Polyploidy

Polyploidy is the presence of one or more complete extra sets of chromosomes in a cell. In humans the common forms are triploidy (three full sets, 69 chromosomes) and tetraploidy (four full sets, 92 chromosomes); both are typically lethal in early development. Polyploidy is distinct from aneuploidy, which involves the gain or loss of individual chromosomes rather than whole sets.

Definition

Polyploidy is a chromosome complement that contains one or more complete additional haploid sets, so that the total chromosome number is a multiple of the haploid number greater than the normal diploid two (for example, triploidy with three sets or tetraploidy with four).

Scope

The topic covers the concept of whole-genome multiplication, the fertilisation and cell-division errors that cause it, the parental origin of the extra set and its developmental consequences, and the contrast between human polyploidy and the constitutive polyploidy seen in some normal tissues and in many plants. It is treated as a reference cytogenetic concept; in humans constitutional polyploidy is generally not a viable clinical entity, and the entry offers no clinical guidance.

Core questions

• How does polyploidy differ from aneuploidy in mechanism and consequence?
• What fertilisation and division errors produce an extra whole chromosome set?
• Why does the parental origin of the extra set matter for the developmental outcome of triploidy?
• Why is constitutional polyploidy generally not viable in humans, despite occurring normally in some cells and many other species?

Key concepts

• Triploidy (69 chromosomes)
• Tetraploidy (92 chromosomes)
• Whole chromosome set versus individual chromosome
• Diandry and digyny (paternal versus maternal extra set)
• Dispermy and failed polar-body extrusion
• Genomic imprinting and parental-origin effects
• Physiological polyploidy in normal tissues

Mechanisms

Triploidy usually arises at fertilisation, either when two sperm fertilise one egg (dispermy) or when a diploid gamete is produced because a meiotic division failed, leaving an unreduced egg or sperm. The extra set is therefore either paternal in origin (diandric) or maternal (digynic), and the parental origin influences the developmental phenotype because of genomic imprinting — the differential expression of genes according to which parent they came from. Tetraploidy typically results from failure of cytokinesis after the first cell division, doubling the diploid set. Whereas aneuploidy unbalances the dosage of one chromosome, polyploidy adds an entire balanced set yet still profoundly disrupts human development; meanwhile, physiological polyploidy occurs normally in certain cell types such as hepatocytes and megakaryocytes.

Clinical relevance

Constitutional triploidy and tetraploidy are recognised chiefly through their role in early pregnancy loss and in pathology specimens; diandric triploidy is associated with partial molar changes of the placenta. This entry describes the concept and its biological basis for reference and evidence appraisal and does not provide diagnostic or management guidance.

Epidemiology

Polyploidy is a frequent cause of spontaneous abortion, with triploidy being the more common form among recognised pregnancies; constitutional polyploidy is essentially absent among surviving liveborns, the rare reported survivors typically being mosaic. The relative frequency of diandric versus digynic triploidy and its association with placental phenotype have been characterised in studies of the parental and meiotic origin of triploid conceptions.

History

Once human chromosomes could be counted reliably from the late 1950s, triploid and tetraploid karyotypes were soon recognised among spontaneous abortions, establishing whole-set anomalies as a major cause of pregnancy loss. Later work used DNA polymorphisms to determine the parental origin of the extra set and linked diandric and digynic triploidy to distinct placental and fetal phenotypes, connecting polyploidy to the broader study of genomic imprinting.

Key figures

• Dorothy Warburton
• Deborah McFadden
• Terry Hassold

Seminal works

• mcfadden-langlois-2000
• hassold-hunt-2001

Frequently asked questions

What is the difference between polyploidy and trisomy?

Trisomy is the gain of a single chromosome (one extra copy of one chromosome), whereas polyploidy is the gain of one or more complete chromosome sets, such as triploidy with 69 chromosomes instead of 46.

Is polyploidy ever normal in human cells?

Yes. Although a polyploid karyotype is not viable for a whole person, certain specialised cell types, such as some liver cells and the platelet-producing megakaryocytes, are normally polyploid as part of their function.