sample.cs

public abstract class FormElement2
    {
        public Guid Id { get; set; }
        public int Order { get; set; }
    }

    public abstract class FieldControl2 : FormElement2
    {
        public string Label { get; set; }
        public string Trailer { get; set; }
        public bool Prepoluate { get; set; }
    }

    public abstract class TextFieldControl2 : FieldControl2
    {
        public string DefaultValue { get; set; }
        public int Size { get;  set; }
    }

    public class TextBoxControl2 : TextFieldControl2
    {
        public int Rows { get; set; }
    }

    public abstract class FormControlBaseDTO2
    {
        public Guid Id { get; set; }
        public int Order { get; set; }
    }

    public class FormElementDTO2 : FormControlBaseDTO2
    {
        public FormElementType ElementType { get; set; }
        public string Label { get; set; }
        public string Trailer { get; set; }

        public string DefaultValue { get; set; }
        public int Size { get; set; }
        public int Rows { get; set; }

        public bool Prepopulate { get; set; }
    }

    public class Scenario2
    {
        public static void Run()
        {
            Mapper.Initialize(cfg => {
                cfg.CreateMap<FormElement2, FormElementDTO2>(MemberList.None)
               .Include<FieldControl2, FormElementDTO2>()
               .Include<TextBoxControl2, FormElementDTO2>();

                cfg.CreateMap<FieldControl2, FormElementDTO2>(MemberList.None)
                    .Include<TextBoxControl2, FormElementDTO2>()
                    .ForMember(dto => dto.Prepopulate, opt => opt.MapFrom(src => src.Prepoluate));

                cfg.CreateMap<TextBoxControl2, FormElementDTO2>(MemberList.Source)
                .ForMember(dto => dto.ElementType, opt => opt.UseValue(FormElementType.TextBox));
            });

            Mapper.AssertConfigurationIsValid();
        }
    }