Mitochondria are thought to be the descendants of certain alpha proteobacteria. they are, however, no longer able to lead independent lives because most genes originally present on their chromosomes have moved to the nuclear genome. which phenomenon accounts for the movement of these genes?

The correct answer is "horizontal gene transfer".

Explanation:

The most common way at which genes are transferred is vertically, from parents to their descendants. However genes could be transferred among species, in what is known as horizontal gene transfer (HGT). HGT is most clearly seen in bacteria, actually HGT is the cause of the phenomenon of the development of antibiotic resistance. The theory of an ancient mitochondria moving their genes to the chromosomes of its host is most likely the result of HGT. This makes sense since an ancient mitochondria most likely behaved as an alpha proteobacteria, an ancient bacteria that already developed its HGT mechanism..

blood enters the heart through two large veins, the inferior and superior vena cava, emptying oxygen-poor blood from the body into the right atrium of the heart. as the ventricle contracts, blood leaves the heart through the pulmonic valve, into the pulmonary artery and to the lungs where it is oxygenated.