Epigenesis and Preformation and Hypothesis Explaining Congenital Twins

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Mutants: On Genetic Variety and The Human Body, By Armand Marie Leroi
Epigenesis is the process by which an organism develops from a fertilized egg through a series of developmental stages, with each stage building on the previous one. This process is regulated by a complex network of genes and environmental factors that influence the expression of those genes. Preformation, on the other hand, is the idea that an organism is fully formed in miniature within the egg or sperm and grows larger during development. This idea was popular in the 17th and 18th centuries but has yet to be discredited by current scientific evidence (Fagan and Maienschein). While both theories have been proposed to explain the development of organisms, the evidence provided in Mutants: On genetic variety and the human body supports the idea of epigenesis.
The examples provided in the book strongly support the idea of epigenesis. For example, experimental models using the fruit fly Drosophila melanogaster have shown that the development of the fly's body plan is regulated by a series of genes called the Hox genes. These genes are activated in a specific order during development, and mutations in these genes can dramatically change the fly's body plan. This supports the idea that development is a step-by-step process regulated by genes rather than a simple unfolding of a preformed organism. Similarly, human diseases such as congenital malformations provide further evidence for the role of genes in development. For example, mutations in the gene Sonic hedgehog (SHH) can cause holoprosencephaly, a condition in which the brain fails to divide into two hemispheres properly. This condition is caused by the failure of SHH to properly regulate the development of the brain properly,...