Spot the correct spelling of a chromosomal “three copies” term.

Correct — C. Build the word from its parts: tri-, 'three', and -somy, the ending taken from chromosome.

Join them with nothing added: tri + somy = Trisomy, three copies of a chromosome instead of the usual two → option (c).

The same ending gives monosomy, a single copy, which is a quick check on the spelling.

Why the others are wrong

• (a)
  Trisomie — The -ie ending is not English. English writes the suffix as -somy, as in monosomy; trisomie is the French spelling of the word.
• (b)
  Trissomy — Doubled s has no source. Tri- ends in a vowel and -somy begins with a single s, so the join supplies one s: tri + somy.
• (d)
  Trissomie — Two errors at once: the doubled s of Trissomy and the -ie ending of Trisomie. English keeps a single s and ends in -y.

Technical spellings are easiest when you split the word into its parts and spell each part as you already know it.

Tri- means three, as in triangle and tricycle. -somy comes from chromosome, whose -some goes back to Greek soma, 'body'.

Compare mis + spell = misspell, where the double s comes from both parts. In tri + somy only -somy has an s, so trisomy keeps one.

Key facts

• Trisomy means three copies of a particular chromosome instead of the normal two.
• Down syndrome, also called trisomy 21, is caused by all or part of a third copy of chromosome 21.
• Monosomy is the matching term for a single copy of a chromosome.
• The -some of chromosome comes from Greek soma, 'body'.

Common traps

• Doubling the s by analogy with misspell, where both parts supply an s
• Accepting Trisomie because it keeps the single s, without checking the ending

No directly related past PYQ was found.