(a) The trait that appears dominant in the progenies when breeds of different traits are crossed are called dominant traits.
The trait which though appears in progenies but is not so conspicuous is called recessive trait.
(b) Consider tallness as a characteristic. Plant height depends on particular plant hormone which in turn will depend on the efficiency of the process for making it. If the enzyme responsible for the production of this hormone is efficient, plant will be tall. If the gene for that enzyme has an alteration that makes enzyme less efficient, the amount of hormone will be less and plant will be short.