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 by microbes (and later by plants) produced all the oxygen in Earthâs atmosphere, and fixed most of the carbon dioxide as complex organic components of living organisms. Other kinds of microbes produce nitrogen gass (N2) and fix nitrogen into forms that plants and animals can use.