In Triturus newts, including the great crested newt, chromosome 1 exists in two forms, and only heterozygotes survive. Homozygotes for either form die during embryonic development. Because every viable adult is a heterozygote, the expected Mendelian outcome is that about half the embryos in each clutch are homozygous and die, a phenomenon described as balanced lethal system.
The consequence is a built-in reproductive tax. A female lays a few hundred eggs across a season, wrapping each individually in a folded pond-plant leaf with her hind feet, and around half of those simply arrest and die regardless of pond quality or predation.
The system is genuinely unusual, and it is sometimes reported as a defect specific to the great crested newt. It is shared across the genus Triturus, and it has persisted for millions of years, which is precisely why it interests evolutionary biologists: a permanently costly arrangement that natural selection has not removed.