In some bryophytes, microchromosomes have been found to co-occur with sex chromosomes and likely impact sex determination. Polyploidization can occur before and after the development of sex chromsomes. It can also occur when two fertilized embryo fuse, producing a chimera that might contain two different sets of DNA one XX and the other XY.
Unlike seed plants, where gametophytes are always unisexual, in bryophytes they may produce male, female, or both types of gamete. An explanation for this theory is that the X-chromosome simply inactivates in the presence of another X-chromosome; this causes XX-chromosome humans to have a lower frequency of the regulatory gene given that both X and Y chromosomes have an equal frequency of the regulator and so the expression of the male trait is prevented from appearing in the phenotype.
Meiotic sex chromosome inactivation MSCI is a repressive mechanism that occurs during meiotic prophase I, and involves elaboration of a specialized heterochromatin domain and transcriptional silencing of heteromorphic sex chromosomes Turner, If it occurs after sex chromosomes are established, dosage should stay consistent between the sex chromosomes and autosomes, with minimal impact on sex differentiation.
Partner choice during meiosis is regulated by Hop1-promoted dimerization of Mek1. A pair of X and Y results in a chromosomal male whereas a combination of X and X results in a chromosomal female. Namespaces Article Talk.
Featured Content. Epigenetic regulation in male germ cells.
Regardless of how different organisms have modified these homology-dependent processes, heterogametic sex chromosomes of worms, mice and chicken all undergo MSCI, a specialized form of meiotic silencing Kelly and Aramayo, ; Maine, ; Turner, Exp Cell Res.
Sex determination and differentiation. Since usually men inherit Y chromosomes, they are the only ones to inherit Y-linked traits. Male sterility likely arises first as an adaptation to prevent selfing. If it occurs after sex chromosomes are established, dosage should stay consistent between the sex chromosomes and autosomes, with minimal impact on sex differentiation.
Paula M. Over time, this altered pattern of recombination would allow sequence divergence, except at the distal-most end, resulting in establishment of a PAR. Cytogenetic and Genome Res.