Part of the show Sex Chromosomes, Genetics and Food Webs
Phil in Milton Keynes asked:
If as a human I share 98% of my genes with a chimpanzee and 60% of my genes with a banana, how come I only share 50% of my genes with my own daughter?
I would say that you actually share more than 98% of your genes with a chimpanzee. I suspect that virtually all of the genes in the human genome also have counterparts in the chimpanzee genome. The most likely explanation for the fact that we are so obviously different from chimpanzees is the way in which these genes are controlled and the way they are switched on and off, and the length of time for which genes are active. You share 50% of your genes with your daughter because she's obviously inherited one genome from you and one genome from her mother. We all have two genomes in our bodies: one from our mother and one from our father, but there are counterparts to all of your genes in the genome that your daughter has inherited from her mother. If you compare a banana with a human, just over half the genes in a banana will do the same job in a banana as they do in a human. However, the genes themselves will not be the same letter for letter; they just perform the same function. In contrast, when you are talking about the genes you share with your daughter, you are not asking how many of the genes have the same function (which is 100%), you are asking how many of those genes are absolutely identical, letter for letter. The probability that any one of those genes came from the father is 50%, and the probability that a gene has come from the mother is also 50%. This is why you share 50% of your genes with your daughter.