Other posts explained it well, so just adding a fun fact.
Basically, in theory for every gene, every parent can give one of their two versions, that differ in a few percent of cases.
If you take into account all possible combinations of alleles given by each parent, you get 10⁵⁰⁸ possibilities for the baby's genome.
That number makes no sense. For comparison, there's about 10⁸⁰ particles in the observable universe...
(Details: there's about 20,000 genes, 6,7% of which are heterozygous, so about 1,340. For every such gene, you have 2 choices: allel 1 or 2? So, total of 2^1340... squared, because there's two parents. This doesn't even take into account mutations that always occur.)

Merit is a mere fiction used to sparkle guilt in the poor and confidence in the dominant classes of society.
As a kid with good grades, I didn't choose to have a stable, loving, caring family with a strong cultural capital, to be interested in reading, counting, learning, solving problems. I didn't choose to be a white kid in a privileged school.
A kid who has bad grades didn't choose that. Didn't choose their social and economical conditions, that their body resists staying silent on a chair 8h/day, to be plagued by dyslexia, to have hobbies that have nothing in common with what school teaches...
Now, as a teacher who reads a few studies, I've never been so sure that there is no such thing as "merit".