Developmental Genes
Weismann also assumed that different species contain different developmental determinants. The lizard's germ plasm naturally contains different developmental instructions than the cow's germ plasm. Only in the late 20th century did scientists learn that developmental genes, now known as homeobox genes, hardly vary at all across vast portions of the phylogenetic tree. Everything from flies to humans share roughly the same set of developmental genes. What differs between the species is not the genes themselves but the way they are expressed.
Read more about this topic: Genetic Determinism
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—Wislawa Szymborska (b. 1923)