Visa Necessary to Connect in an Airport in Canada?

Yes. Countries/Territories requiring visas

why would you need a US/Schengen Visa if you're connecting in a Canadian airport? Canada is an independent country...

A Transit Visa is a type of Temporary Resident Visa (TRV) required by anyone from a non-visa-exempt country who is travelling through Canada to another country and whose flight will stop in Canada for less than 48 hours. There is no cost for a transit visa but the application process is the same as that for the TRV. Apply for a transit visa for Canada at least 30 days in advance of your departure or allow eight weeks if mailing it in.

Visitors must apply for a transit visa for Canada from their country of residence. You may not apply for a visa upon your arrival in Canada
Unless told otherwise, travel agents or cruise lines will not take care of your transit visa - it is your responsibility.

The Temporary Resident Visa (TRV) has three types: single entry, multiple entry and transit. To apply for any one of these types of TRV, fill out the two-page application for a Temporary Resident Visa Made Outside of Canada (download PDF document) or call the nearest Canada Visa Office. At the top of the application, you will choose the box named "Transit." Gather the necessary documents and mail in or take the application to a Canada Visa Office. You will not have to include a payment as the Transit Visa is free.

Yes a Transit Visa is needed for all nationals that require a Visitors(Tourist) Visa which included dominican nationals. On a Positive side The Transit Visa is usually Free so you dont lose any money trying and its fairly easy to obtain if you have a Legit reason for going through Canada. For instance flying to a Destination only Air Canada serves or going to a Faraway destination which the dominican holds a visa for already such as Australia,New Zealand,Or Going through Canada is much quicker.On a Downside this Transit Visa only enables the Dominican to Transit (Meaning they are not allowed to leave the Airport) and may only Transfer between flights without actually entering the country.

My understanding is that without a visa or a passport issued from a non-visa-exempt country persons are not allowed to board planes flying out of the DR landing in Canada.