Imagine starting with a pure Neanderthal and a pure Denisovan having a child.

That child — a first-generation hybrid — would inherit exactly one copy of each chromosome from each parent.

Since humans have 23 pairs of chromosomes, the child would have 23 chromosomes from the Neanderthal parent and 23 from the Denisovan parent.

The genome would be, in the aggregate, exactly 50 percent Neanderthal and 50 percent Denisovan.

Now imagine that child having a child with a Neanderthal.

The second-generation child would inherit 50 percent of their genome from the first-generation hybrid — which was itself 50 percent Neanderthal and 50 percent Denisovan — and 50 percent from a pure Neanderthal parent.