what change a population would expect to see if a selection pressure was against thetrait of dominant allele?

Answer 1

If selection pressure was against the dominant trait, then those individuals who were homozygous for the recessive trait would have an advantage over the others. Hence this would lead to an increase in the numbers of homozygous individuals. The dominant trait would still be dominant, but there would be far fewer individuals carrying it (either as hetero- or homozygotes), and it may eventually disappear altogether. The recessive trait would not become dominant in the genetic sense, it would just become more prominant. Look up Peppered Moth genetics in wikipedia, for a brief description of how this has happened in the past.

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Answer 2

We would see an allele losing it's dominance. Take a selection pressure such as an ice age. The dominate allele for short coated dogs would surely go to extinction. Dogs whose coats could incrementally increase by assortive mating would be the new ancestors of the long coated dog.

Answer 3

Less fitness of offspring with that allele. Increasing frequency of other alleles over time.