LONDON — They have the same shock of blindingly blond and slightly disheveled hair. They grew up in the same hypercompetitive, overachieving family that held dinner table debates where cleverness was the coin of the realm. They were on opposite sides of the 2016 Brexit referendum but patched things up afterward, as the right sort of people do.

In short, the brothers Johnson, Boris and Jo, always had each other’s back. Until Thursday, anyway, when Jo Johnson announced his resignation, both from his seat in the British Parliament and from the Conservative government that his brother leads.

His resignation, coming near the end of a disastrous week for his brother, Prime Minister Boris Johnson, seemed timed to inflict maximum pain.

It felt Shakespearean, even Freudian, but in the end it may have been a simple act of conscience.

“In recent weeks I’ve been torn between family loyalty and the national interest,” Jo Johnson, 47, wrote in a Twitter post. “It’s an unresolvable tension & time for others to take on my roles as MP & Minister.”