Chromosome Number Mutations

1. Definition:

Chromosome number mutations are changes in the number of chromosomes that occur in one or more chromosome pairs in a cell.

2. Types of chromosome number mutations:

a) Aneuploidy:

• Trisomy (2n + 1): There are 3 chromosomes in one chromosome pair.

• Monosomy (2n – 1): One chromosome is missing from one chromosome pair.

• Tetrasomy (2n + 2): There are 4 chromosomes in one chromosome pair.

• Nullisomy (2n – 2): Two chromosomes are missing from one chromosome pair.

• Double monosomic (2n – 1 – 1): One chromosome is missing from two different chromosome pairs.

• Double trisomic (2n + 1 + 1): One extra chromosome is present in two different chromosome pairs.

b) Polyploidy:

• Autopolyploidy: The chromosome set is a multiple of the haploid chromosome set of the species (3n, 4n, 5n…).

• Allopolyploidy: The chromosome set is a multiple of the haploid chromosome set of two different species.

3. Mechanism of occurrence:

• Due to chromosome non-disjunction during meiosis:

• If one chromosome pair does not separate during meiosis I: It produces gametes (n + 1) and (n – 1), which through fertilization create mutant gametes.

• If one chromosome pair does not separate during meiosis II: It produces gametes (n + 1) and (n – 1), which through fertilization create mutant gametes.

• Due to chromosome non-disjunction during mitosis:

• If one chromosome pair does not separate during mitosis: It produces cells (2n + 1) and (2n – 1), forming a mosaic.

4. Consequences:

• Changes in phenotype: Most are harmful and often do not have sexual reproduction.

• Cause disease: For example:

• Down syndrome: Has 3 chromosomes number 21 (2n + 1).

• Turner syndrome: Has 1 sex chromosome XO (2n – 1).

• Klinefelter syndrome: Has 3 sex chromosomes XXY (2n + 1).

5. Significance:

• Raw material for evolution: Aneuploidy can create new genotypes, leading to biodiversity.

• Breeding: Polyploidy can create crop varieties with higher yield and better quality.

• Determining linkage groups: Studying aneuploidy helps to determine the location of genes on chromosomes.

Note:

• Chromosome number mutations are more common in plants than animals.

• Aneuploidy in each chromosome pair will have different manifestations.

• Polyploidy can be applied in production, for example, to create crop varieties with higher yields.

6. Exercises:

a) Mutant gametes:

• How many types of mutant gametes are there? What are they?

• There are two types of mutant gametes:

• Gamete with an extra chromosome (n + 1).

• Gamete with one missing chromosome (n – 1).

b) Gamete ratio:

• A diploid organism has n sperm cells undergoing meiosis. During meiosis I, m cells mutate, and meiosis II is normal. What is the ratio of non-mutant gametes?

• The ratio of non-mutant gametes is (n – m)/n.

• A diploid organism has n sperm cells undergoing meiosis. During meiosis I, m cells mutate, and meiosis II is normal. What is the ratio of gametes with an extra chromosome (n + 1) and one missing chromosome (n – 1)?

• The ratio of gametes with an extra chromosome (n + 1) and one missing chromosome (n – 1) are equal and equal to m/2n.

c) Zygote:

• What does a non-mutant zygote equal?

• Non-mutant zygote = non-mutant male gamete × non-mutant female gamete.

• What does a mutant zygote equal?

• Mutant zygote = 1 – non-mutant zygote.

d) Difference between autopolyploidy and allopolyploidy:

• What is the fundamental difference between autopolyploidy and allopolyploidy with the same number of chromosomes in the cell?

• Chromosome origin:

• Autopolyploidy has chromosomes from the same species.

• Allopolyploidy has chromosomes from two different species.

e) Mechanism causing mutation:

• What substance prevents spindle formation, so the chromosomes duplicate but do not separate?

• Colchicine (consixin).

f) Formation of cell lines:

• If a pair of homologous chromosomes does not separate in a group of somatic cells of an organism during mitosis, how many cell lines does the organism have? What are they?

• The organism has 2 cell lines:

• Normal cell line.

• Mutant cell line.

g) Allopolyploidy:

• Where is allopolyploidy commonly found?

• Allopolyploidy is commonly found in animals.

7. Other terms:

• Mosaic: An organism with different genotypes in its body.

• Somatic cell: Cells that make up the body.

• Germ cell: Specialized cells that produce gametes.

• Meiosis: The process of cell division in germ cells to produce gametes.

• Mitosis: The process of cell division in somatic cells to produce identical daughter cells.

Note:

• This article is only a part of the knowledge about chromosome number mutations. To learn more, you should refer to specialized materials.

• Pay attention to concepts, examples, and exercises to reinforce knowledge.

• Good luck!