Final answer:
The concept where sperm cell contributes mitochondria to the oocyte is called 'paternal leakage'. While typically mitochondria are maternally inherited, paternal leakage is a rare instance where paternal mitochondria contribute to the offspring's mtDNA.
Step-by-step explanation:
If the sperm cell contributes mitochondria to the oocyte, it is referred to as a) paternal leakage. Although it is typically rare, paternal leakage refers to the phenomenon where the paternal mitochondria, which are typically marked for destruction within the embryo, somehow escape this process and are retained, thus contributing to the mitochondrial gene pool of the offspring. This is in contrast to the usual process where mitochondria and mitochondrial DNA (mtDNA) are inherited exclusively from the mother, a concept known as maternal inheritance.
In most cases, following fertilization, the sperm contributes only its DNA through the nucleus, and the cytoplasm as well as the cytoplasmic organelles, including mitochondria and their mtDNA, come solely from the oocyte. Maternal inheritance of mitochondria has been a crucial concept in understanding human ancestry and genealogy, as it enables the tracing of lineage through the female line. However, on the rare occasions when paternal mitochondrial DNA is found in the offspring, it can provide additional insights into genetics and inheritance.