Artificial selection is the breeding of animals, plants, or other organisms in which the parents are individually selected by humans in order to perpetuate certain desired traits and eliminate others from the captive population. By this means, most of our domestic crops, livestock, and pets have arisen. The many breeds of dog have been developed by artificial selection. Crops such as wheat have been produced by artificial selection over thousands of years. Artificial selection can be accelerated today by techniques such as plant tissue culture, in which selected plants can be cloned (see cloning) and grown directly into new plants without having to wait for seed production and germination; and artificial insemination of livestock. The cloning of livestock embryos has also become possible. Genetic engineering is the ultimate advance in artificial selection, allowing humans to combine specific sequences of DNA to combine desired genes.