Main Difference – Euploidy vs Aneuploidy
Each organism has a defined chromosome number in their genome. The number of chromosomes, as well as the number of chromosome sets, can vary due to various mechanisms that occur in the sexual reproduction. Euploidy, aneuploidy, and monoploidy are three terms used to describe the variation of chromosome number in the genome. The main difference between euploidy and aneuploidy is that euploidy is the increase of the number of chromosome sets in the genome whereas aneuploidy is the variation in the number of a particular chromosome within the set. Monoploidy is the loss of an entire set of chromosomes from the genome.
Key Areas Covered
1. What is Eupolidy
– Definition, Variations, Causes
2. What is Aneuploidy
– Definition, Variations, Causes
3. What are the Similarities Between Euploidy and Aneuploidy
– Outline of Common Features
4. What is the Difference Between Euploidy and Aneuploidy
– Comparison of Key Differences
Key Terms: Alloploidy, Allopolyploidy, Aneuploidy, Autopolyploidy, Chromosome Number, Chromosome Sets, Complete Nondisjunction, Euploidy, Meiotic Nondisjunction, Monosomy, Nullisomy, Trisomy
What is Eupolidy
Euploidy refers to the state of having a chromosome number that is an exact multiple of a basic chromosome set. This means the number of chromosome sets is increased in euploidy. The somatic chromosome number of a particular organism is defined as n. Based on the number of chromosome sets, the euploid genome can be classified as monoploids, diploids, and polyploids. Monoploids (n) consist of a single set of chromosomes while diploids (n) consist of two sets of chromosomes. Polyploids consist of more than two chromosome sets. They can be triploid (3n), tetraploid (4n), pentaploid (5n), hexaploid (6n), etc. Individuals with an odd number of chromosomes are usually sterile. A variable number of chromosome sets are shown in figure 1.
Figure 1: Euploidy
Euploidy mainly occurs in plants. Complete nondisjunction is the mechanism that leads to euploidy in which all the chromosome in a set migrates to one daughter cell. The main natural method that leads to eupolidy is interspecies crosses, a cross between distinct species. Autopolyploidy, alloploidy, and allopolyploidy are the three mechanisms that lead to euploidy in interspecies crosses. Autopolyploidy is the possession of more than two chromosome sets derived from the same species. Alloploidy is the presence of two or more sets of chromosomes derived from two different species. Allopolyploidy is a combination of both autopolyploidy and alloploidy in which the genome may be made up of the chromosome sets of one or more species. Euploidy through the interspecies crosses leads to sympatric speciation that occurs in related species with similar physical characteristics and similar niches.
What is Aneuploidy
Aneuploidy refers to a condition in which one or a few chromosomes are added or deleted from the normal chromosome number. Hence, the number of chromosomes in aneuploidy can be greater or smaller than the number of chromosomes in the wild type, a strain that prevails among individuals in natural conditions. Various types of aneuploidy can be identified as nullisomy, monosomy, and trisomy. Nullisomy (2n-2) is the loss of both chromosomes of the homologous pair. This conditions may be lethal in most organisms. Monosomy (2n-1) is the loss of a single chromosome of the homologous pair. The human genome is diploid (2n), consisting of 44 autosomes and two sex chromosomes. Turner syndrome (44+XO) is an example of monosomy in humans. Trisomy is the gain of an extra chromosome (2n+1). Klinefelter syndrome (44+XXY/XYY) and Down syndrome are the examples of trisomy. The chromosome arrangement of Down syndrome is shown in figure 2.
Figure 2: Down Syndrome (the presence of extra chromosome 21)
Meiotic and mitotic nondisjunction are the main causes of aneuploidy. The failure of homologous chromosomes to separate during the anaphase 1 of meiosis results in gametes with greater or lesser number of chromosomes. During mitosis, the failure of sister chromatids to separate from each other may also result in the abnormal number of chromosomes in the daughter cells. Chromosome loss is another cause of aneuploidy in which one of the sister chromatids does not migrate to the pole during mitosis. Aneuploidy leads to the production of an imbalanced amount of gene products due to the presence of an abnormal number of genes that encode them.
Similarities Between Euploidy and Aneuploidy
- Euploidy and aneuploidy are two types of variations in the chromosome number in the genome of a particular organism.
- In both euploidy and aneuploidy, the amount of genetic material of the cell is changed.
- Both euploidy and aneuploidy may occur during the formation of gametes in sexual reproduction.
Difference Between Euploidy and Aneuploidy
Definition
Euploidy: Euploidy refers to the state of having a chromosome number that is an exact multiple of a chromosome number.
Aneuploidy: Aneuploidy refers to a condition in which one or a few chromosomes are added or deleted from the normal chromosome number.
Type of Variation
Euploidy: Euploidy is a large variation in which the amount of genetic material increases by means of chromosome sets.
Aneuploidy: Anueploidy is a comparatively small variation in which the amount of genetic material varies by means of the number of chromosomes.
Variations
Euploidy: Diploid (2n), triploid (3n), and tetraploid (4n) are the variations in euploidy.
Aneuploidy: Nullisomy, monosomy, trisomy, and tetrasomy are the variations in aneuploidy.
Occurrence
Euploidy: Euploidy frequently occurs in plants and rarely in animals.
Aneuploidy: Aneuploidy occurs in both animals and plants.
Causes
Euploidy: Complete nondisjunction and interspecies crosses lead to euploidy.
Aneuploidy: Meiotic nondisjunction, mitotic nondisjunction, and chromosome loss lead to aneuploidy.
Role
Euploidy: Euploidy may lead to the formation of new species.
Aneuploidy: Aneuploidy leads to the imbalance in the number of gene products.
Conclusion
Euploidy and aneuploidy are two types of chromosome variations that occur during the formation of gametes in sexual reproduction. Euploidy is the presence of additional chromosome sets while aneuploidy is the presence of a variable number of chromosomes in the genome. Therefore, the main difference between euploidy and aneuploidy is the type of variation in each type of genome conditions.
Reference:
1. “Euploidy: Meaning and Types | Cell Biology.” Biology Discussion, 14 July 2016, Available here.
Image Courtesy:
1. “Haploid, diploid ,triploid and tetraploid” By Haploid_vs_diploid.svg: Ehambergderivative work: Ehamberg (talk) – Haploid_vs_diploid.svg (CC BY-SA 3.0) via Commons Wikimedia
2. “Down Syndrome Karyotype” By Courtesy: National Human Genome Research Institute – Human Genome Project (Public Domain) via Commons Wikimedia
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