this is what ive got...dont take it as gospel though
gene cascades are chain reactions- domino effects in terms of gene expression. genes called transcription factor genes have the code that produces proteins that activate or repress the expression of other genes. in a cell, as one gene activates, it produces, along with the proteins it must produce for its function, chemicals which activate other genes within itself or other cells. this is important in limb formation of birds and animals. as the embryo develops, “buds” form. gene’s have been expressed in these buds, causing the formation of particular limbs. As each subsequent gene is expressed in one particular pathway, the next expresses and the chain reaction causes the formation of a particular limb. for this reason, if a bud for leg formation in a bird was transplanted to where the wing should be, the gene cascade, having already started, would result in a domino effect and a leg would form where the wing should be- these cascades involved in limb development are known as homeobox genes
