Why the estimate of human genes is from 20000 to 25000. Can two human being have so different genes numbers, why there is no absolute estimate of the number of genes present in human genome?
There are some gaps of the human genome for which the real sequence has not been elucidated yet. Therefore, some "genes" could still be "hidden" within the genome. Additionally, there is the issue of determining what a gene is, since there are several genomic elements with the potential to be referred as genes (e.g. ORFs, RNA coding genes, protein coding genes, etc.).
For example have a look at this paper:
https://www.nature.com/articles/s41588-018-0273-y
They found in people of African descent ~10% more DNA than in current genome assembly, that is 300 mln bp.
Not all genes are known or confirmed. So some are estimated from computational analyses of the sequence data. Since there are different ways of estimating putative genes (not to mention issues with deciding what a "gene" is or can be), there is a range of numbers of possible, yet to be confirmed, genes. Combine that with the fact that "the" human genome is not "a" genome, but the consensus genome of multiple individuals, and the problem of identifying unknown genes gets even more complicated.
The numbers for the reference genome assembly are a bit fun: 70% of its sequence comes from 1 individual, 23% from 10 individuals, and only 7% from >60 individuals. So actucally a "consensus" applies to a minority of its sequence. But the true consensus genomes and graph genomes are making their way.
This is a nice read:
Article Is it time to change the reference genome?
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