Differences Between Germline Genomes of Monozygotic Twins
Identical (monozygotic) twins are genetically similar but are not 100% the same. Over time, mutations occur in the individual’s blood cells. People have been wanting to track the number and timing when these mutations emerge compared to the other twin, also to see if it has been passed on or not. It has been determined that these changes are postzygotic mutations, meaning the cells change over the lifespan and not by genetics. Some mutations can be found in somatic tissues to determine if it is a postzygotic mutation or not, but only at low frequency. In Addition, the number of postzygotic mutations increases with the age of the individual.
When finding postzygotic mutations, comparing whole-genome sequencing (WGS) is a must. This allows researchers to compare each complete DNA sequence to see the similarities or differences that would be pointed out. The work allows people to look into the early stages of embryonic progress. This work has concluded two things; either both of them have the same cells that have been evenly distributed from their parents or not. It helps determines the number of mutations, the type, and the time in which they were formed.
The variant allele frequency (VAF) double checks in the postzygotic mutations are correct or not by checking if the cells are somatic or inherited from the unequal or equal distribution of the genetic code. A twin with ten or more mutations is considered to have a low VAF than a twin with less than 10. These alterations are sourced more commonly in the blood compared to the oral cavity. One twin can have different postzygotic mutations compared to the other but finding one can help determine the development of the other one too. Similar somatic VAFs mean that both twins have the same cellular makeup. The main differences between twins are based on the genetic lineages in their early stages of growth.
To conclude more information, a study of a parent twin producing an offspring of twins was conducted. Most mutations that the parent had were also passed on to the children. It is believed 21 mutations to be passed on to their immediate descendants, which would originally show up during pre-twinning. Yet, the majority of the young twins had a difference in their mutations. When a twin has different cells, one twin could have an almost constructed mutation while the other does not have it at all. Additionally, damaging or eliminating cells can also increase the twin’s differences because repair to DNA is reduced during the early stages of life. This will cause more mutations, as well as the more times a cell is divided. Mutations are usually more nonexistent to the twin with lower VAF. A lower VAF can also mean the mutation took a while to be fully formed. Also, from a parent who has a high VAF then the number of mutations expected from the children is reduced.