This (only the head of the sperm enters the ovum) was once thought to be true but it is now known that the entire sperm cell, including the mitochondria and flagellum, enters the egg at fertilization.

Misconceptions about mitochondria ...

Nature - eliminating paternal mitochondrial dna

The egg contains over 100,000 mitochondria while the sperm contains 50 - 75. This could mean that the paternal mtDNA are either diluted out of the embryo or eliminated early in oocyte development.

The first paper suggests the paternal mtDNA could simply be diluted beyond detection:

>We simply do not yet know what happens to the paternal contribution of mtDNA in humans, but the simplest explanation is that it is diluted beyond recognition by researchers using relatively low resolution techniques of molecular biology.

The second one suggests elimination of the paternal mtDNA

>In mammals, the inheritance of mitochondrion and its DNA (mtDNA) is strictly maternal, despite the fact that a sperm can inject up to 100 functional mitochondria into the oocyte during fertilization. The mechanisms responsible for the elimination of the paternal mitochondria remain largely unknown.