Human Genetic Mutations
JBS Haldane arrived at his number of 150 new human genetic mutations in each generation after studying men with haemophilia. He speculated that there was a 1 in 50,000 incidence of mutations causing haemophilia in the gene affected. That maps out to a mutation rate of 1 in 25 million nucleotides across the genome. Other studies have looked at the mutation rates of other genes and scientists have also compared human and chimp DNA to try and come up with a number.
The actual mutation rate is closer to 1 mutation in 15-30 million nucleotides, which is the equivalent of 100-200 new mutations in a genome.
An international team of scientists came up with this figure by comparing thousands of nucleotides from the same region of the Y chromosome from two Chinese men who were separated by 13 generations. They shared a common ancestor who was born in 1805.
The two men were members of a family that had lived in the same Chinese village for centuries. Researchers were able to look at genetic mutations and the rate that these mutations occurred by focusing on a section of the Y chromosome. The Y chromosome was studied because it passes from father to son relatively unscathed, and so the mutation rate is low.
The two men are distantly related and scientists compared 10,149,085 letters of the genetic code. Of those, 10,149,073 were identical. There were only 12 differences in the DNA studied. In other words, the Y chromosomes were virtually identical. Of the 12 differences, four were found to be true genetic mutations. By relating this number to the size of the genome, and knowing the length of the Y chromosome that was sequenced, plus the number of generations separating the two men, scientists were able to calculate the mutation rate.