With increasing maternal age, the chances of observing trisomies increase significantly. Increasing paternal age is associated with de novo point mutations. Why?

Mother: higher risk of an improper chromosome division

Father: higher risk to suffer point mutations in sperm cells

Explanation:

A trisomy is an alteration in the chromosome number (i.e., an aneuploidy), and occurs when a cell receives three chromosomes instead of two. The risk of aneuploidies -including trisomies- increases with the mother's age due to the fact that older eggs have a higher chance of dividing unequally, thereby leading to an imbalanced number of chromosomes between daughter cells. Moreover, a point mutation is defined as a genetic modification where a single nucleotide is changed in the DNA sequence. It has been shown that point mutations in children are determined to a large extent by the father’s age, which are due to mutations in germline cells (i.e., sperm cells) that were passed to the progeny. A child born from a father who is 20 years old exhibits on average 40 point mutations, whereas a child born from a father who is 40 years old has typically twice the point mutations on its genome.