The story
- Proto-Indo-European*h₁éǵʰ
- Proto-Indo-European*h₁éǵʰs
- Proto-Indo-European*h₁eǵʰsteros
- Proto-Italic*eksteros
- Latinexter
- Latinextrā
- Englishextra-
- Englishchromosome
- Proto-Indo-European*h₂el-
- Proto-Indo-European*h₂élisder.?
- Englishchromosomal
- English todayextrachromosomal
From extra- + chromosomal.
Etymology from Wiktionary, CC BY-SA 4.0.