Not all chromosomes experienced a crossover and, as such, 11 chromosomes are either completely European or Micronesian.
This further suggests that in their evolution, promoters located on these chromosomes experienced few point mutations and accumulated more Slipped Strand Mispairing (SSM) mutations [ 53].
The "Darwin 2009" project was flexible and adaptable to all ages and school levels (primary school did not take part in the chromosome experience) because it was aimed at building framing concepts and not at "transmitting" information or explanations about evolution; this was possible also without precise previous scientific knowledge.
This is due to the fact that the X chromosome experiences sex-specific selection pressure [ 59].
Second, in many taxa, the X chromosome experiences meiotic sex chromosome inactivation (MSCI), the facultative heterochromatinization and transcriptional silencing of the sex chromosomes prior to the autosomes during early meiosis I (Lifschytz 1972; Turner 2007; Namekawa and Lee 2009).
Although the subsequent double-stranded DNA breaks (DSBs) could simply reflect the mechanical forces applied by the spindle to the dicentric chromosome, breakage of the dicentric is observed when the chromosome experiences forces of ∼1 piconewton (pN) (Fisher et al. 2009), much less than the force required to mechanically break a double-stranded DNA molecule (∼480 pN; Bensimon et al. 1995).
Interestingly, the two shorter linkage groups correspond to chromosomes having experienced few chromosomal fusion events (three for PsI and two for PsVI) during evolution, while longer linkage groups correspond to chromosomes having experienced more fusions (at least six for PsIII and four for PsV), suggesting a link between genome evolution and recombination distribution.
Thus, in addition to the gradient in gene dosage from the single replication origin to the terminus found on any single bacterial chromosome (Sharp et al. 1989), genes on the secondary chromosome will experience reduced copy number than those on the primary chromosome, on average.
A teleost fish specific whole genome duplication resulted in duplication of the Mhc region to two different chromosomes, each experiencing subsequent rearrangements and gene losses [ 32, 33].
Third, the X chromosome may experience a faster rate of adaptive evolution than the autosomes which could, in turn, contribute to the evolution of more X-linked hybrid incompatibilities (Charlesworth et al. 1987; Coyne and Orr 1989; Presgraves 2008b; Hvilsom et al. 2012; Llopart 2012; Meisel et al. 2012; Meisel and Connallon 2013; Kousathanas et al. 2014).
