This study compared parent and offspring genotypes in 78 sets of Icelandic families and revealed a strong influence of paternal age on spontaneous DNA mutation in offspring.  They found that a newborn’s DNA contains an average of 60 new small-scale mutations, and that this number is correlated with the age of the father at the time of conception. Their data showed that on average, a 20 year old father transmits approximately 25 de novo mutations to his child. By comparison, a 40 year old father transmits approximately 65 mutations. The number of de novo mutations transmitted by the mother is approximately 15, regardless of age. It has been previously reported that up to 10% of point mutations are deleterious in humans and a causal role of de novo mutations has been demonstrated in autism. Some brain disorders appear to be particularly susceptible to the paternal-age effect. These include autism, schizophrenia, dyslexia and lower intelligence. Kondrashov has noted that this is consistent with the fact that more genes are expressed in the brain than in any other organ, so that the fraction of new mutations that will affect its function is the highest.

Kong A, Frigge ML, Masson G, Besenbacher S, Sulem P, Magnusson G, Gudjonsson SA, Sigurdsson A, Jonasdottir A et al: Rate of de novo mutations and the importance of father’s age to disease risk. Nature 488(7412): 471-475 (2012).

http://www.ncbi.nlm.nih.gov/pubmed/22914163

See also: Kondrashov A: Genetics: The rate of human mutation. http://www.nature.com/nature/journal/v488/n7412/full/488467a.html