For the most part of a cell's cycle, each chromosome has only one chromatid.
Before entering meiosis, a diploid cell (which contains a set of chromosomes from one parent and another set from the other parent) first doubles its DNA, and the cell will start division with each chromosome having 2 sister chromatids.
During meiosis I homologous chromosomes (the maternal and paternal sets) separate, and during meiosis II the sister chromatids separate. As a result, 4 daughter cells with half the original diploid chromosome number and one chromatid per chromosome are produced.
The organs are known as Vestigial organs. The evolution from land-dwelling animal to the marine animals has led whales as well as dolphins, to lose hind limbs. The pelvic and leg bone serves no purpose in them and just provide the anchorage to muscles
