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 uses energy from the sun to split water into oxygen and hydrogen. The oxygen passes into the atmosphere and the hydrogen joins with carbon dioxide to produce biomass. When living things need energy they take in oxygen for respiration. The oxygen returns to the atmosphere in the form of carbon dioxide. Oxygen gas is fairly soluble in water, which makes aerobic life in rivers, lakes and oceans possible.