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, a process undergone by the chloroplasts of the cells of plants, captures the light energy radiated by the sun and utilizes it for the synthesis of glucose from carbon dioxide and water. A byproduct of this synthesis, oxygen (O2), is filtered out of these plants and into the surrounding atmosphere. Animals then inhale this oxygen and, using it and water, perform cellular respiration to extract energy from food in the form of ATP (Adenosine triphosphate).