Difference Between Aneuploidy and Polyploidy

Main Difference – Aneuploidy and Polyploidy

Aneuploidy and polyploidy describe the changes in the chromosome and chromosome sets number of a cell respectively. Different numbers of chromosomes are born by different species. Aneuploidy is the presence or the absence of chromosomes other than its normal number, which causes chromosomal abnormalities in the cell. Polyploidy is the acquisition of one or more chromosome sets additionally by a normal diploid cells. The key difference between aneuploidy and polyploidy is that aneuploidy is the numerical change in cell’s usual chromosomes and polyploidy is the numerical change in a cell’s usual chromosome sets.

This article looks at,

1. What is Aneuploidy
      – Definition, Characteristics, Disorders
2. What is Polyploidy
      – Definition, Characteristics, Examples
3. What is the difference between Aneuploidy and Polyploidy

What is Aneuploidy

The presence of an abnormal number of chromosomes in a cell is referred to as the aneuploidy. This includes the presence of extra chromosomes or absence of the usual number of chromosomes. Aneuploidy causes genetic disorders including birth defects in human beings. It may also cause cancers. During the production of gametes, improper segregation of chromosomes between the cells leads to aneuploidy. Genetic disorders arising from the aneuploidy can be divided into two: autosomal disorders and sex chromosome disorders. 

Human cell contains 46 chromosomes – 22 homologous pairs and 2 sex chromosomes. During meiosis, homologous pairs are segregated into each cell one by one. The two sex chromosomes are also segregated to two daughter cells. But, some pairs remain unsegregated and located in one gamete. The resulting one gamete thereby contains extra chromosome whereas the other is missing a chromosome. In some conditions, these abnormal chromosome numbers can be tolerated. But some may be lethal (miscarriage).

Aneuploidy can be described under different terminologies such as nullisomy, monosomy, disomy, trisomy and tetrasomy. Nullisomy is a missing pair of homologous chromosomes. Monosomy is the lack of one chromosome in a diploid nucleus. Disomy is the presence of the two copies of a chromosome, which is the normal condition of human somatic cells. Trisomy is the presence of three copies of a chromosome. Tetrasomy/ Pentasomy is a rare condition with autosomes where presenting four or five copies of chromosomes. Chromosomal disorders occur in humans are described in table 1.   

Table 1: Chromosomal Disorders

Down syndrome is the most common type of chromosomal disorders in live-born humans. The chromosome appearance in Down syndrome’s karyotype is shown in figure 1. The syndrome consists of three copies of chromosome 21. 

Figure 1: Down syndrome karyotype

Somatic mosaicism for chromosome 21 can occur in neurons of the brain due to defects of neuronal precursor cells in cell division. Somatic mosaicism also occurs in all virtual cancers such as chronic lymphocytic leukemia (CLL) in trisomy 12 and acute myeloid leukemia (AML) in trisomy 8. Aneuploidy can be detected through karyotyping.

What is Polyploidy

Consisting of more than two homologous chromosome sets in a cell is referred to as polyploidy. Human is a diploid organism, consists of two homologous chromosome sets. But in ferns and flowering plants, polyploidy is observed frequently. Epulopiscium fishelsoni like large bacteria containing more than two chromosome sets in their cytoplasm. Endoplolyploidy is the polyploidy in diploid human’s muscle cells, liver and bone marrow.

Polyploidy can occur due to the abnormalities in cell division, both during mitosis or metaphase I in meiosis. It is rare in humans other than in differentiated tissues. But, triploidy, designated as 69, XXX and tetraploidy, designated as 92, XXXX occur in humans. Triploidy can be either digyny or diandry. Digyny occurs due to two haploid sets from mother. Diandry occurs due to two haploid sets from the father. Most of the time, polyploidy in humans end up with miscarriage.

Treating the seeds with a chemical called colchicine induces polyploidy in crops. Polyploidy of crops can be used either to overcome hybrid species’ sterility or in some cases to achieve sterile fruits. Examples of crops for polyploidy is included in Table 2.  

Table 2: Examples for the Polyploidy Crops

Terminology

Autopolyploidy

Polyploidy which is derived from the chromosomes of a single species is referred to as autopolyploidy. It occurs due to natural genome doubling. Ex: Potato, Banana, Apple

Allopolyploidy

Polyploidy derived from different species. Ex: Triticale (6n) is derived from Wheat (4n) and Rye (2n), Cabbage

Paleopolyploidy

Ancient genome duplications through mutations and gene translocations. Ex: Baker’s yeast, Rice

Figure 2: The correlation between chromosome sets

Difference Between Aneuploidy and Polyploidy

Definition

Aneuploidy: Aneuploidy is the presence of an abnormal number of chromosomes.

Polyploidy: Polyploidy is the presence of more than two homologous chromosome sets.

Occurrence in Humans

Aneuploidy: It is more common in humans.

Polyploidy: It is rare in humans.

Types

Aneuploidy: Nullisomy, Monosomy, disomy, trisomy and tetrasomy.

Polyploidy: Triploid, Tetraploid, Hexaploid, Octaploid, Dodecaploid etc.

Influence in Human

Aneuploidy: It causes chromosomal disorders. Some of disorders are lethal.

Polyploidy: Triploid and tetraploid situations are lethal.

Human Somatic Cells

Aneuploidy: Chronic lymphocytic leukemia (CLL) in trisomy 12, Acute myeloid leukemia (AML) in trisomy 8, Somatic mosaicism for chromosome 21 in neuron cells in brain.

Polyploidy: Differentiated cells such as liver, muscle and bone marrow.

Conclusion

Aneuploidy is described in chromosome numbers. It causes abnormalities in chromosomes of humans. Some of them are tolerable whereas the others end up with miscarriage. Polyploidy is the abnormal number of chromosome sets. Triploid and tetraploid are the possible situations in humans. But, both of them end up with miscarriage. Therefore, the key difference between aneuploidy and polyploidy is the abnormal numbers of chromosomes and the chromosome sets.

Reference:
1. “Aneuploidy”. Wikipedia, the free encyclopedia, 2016. Accessed 21 Feb 2017
2. Griffiths AJF, Miller JH, Suzuki DT, et al. “Aneuploidy”, An Introduction to Genetic Analysis. 7th edition. New York: W. H. Freeman, 2000. NCBI Bookshelf. Accessed 21 Feb 2017
3. O’Connor, C. “Chromosomal abnormalities: Aneuploidies”. Learn at Scitable, Nature Education, 2008, 1(1):172. Accessed 21 Feb 2017
4. “Polyploid”. Wikipedia, the free encyclopedia, 2017. Accessed 21 Feb 2017
5. Woodhouse, M., Burkart-Waco, D. and Comai, L. “Polyploidy”. Learn at Scitable, Nature Education, 2009, 2(1):1. Accessed 21 Feb 2017
6. “Polyploidy”. NEW WORLD ENCYCLOPEDIA, 2008. Accessed 21 Feb 2017

Image Courtesy:
1. “Down Syndrome Karyotype” Courtesy: National Human Genome Research Institute – Human Genome Project, (Public Domain) via Commons Wikimedia
2. “Haploid, diploid, triploid, tetraploid.svg”. By By Haploid_vs_diploid.svg: Ehambergderivative work: Ehamberg (talk) – Haploid_vs_diploid.svg (CC BY-SA 3.0) via Commons Wikimedia