One of the major components of teeth is enamel. Enamel caps teeth and is the hardest known substance in the human body, an understandable feature considering the amount of grinding that teeth are used for.
Several genes are crucial for enamel development, including some that encode proteins that build a scaffold for the deposition of mineralized enamel and others that breakdown these proteins.
Baleen whales, which lack teeth entirely, are thought to be derived from ancestors that possessed enamel-capped teeth, as demonstrated by evidence from the fossil record and their genetic relationships to other toothed mammals. This leads to the prediction that genes important for enamel formation have become nonfunctional in baleen whales and preserved as pseudogenes.
Sure enough, Demere et al.  discovered that the enamel genes ENAM (enamelin) and AMBN (ameloblastin) are inactivated in baleen whales while remaining functional in other toothed mammals, including toothed whales. Meredith et al.  also demonstrated that the pygmy and dwarf sperm whales, two toothed whale species that lack enamel, have shared mutations in ENAM. Another paper by Meredith et al.  discovered that the enamel gene MMP20 (enamelysin) has been disrupted by an inactivating retrotransposon (“jumping gene”) shared in all baleen whales, suggesting that enamel formation was likely inactivated in the last common ancestor of baleen whales.
A figure from Meredith et al.  showing a summary of enamel gene mutations across a whale phylogeny and transitions from toothed to toothless species incorporating the fossil record. Nodes that are black denote toothed species, gray are enamelless but toothed species, and white are toothless.
Questions for Creationists
Why would God create nonfunctional enamel genes in whales that completely lack teeth? Is it a coincidence that all baleen whales and pygmy and dwarf sperm whales have shared inactivating mutations, respectively? Is it a coincidence that the presence of tooth pseudogenes was predicted by molecular phylogenetics and the fossil record?
1. Deméré, T. A., McGowen, M. R., Berta, A., & Gatesy, J. (2008). Morphological and molecular evidence for a stepwise evolutionary transition from teeth to baleen in mysticete whales. Systematic Biology, 57(1), 15-37.
2. Meredith, R. W., Gatesy, J., Murphy, W. J., Ryder, O. A., & Springer, M. S. (2009). Molecular decay of the tooth gene enamelin (ENAM) mirrors the loss of enamel in the fossil record of placental mammals. PLoS genetics, 5(9), e1000634.
3. Meredith, R. W., Gatesy, J., Cheng, J., & Springer, M. S. (2010). Pseudogenization of the tooth gene enamelysin (MMP20) in the common ancestor of extant baleen whales. Proceedings of the Royal Society B: Biological Sciences, rspb20101280.