chromosome

Picture every blood test as a dot on a large map. Plot thousands of those dots and they won't scatter at random: they line up along two main roads. One road separates men from women, the other stretches from youth to old age. Follow these roads and you can see how a person grows up, lives and grows old.

Dutch family archives hold marriage records going back to 1600. From these records, like entries in an old ledger, you can extract two columns: how many children a person had and how many years they lived. Evolutionary biologists have been arguing for half a century over whether these columns are connected.

In a yeast cell, the nucleus takes up about six percent of the volume, like a small ball in a room. A hundred years ago, biologists noticed that bigger cells have bigger nuclei, but why this is so remained a mystery. Some talked about protein motors, others about envelope tension, still others about shape.

In the orchestra of the immune system, B cells play the antibody part, while the sex chromosomes and gonads act as conductors setting a different tempo. Until recently, scientists could not work out which of them calls the shots: the genes written into the chromosomes, or the hormones produced by the gonads.

On a crowded train, some passengers ride in silence while others push toward the exit, shoving their neighbors aside. In every person's blood, mutant cells appear with age, and their fate is much the same: most changes are neutral, like passengers who are simply along for the ride, but some give a cell an edge, and it starts dividing faster than the rest.