C# new interface method

gistfile1.cs

class Program
{
    public interface IPerson { string Speak(); }
    public interface IMale : IPerson { string GrowABeard(); new string Speak();}
    public interface IFemale : IPerson { string Gossip(); new string Speak();}

    public class XY : IMale
    {
        private string name = "";
        public XY(string name) { this.name = name; }
        string IMale.GrowABeard() { return string.Format("{0} grows a beard like a male", name); }
        string IMale.Speak() { return string.Format("{0} speaks like a male", name); }
        string IPerson.Speak() { return string.Format("{0} speaks like a male person", name); }
    }
    public class XX : IFemale
    {
        private string name = "";
        public XX(string name) { this.name = name; }
        string IFemale.Gossip() { return string.Format("{0} gossips like a female", name); }
        string IFemale.Speak() { return string.Format("{0} speaks like a female", name); }
        string IPerson.Speak() { return string.Format("{0} speaks like a female person", name); }
    }
    public class XXY : IMale, IFemale
    {
        private string name = "";
        public XXY(string name) { this.name = name; }
        string IMale.GrowABeard() { return string.Format("{0} grows a beard like a male", name); }
        string IMale.Speak() { return string.Format("{0} speaks like a person male", name); }
        string IPerson.Speak() { return string.Format("{0} speaks like a person", name); }
        string IFemale.Speak() { return string.Format("{0} speaks like a person female", name); }
        string IFemale.Gossip() { return string.Format("{0} gossips like a female", name); }
    }
    static void Main(string[] args)
    {
        var sb = new StringBuilder();
        foreach (var person in new List<IPerson>() { new XXY("Kai"), new XX("Olivia"), new XY("Liam") })
        {
            if (person is IPerson)
                sb.AppendLine((person as IPerson).Speak());
            if (person is IMale)
                sb.AppendLine((person as IMale).Speak());
            if (person is IFemale)
                sb.AppendLine((person as IFemale).Speak());
        }
        Console.WriteLine(sb.ToString());
        Console.ReadLine();
    } 
}