Select multiple rows with the same value(s)

I have a table, sort of like this:

ID  |  Chromosome | Locus | Symbol | Dominance |
===============================================
1   |      10     |   2   |   A    |   Full    |
2   |      10     |   2   |   a    |   Rec.    |
3   |      10     |   3   |   B    |   Full    |
4   |      10     |   3   |   b    |   Rec.    |

I'd like to select all rows with the same locus and chromosome. For example, rows 3 and 4. There may be more than 2 at a time and they may not be in order.

I tried this:

SELECT *
FROM Genes
GROUP BY Locus
HAVING Locus='3' AND Chromosome='10'

But it always returns row 3, never row 4, even when repeated. I think I'm missing something obvious and simple, but I'm at a loss.

Can someone help?