blind Barrera, no matter how impressively he dominated the Mexican legend. Khan is a work in progres and perhaps Freddie Roach will refine his skills, defence and footwork enough to make the Brit a genuine force on the world stage. But how long before he faces another puncher? He has been wisely nudged up a weight so as to avoid the animals who await him at lightweight and would lick their lips at the prospect of fighting the fragile Bolton boy (Marquez, Valero, Katsidis, Casamayor etc). Khan doesn't belong in that class, yet, and maybe he will have some joy at light-welter, starting with the tough, but slightly one-paced WBA champion Andrej Kotelnik. It is possible that Khan could outbox him if he stuck to a game plan, but what then? Juan Urango? Kendall Holt? Timothy Bradley? And pigs will fly.