Dr. S: Hats off to your work in showing the flaws of evolution! I wanted to ask: Is there any truth to the claim that whales possess genes for hind legs but are not expressed or are dormant in whales. If this is true how would the creation model explain such a fact? I know that supposed vestigial bones have been shown to be functional but i am asking about supposed leg genes here. Thank You
CMI denied it
First, the short dry answer: No, whales do not have ‘leg’ genes.
CMI is 100% wrong as usual. Cetaceans as well as other marine mammals do have the unexpressed Tbx4 genes for rear limbs
Abstract: The amino-acid sequences of the T-domain region of the Tbx4 gene, which is required for hindlimb development, are 100% identical in humans and mice. Cetaceans have lost most of their hindlimb structure, although hindlimb buds are present in very early cetacean embryos. To examine whether the Tbx4 gene has the same function in cetaceans as in other mammals, we analyzed Tbx4
sequences from cetaceans, dugong, artiodactyls and marine carnivores. A total of 39 primers were
designed using human and dog Tbx4 nucleotide sequences. Exons 3, 4, 5, 6, 7, and 8 of the Tbx4
genes from cetaceans, artiodactyls, and marine carnivores were sequenced. Non-synonymous
substitution sites were detected in the T-domain regions from some cetacean species, but were not
detected in those from artiodactyls, the dugong, or the carnivores. The C-terminal regions
contained a number of non-synonymous substitutions. Although some indels were present, they
were in groups of three nucleotides and therefore did not cause frame shifts. The dN/dS values for
the T-domain and C-terminal regions of the cetacean and artiodactylous Tbx4 genes were much
lower than 1, indicating that the Tbx4 gene maintains it function in cetaceans, although full expression leading to hindlimb development is suppressed.
Perhaps @PDPrice can explain why CMI posts such demonstrably wrong information.
ETA: Turns out the Joshua S who asked this at CMI was not our Dr. Swamidass but another Joshua S.
Disregarding the fact that fossils are rarely thought to represent direct ancestors(it’s more likely to be a sister taxon closely related to a direct ancestor), the transitional status of this fossil is not an assumption but an evidentially derived conclusion.
This gene is a member of a phylogenetically conserved family of genes that share a common DNA-binding domain, the T-box. T-box genes encode transcription factors involved in the regulation of developmental processes. This gene is the human homolog of mouse Tbx4, which is closely linked to Tbx2 on mouse chromosome 11. Similarly this gene, like TBX2, maps to human chromosome 17. Expression studies in mouse and chicken show that Tbx4 is expressed in developing hindlimb, but not in forelimb buds, suggesting a role for this gene in regulating limb development and specification of limb identity.
Tbx4 expression is suppressed in cetaceans which is why they no longer develop hind limbs even though they still carry the functioning Tbx4 gene. It’s why we occasionally find atavistic hind limbs on whales when a mutation expresses the gene. Read the paper provided in the OP.
Do you really want to stoop to CMI’s level and argue the semantics of “limb” versus “leg”?
But Tbx4 didn’t arise in tetrapods. As @scd pointed out, Tbx4 also exists in zebrafish, indicating that the gene existed at least far back as the Actinopterygii/Sarcopterygii split, during which tetrapod legs did not yet exist. Do zebrafish also have a “leg gene”?
It’s really silly to talk about “leg genes” or, as it’s commonly stated, “genes for legs”. There are genes expressed during leg development, which may also be expressed in other places or may not. When expressed they may have varying downstream effects depending on whatever other genes are developmentally downstream. That’s how transcription factors work.
