A cutting edge study conducted by researchers at the University of Southampton, published in Nature Communications, has identified the power struggle between parental chromosomes which determines the traits inherited by offspring.
Immature egg cells, which have not yet undergone meiosis, contain two sets of chromosomes. Within these sets, one is inherited from the mother, one from the father, and it has been suggested that there is a struggle for survival between the two. Only one chromosome is able to survive and form part of the mature egg which can then be fertilised. The study involved visualisation of the maternal and paternal chromosomes. This uncovered that the chromosome with a larger centromere, a region on the chromosome that plays a critical role in cell division, is more likely to survive than its counterpart.
It has long been believed that the surviving chromosome is selected at random, and this has been supported by the seminal work of Gregor Mendel. Mendel’s law of independent assortment states that characteristics are inherited independently, and thus, gametes have an equal likelihood of inheriting either copy of a gene. Despite this, the study has shown that chromosomes can violate the laws of Mendelian genetics. The ‘selfish’ chromosomes favour their own retention within the egg, whilst the other is lost within a polar body, a meiotic by-product that is destined for degradation. This process is an example of meiotic drive, where one gene has been transmitted in preference over another.
In eggs, when only some chromosomes survive the creation of a new egg, we may have to think about the process as a struggle for survival and suddenly, at the level of the chromosome, whoever is suited to win that battle will win out overall.
It is not uncommon for embryos to have an abnormal number of chromosomes due to poor chromosomal division. Although currently lacking empirical evidence, this study has uncovered a potential link between the aforementioned power struggle and chromosomal disorders such as Trisomy 21, more commonly known as Down’s Syndrome.
While the findings of this study were based on observations in mice, it may lead to a change in our fundamental understanding of cell division and genetic inheritance in humans.