Scientists have uncovered a surprising mechanism behind how genetic variation arises in human sperm, showing that some DNA modifications happen earlier in sperm development than previously thought.
This discovery, detailed in the journal Nature, was made by researchers from the Wellcome Sanger Institute, the University of Cambridge, and other scientific institutions. The findings may enhance our understanding of human fertility, inherited genetic conditions, and how the human genome changes across generations.
Every individual inherits a set of chromosomes from their mother and another from their father. However, the DNA handed down isn’t just an exact copy of the parent’s genome. During the formation of sperm and eggs, chromosomes undergo a process called recombination, which shuffles genetic material and contributes to the genetic uniqueness of each child.
Traditionally, scientists believed that most of this genetic mixing, known as recombination, takes place during meiosis—the specialized cell division forming sperm and eggs. Recombination can occur in different ways: one involves crossover events, where large segments of DNA are exchanged between chromosomes; another involves non-crossover gene conversion, a smaller process where a brief segment from one chromosome is copied onto its homolog.
Detecting these tiny gene conversions has been challenging, leaving questions about the precise timing of these events. To explore this, researchers analyzed 15 sperm samples from 13 men aged between 24 and 74. They employed high-precision long-read DNA sequencing technology, which allows detailed examination of long stretches of genetic material.
Through this analysis, they identified over 7,100 crossover events and approximately 2,400 non-crossover gene conversions directly in sperm DNA. Surprisingly, many of the gene conversions appeared to occur before meiosis, during normal cell divisions that replenish sperm-producing cells throughout a man’s life.
This indicates that genetic recombination in sperm can take place in at least two stages: prior to meiosis and during meiosis itself. The early gene conversions displayed distinct molecular features resembling DNA repair processes seen in regular body cells, suggesting that DNA repair before sperm development may be an overlooked source of genetic diversity.
Moreover, the number and locations of these gene conversions varied between individuals, even among identical twins who share nearly identical DNA. This variation implies that the genetic diversity passed to offspring is shaped not only by inherited DNA but also by biological processes occurring within a person’s lifetime.
These insights could also have implications for inherited diseases. Certain DNA-copying events happen in regions of the genome prone to damage or breaks. Errors during repair in these regions could potentially lead to harmful genetic mutations passed on to children.
By directly observing thousands of these events, researchers have opened new avenues to study how the human genome balances maintaining DNA stability with generating the genetic variability needed for evolution and individual differences.
Overall, the results show that the process contributing to each person’s unique genetic blueprint begins earlier than previously understood, adding a new dimension to the complex mechanisms behind human genetic diversity.