The principle behind it is that, in a population where certain conditions are met (see below), the frequency of the alleles in the gene pool will be constant. To read more about the Hardy-Weinberg Theory and Equation, check out this page from Nature.
Although meningiomas are referred to as brain tumors, they do not grow from brain tissue. They arise from the meninges, which are three thin layers of tissue covering the brain and spinal cord. These tumors are most often found near the top and the outer curve of the brain. Tumors may also form at the base of the skull. Meningiomas usually grow inward, causing pressure on the brain or spinal cord. They also can grow outward toward the skull, causing it to thicken. Most meningiomas are noncancerous, slow-growing tumors. Some contain sacs of fluid (cysts), mineral deposits (calcifications), or tightly packed bunches of blood vessels.
Three layers of membranes, called meninges, lying just under the skull, protect the brain and spinal cord. From the outermost layer inward they are: the dura mater, arachnoid mater, and pia mater.A meningioma grows from the arachnoid cells that form the middle layer, and are firmly attached to the dura. meningioma is a type of tumor that grows from the protective membranes, called meninges, which surround the brain and spinal cord. Most meningiomas are benign (not cancerous) and slow growing; however, some can be malignant.