What’s the Plural of Locus: Understanding Grammatical Number in Biology
Quick Answer
The plural of locus is loci (say it like “loh-sigh”). Do not say “locuses” — that is not correct.
One: The locus of the problem is in the system.
More than one: Scientists found several loci on the map.
Locus, Loci — See the Pattern
Look at these words. Can you see what happens?
- one locus → two loci
- one focus → two foci
- one fungus → two fungi
- one cactus → two cacti
Did you see the pattern? All these words come from Latin. The ending -us changes to -i when there is more than one.
| Singular (one) | Plural (more than one) |
|---|---|
| locus | loci |
| focus | foci |
| fungus | fungi |
| cactus | cacti |
When to Use Locus and When to Use Loci
Use locus when you talk about one place, point, or position.
- The locus of the earthquake was near the coast. (one place)
- This gene has a specific locus on the chromosome. (one position)
Use loci when you talk about two or more places, points, or positions.
- The scientists studied three different loci on the chromosome. (more than one)
- Several loci of activity were found in the brain. (more than one)
Easy way to remember: One locus, two loci. Just like one focus, two foci. Latin words that end in -us change to -i.
The Rule in One Line: One locus, two or more loci — change -us to -i.
Real-Life Examples With Locus and Loci
- The doctor said the locus of her pain was in her lower back. (the exact place where it hurts)
- The park became the locus of all community events. (the centre of activity)
- Researchers found two loci that affect eye colour. (two positions on a gene)
- The city has several loci of cultural activity. (more than one centre)
- Each locus on the map shows where a sample was taken. (one point)
You are doing great. Now let us look at the mistakes many learners make.
The Most Common Mistake With Locus
Even advanced learners sometimes write “locuses” because it sounds like normal English. If you have done this, do not worry — you are not alone. Most English plurals just add -s or -es, so it is a very natural mistake.
✗ The scientists studied several locuses on the gene.
✓ The scientists studied several loci on the gene.
✗ Two locuses of infection were found.
✓ Two loci of infection were found.
How to remember: Think of focus → foci. The same pattern works for locus → loci. If you remember one, you remember both.
Other words that follow this Latin pattern: radius → radii, stimulus → stimuli, syllabus → syllabi.
Test Yourself: Locus or Loci?
Choose the correct answer for each sentence. Click Check to see if you are right.
1. The scientists found a single _______ on the chromosome.
2. Researchers studied three different _______ linked to the disease.
3. The park is the _______ of all the town’s events.
4. Several _______ of activity were found in the brain scan.
5. Each _______ on the map shows where a sample was collected.
Keep Going — You Are Building Something
You just learned the plural of locus. That is one more Latin plural you will never get wrong again.
But here is a question: what about the word vertex? It also comes from Latin, but the plural is not “vertexes.” Do you know what it changes to? And does it follow the same -us → -i rule, or a completely different one?
Next lesson: What’s the Plural of Vertex?
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My name is Khamis Maiouf. I am the creator of the English Teacher Site, dedicated to providing valuable resources and insights for students around the world. With a passion for education and a commitment to helping students enhance their skills, I aim to make English teaching more effective and enjoyable for both educators and students.
