The human Y chromosome is shrinking. In the next 5 million years or so, some geneticists think the sex-determining chromosome will vanish completely from our species. In the meantime, we have a bigger concern at hand. As some men age, they are losing the Y chromosome in their blood, brain, or immune cells, and that could have serious health effects. A loss of the Y chromosome has surprising connections to cancer , kidney disease , heart disease , and Alzheimer's . For decades, researchers have