Einkorn is a diploid wheat. It has 14 chromosomes. It’s genome constitution is called AA.
Emmer is a tetraploid wheat. It has 28 chromosomes. It’s genome constitiution is called AA BB.
Speltz is a hexaploid what. It has 42 chromosomes. It’s genome constitution is called AA BB DD.
Most modern wheats are hesaploids same as speltz. They have 42 chromosomes. Their genome constitution is called AA BB DD, same as speltz.
When diploid plants set seed, two cells unite and in the process half of the chromosomes are lost so that the new plant still has only 14 chromosomes.
Emmer is known as a plant that originated from one cell from an einkorn plant, and once cell from another grass, that united and kept all the genes from both parent plants thus producing a 28 chromosome plant.
Speltz is known as plant that originated from one cell from an emmer plant, which has 28 chromosomes, uniting with one cell from another grass, which has 14 chromosomes, again, keeping all the chromosomes from both parent plants, thus producing a 42 chromosome plant.
Speltz and modern wheat are now classified as the same spcies. Each is considered a subspecies of Triticom aestivum.
Normally, when a hybrid from two different plant species unites and sets seed, the sed is sterile and does not grow. But there are plants that originate from such crosses and reproduce, for example wheat and triticale.
The advantage of having more chromosomes in one plant is that the seed and plant are bigger.