What are aneuploids? Give a brief account of their meiotic behavior.

Aneuploidy refers to a condition in which an organism or cell has an abnormal number of chromosomes that is not an exact multiple of the haploid set. Aneuploids have either extra chromosomes or are missing chromosomes, differing from the euploid condition, where the chromosome number is a whole multiple of the basic set (such as diploid 2n, triploid 3n, etc.). Aneuploidy is commonly observed in both plants and animals and can result in a variety of developmental and physiological disorders.

Types of Aneuploidy

There are several types of aneuploidy based on the gain or loss of individual chromosomes:

1. Monosomy (2n - 1): One chromosome is missing from the normal diploid set. For example, Turner syndrome (XO) in humans is a monosomic condition.
2. Trisomy (2n + 1): One extra chromosome is present. A well-known example is Down syndrome (Trisomy 21).
3. Nullisomy (2n - 2): Both homologs of a particular chromosome pair are missing. Nullisomics are generally inviable in diploids but can survive in polyploid plants.
4. Tetrasomy (2n + 2): Two extra copies of a single chromosome are present.
5. Double trisomy (2n + 1 + 1): Extra copies of two different chromosomes.

These chromosomal changes usually arise due to errors in meiotic division, particularly non-disjunction.

Causes of Aneuploidy

Aneuploidy typically results from non-disjunction, an error during meiosis I or meiosis II, where homologous chromosomes or sister chromatids fail to separate properly. This leads to gametes with abnormal chromosome numbers:

• In meiosis I, non-disjunction of homologous chromosomes results in two gametes with an extra chromosome (n + 1) and two with one less (n - 1).
• In meiosis II, failure of sister chromatids to separate causes one gamete with (n + 1), one with (n - 1), and two normal (n) gametes.

When such abnormal gametes fuse with normal ones during fertilization, they form zygotes with aneuploid chromosome numbers.

Meiotic Behavior of Aneuploids

The behavior of aneuploids during meiosis is often aberrant and irregular, particularly in organisms with missing or extra chromosomes. The pairing and segregation of chromosomes are affected, leading to gametes with unbalanced chromosome sets.

1. Monosomics (2n - 1):

• In monosomics, a particular chromosome is missing its homologous partner.
• During meiosis, the unpaired chromosome (called a univalent) does not form a normal bivalent.
• The univalent may be randomly lost, lag behind, or occasionally segregate with a random chromosome.
• This leads to gametes that are highly unbalanced and usually nonviable.

2. Trisomics (2n + 1):

• Trisomics have one extra chromosome, resulting in three homologous chromosomes.
• During meiosis, these three chromosomes form a trivalent (three chromosomes pairing together) or a bivalent and a univalent.
• The trivalent can segregate in various ways: 2:1 segregation: Two chromosomes go to one pole, and one to the opposite. 1:2 segregation: The reverse of the above.
• These irregular segregations result in gametes with either extra or missing chromosomes, reducing fertility.

3. Nullisomics (2n - 2):

• Both chromosomes of a homologous pair are missing.
• In diploids, this condition is generally lethal, as essential genes on that chromosome pair are missing.
• In polyploids (e.g., wheat), nullisomics can survive due to multiple chromosome sets that can compensate for the loss.

4. Tetrasomics (2n + 2):

• These have four copies of a particular chromosome.
• During meiosis, quadrivalents (groupings of four homologous chromosomes) may form.
• Their segregation is complex and often results in unbalanced gametes, reducing fertility.

Consequences and Significance

Aneuploidy often results in reduced viability or fertility due to gene dosage imbalances. In animals, especially humans, most aneuploid conditions are lethal, except for a few (like trisomy 21). In plants, however, many aneuploids survive and are used in genetic research and breeding to study gene function and chromosome mapping.

For example, in wheat genetics, nullisomic and monosomic lines are instrumental in identifying genes associated with particular chromosomes. In medical genetics, studying aneuploidy has been crucial in understanding chromosomal disorders, prenatal diagnostics, and cancer biology.

Conclusion

Aneuploidy represents a deviation from the normal chromosomal number and plays a vital role in both genetics and evolution. Though often detrimental, it provides insight into chromosome behavior during meiosis and gene function. The irregular meiotic behavior of aneuploids leads to abnormal gametes and altered phenotypes, making them valuable tools in cytogenetics, plant breeding, and human genetic studies.

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