"Are you a doctor?"
Translation:Meddyg dych chi?
No. See rmcode's answer above.
It has a question mark to show that it is a question and when spoken it has a rising intonation at the end, just as a question in English tends to have.
In more formal Welsh, an emphatic question such as this would be started with the particle Ai...? (Ai meddyg ydych chi?). This has generally been dropped from colloquial Welsh. People do sometimes use it (or the form Ife...? in informal speech), but it is not very widespread and is not often taught - it is perhaps more common in parts of west Wales. It is not included on this course.
In the register of Welsh taught on Dysgu Cymraeg courses in Wales ...dych chi is fine in this context. If you want to be slightly more formal then, yes, ...ydych chi is the form to use, and that is also accepted on this Duo course.