Human supernumeraries: are they B chromosomes?

Abstract

In humans, the presence of supernumerary chromosomes is an unusual phenomenon, which is often associated with developmental abnormalities and malformations. In contrast to most animal and plant species, the extensive knowledge of the human genome and the ample set of molecular and cytogenetic tools available have permitted to ascertain not only that most human supernumerary chromosomes (HSCs) derive from the A chromosome set, but also the specific A chromosome from which most of them arose. These extra chromosomes are classified into six types on the basis of morphology and size. There are both heterochromatic and euchromatic HSCs, the latter being more detrimental. Most are mitotically stable, except some producing individual mosaicism. No information is available on the HSC transmission rate since extensive familial studies are not usually performed generally because of death of the relatives or lack of cooperation. The main B chromosome property failing in HSCs seems to be their population spread as polymorphisms, since most HSCs seem to correspond to extra A chromosomes or centric fragments spontaneously arisen in the analysed individual or one of his/her parents. However, we cannot rule out at this moment, that more intensive studies on population distribution and frequency of those HSCs most closely resembling B chromosomes (i.e. those heterochromatic and thus less detrimental) would reveal possible HSCs polymorphisms. Although HSCs cannot be considered B chromosomes, some of them might be a source for future B chromosomes. The best candidates would be heterochromatic HSCs, which might manage to drive in either sex. To ascertain this possibility, research on inheritance and population studies would be very helpful in combination with the powerful cytogenetic and molecular tools available for our species.

Trends in Population Sex Ratios May be Explained by Changes in the Frequencies of Polymorphic Alleles of a Sex Ratio Gene

“…If the female-producing tendency were due to separate genes that suppress the meiotic-drive gene then continual variation would not be seen. Variable dosage of a sex-determining supernumerary B chromosome has been suggested to explain male-specific inheritance and continual variation in B. schaefferi (Beladjal et al 2002), but supernumerary chromosomes are unusual in humans and often associated with malformations (Fuster et al 2004). Another hypothesis is that a polygenic system gives the father zygotic control over sex, so after fertilisation the equal sex ratio imposed by a major sex-determining gene in the mother is modified (Premoli et al 1996), but this idea has been criticised on the basis that the polygenic system would presumably also be transmitted through females (Voordouw et al 2005).…”

B chromosome ancestry revealed by histone genes in the migratory locust

“…In human beings, the presence of supernumerary marker chromosomes is frequently reported, which, in most cases, have been shown to derive from one of the human A chromosomes (Fuster et al 2004;Liehr et al 2008). These extra chromosomes are young and their precise origin can be traced back with the powerful tools that molecular cytogenetics provides in our species.…”

Small supernumerary marker chromosomes (sSMC) in humans; are there B chromosomes hidden among them

“…There are several similarities between sSMC and B-chromosomes: they represent a heterogeneous collection of chromosomes added to the standard karyotype, they are small in size, may consist of heterochromatic and/or euchromatic material, there is predominance of maternal transmission and they demonstrate a tendency to mitotic instability [ 1 , 3 - 5 , 11 - 13 ]. Most human sSMC seem to be evolutionary young elements, as still their origin may be tracked to another human chromosome through molecular analyses.…”