Polyploidy

Non Disjunction

Non Disjunction

• Non-disjunction refers to the chromosomes failing to separate correctly, resulting in gametes with one extra, or one missing, chromosome (aneuploidy)

• occurs during meiosis 1

Chromosomes don't separate due to:

• Failure of homologues to separate in Anaphase I (resulting in four affected daughter cells)

• Failure of sister chromatids to separate in Anaphase II (resulting in only two daughter cells being affected)

Non-disjunction in Meiosis 1

results from the failure of chromosomes to separate during cell division because the spindle does not function properly. As a result, all the chromosomes finish up in the same nucleus, which has twice as many chromosomes as it should have. This can happen in two ways. Either way, the normal halving of the chromosome number fails to occur and two diploid nuclei result, which later give rise to diploid gametes.

Polyploidy

results from hybridisation between species. This occurs when chromosomes do not separate properly and so gametes have a diploid number of chromosomes.

Autopolyploidy

occurs when a polyploid offspring is derived from a single parental species (usually via self fertilisation). Involves the multiplication of the entire genome within a single species. Thus an autotetraploid has 4 sets of chromosomes. Results from the failure of chromosomes to separate during cell division because the spindle does not function properly. As a result, all the chromosomes finish up in the same nucleus, which has twice as many chromosomes as it should have. This can happen in two ways; either way, the normal halving of the chromosome number during meiosis fails to occur and two diploid nuclei result, which later give rise to diploid gametes. If a diploid gamete subsequently fuses with another diploid gamete, a triploid zygote is produced.

Allopolyploidy

Allopolyploidy occurs when a polyploid offspring is derived from two distinct parental species. Normally different species cannot interbreed because they have different chromosome numbers and cannot form diploid pairs. Offsprings produced by such unions are typically sterile due to this uneven chromosome pairing. If cytokinesis fails to occur in one of the gametes, the hybrid offspring will have paired chromosomes from that parent species. If the hybrid interbreeds with a member of the other parent species, all chromosomes from both parent species will be paired. The resulting offspring will now be fertile and have the combined chromosome composition of both parental species