HD is called an autosomal dominant disorder because only one copy of the defective gene, inherited from one parent, is necessary to produce the disease. The genetic defect responsible for HD is a small sequence of DNA on chromosome 4 in which several base pairs are repeated many, many times.
Carbon dioxide cannot pass through the protective waxy layer covering the leaf (cuticle), but it can enter the leaf through an opening (the stoma; plural = stomata; Greek for hole) flanked by two guard cells. Likewise, oxygen produced during photosynthesis can only pass out of the leaf through the opened stomata.
Photosynthesis is the process by which plants use the energy of light to produce carbohydrates and molecular oxygen (O 2 ) from carbon dioxide (CO 2 ) and water: Virtually all ecosystems on Earth depend on photosynthesis as their source of energy, and all free oxygen on the planet, including that in the atmosphere, originates from photosynthesis. The overall reaction is the reverse of respiration, which releases energy by oxidizing carbohydrates to produce CO 2 and water.