Final answer:
For a human offspring to express a gene from paternal mitochondrial DNA, an atypical event called paternal leakage must occur, which is rare since mitochondrial DNA is usually inherited from the mother.
Step-by-step explanation:
In human reproduction, mitochondrial DNA (mtDNA) is typically inherited exclusively through the maternal line.
The vast majority of an individual's mitochondria come from the mother's egg cell, with the mitochondria from the father's sperm usually being destroyed prior to or shortly after fertilization.
Therefore, for a human offspring to express a gene from the paternal mitochondria, there must be an exception to the typical pattern of inheritance.
This rare phenomenon is known as paternal leakage.
Paternal leakage refers to the atypical event where mitochondrial DNA from the sperm contributes to the zygote's genetic makeup, allowing the offspring to express mitochondrial genes inherited from the father.