Lecture 8: Cytogenetics 4 Variations in Chromosome Number and Structure 1

Question 1

Variation in chromosome number: Euploid vs Aneuploid - define them.

Answer 1

Variation in Chromosome Numbers:
Refers to changes in the number of chromosomes in an organism’s cells.
Can occur through various mechanisms, including polyploidy, aneuploidy, and chromosomal aberrations.

Variation in chromosome number can take 2 forms:

Euploid - that which involves whole sets (genomes) of chromosomes. E.g. wild, durum and bread wheat

Aneuploid - that which involves part of the whole set of chromosomes, e.g. a single chromosome – Down’s syndrome

Question 2

Explain Euploid chromosome number variation
- 3 = monoploid, diploid, polyploid.

Answer 2

1. Monoploid
   - 1 copy of a genome
   common gametic chromosome number
2. Diploid
   - 2 copies of the genome
   common somatic chromosome number
3. Polyploid
   - 3 or more copies of a genome
   well - also a common chromosome number, especially in plants.

Question 3

Explain Variation in chromosome numbers: Autopolyploidy = 3

Answer 3

1. • The addition of one or more extra set(s) of chromosomes identical to the normal haploid complement of the same species
     * E.g. triploidy, tetraploidy, pentaploidy, hexaploidy
2. • Autotetraploids like potato and alfalfa show
     tetrasomic inheritance
     * Odd numbered polyploids are usually seedless (watermelon, grapes, etc.) with commercial value
3. • A natural compound – colchicine (an alkaloid derived from Autumn crocus) may be used to double the chromosome sets experimentally

1. Polyploidy:
   - Definition: The addition of one or more extra set(s) of chromosomes identical to the normal haploid complement of the same species.
   - Examples: Triploidy, tetraploidy, pentaploidy, and hexaploidy.
2. Autotetraploids and Tetrasomic Inheritance:
   - Autotetraploids, like potato and alfalfa, show tetrasomic inheritance.
   - Tetrasomic inheritance refers to the inheritance of genes in pairs due to the presence of four homologous chromosomes.
   - Odd-numbered polyploids (e.g., triploids) are usually seedless, which is valuable for crops like watermelon and grapes.
3. Colchicine and Doubling Chromosome Sets:
   - Colchicine is a natural compound derived from Autumn crocus.
   - It can be used experimentally to double the chromosome sets.
   - Doubling the chromosome sets results in polyploidy and can be used to create new plant varieties with desirable traits.
   - it disrupts microtubule polymerization, which affects spindle formation indirectly.

Question 4

Explain VARIATION IN CHROMOSOME NUMBERS: AUTOPOLYPLOIDY: MITOSIS - Colchicine can inhibit spindle formation. how?

Answer 4

Mitosis and Autopolyploidy:
In autopolyploidy, mitosis plays a crucial role in the replication and distribution of the duplicated chromosomes to daughter cells.

Colchicine and its Effects:
- 1. Colchicine is a natural compound derived from plants, such as the Autumn crocus.
- 2, It acts by disrupting microtubule polymerization, which is necessary for the proper formation and functioning of the mitotic spindle.
- 3. By inhibiting microtubule polymerization, colchicine prevents the separation of duplicated chromosomes during mitosis.
- 4. This results in the formation of cells with multiple sets of chromosomes, leading to the generation of autopolyploid organisms.

the effects of colchicine can vary depending on the specific organism and the stage of the cell cycle at which it is administered.
It is commonly used in laboratory settings to induce polyploidy artificially and create new plant varieties with desirable traits.

Question 5

Variation in chromosome numbers: Allopolyploidy
- 3

Answer 5

• Results from hybridisation of two closely related species
• Examples include cultivated cotton (tetraploid), durum wheat (tetraploid), bread wheat (hexaploid), canola (tetraploid)
• Typically disomic inheritance but often some illegitimate pairing between ancestrally related HOMOEOLOGUES

Question 6

Here’s a comparison of autopolyploidy and allopolyploidy in an easy flashcard format:

Answer 6

Comparison:

• Source of Chromosome Sets:
  
  • Autopolyploidy: Chromosome sets are duplicated within the same species.
  • Allopolyploidy: Chromosome sets are derived from different species through hybridization.
• Ploidy Levels:
  
  • Autopolyploidy: Can result in various ploidy levels, such as diploid, triploid, tetraploid, etc., within the same species.
  • Allopolyploidy: Often leads to tetraploidy (2n + 2n) due to the fusion of diploid gametes from different species.
• Origin:
  
  • Autopolyploidy: Arises from within the same species through errors in mitosis or unreduced gamete fusion.
  • Allopolyploidy: Occurs through hybridization between different species.
• Examples:
  
  • Autopolyploidy: Examples include polyploid plants, animals, and fungi that have multiple sets of chromosomes within the same species.
  • Allopolyploidy: Commonly observed in plants, where the combination of chromosome sets from different species contributes to their genetic diversity.

Question 7

explain polyploidy and plant breeding - a genetic process =5.

Answer 7

Plant Breeding and Polyploidy:
1. Genetic Variation:
- Polyploidy introduces increased genetic variation into plant populations, as it results in the duplication of the entire set of chromosomes.
- This genetic diversity offers breeders a wider range of traits and characteristics to select from during breeding programs.

2. Hybridization:
   - Polyploidy plays a crucial role in plant hybridization programs.
   - Hybridization involves crossing different plant varieties or even species to combine desired traits.
   - Polyploid plants are often used as parent plants in hybridization because they can act as a barrier to prevent genetic recombination and maintain the desired traits.
3. Crop Improvement:
   - Polyploidy has been instrumental in developing improved crop varieties with enhanced characteristics.
   - Polyploid plants often exhibit desirable traits such as increased size, vigor, disease resistance, and adaptability.
   - By utilizing polyploidy in breeding programs, breeders can create new cultivars that have improved agronomic traits, yield, quality, and other beneficial characteristics.
4. Crop Domestication:
   - Polyploidy has been associated with the domestication of many important crop plants.
   - It has played a role in the evolution of crops such as wheat, oats, cotton, potatoes, and many other agricultural species.
   - Polyploidization events in wild ancestors of crops have led to the development of polyploid crop species with enhanced traits.
5. Seedless Varieties:
   - Polyploidy is often responsible for the production of seedless or parthenocarpic fruit varieties.
   - Seedless fruits are highly valued commercially due to their improved quality, convenience, and reduced seed content.
   - Polyploidy-induced sterility can lead to seedless fruit production, offering benefits for both growers and consumers.

Question 8

The consequence of polyploidy

Answer 8

Consequence – polyploids are generally bigger than diploids and they are better adapted to harsh environments

Question 9

Variation in chromosome numbers: Aneuploidy
- 3

Answer 9

-1. An organism gains or loses one or more chromosomes, but not a complete set

2. Aneuploidy results from nondisjunction – the failure of paired homologues or sister chromatids to disjoin during segregation
3. Nondisjunction leads to the formation of gametes with more or fewer chromosomes than normal

Question 10

aneuploidy: how does it occur in meiosis
in first-division nondisjunction?
vs
Second division nondisjunction?

Answer 10

1. First Division Nondisjunction:
   - During the first division of meiosis (Meiosis I), homologous chromosomes fail to separate properly, resulting in the unequal distribution of chromosomes to the daughter cells.
   - If homologous chromosomes fail to separate during anaphase I, one daughter cell will receive both homologous chromosomes (two copies), while the other daughter cell will not receive any (zero copies).
   - As a result, one daughter cell will have an extra chromosome (trisomy), and the other daughter cell will have a missing chromosome (monosomy).
2. Second Division Nondisjunction:
   - During the second division of meiosis (Meiosis II), sister chromatids fail to separate properly, leading to the unequal distribution of chromatids to the daughter cells.
   - If sister chromatids fail to separate during anaphase II, one daughter cell will receive both sister chromatids (two copies), while the other daughter cell will not receive any (zero copies).
   - This can result in one daughter cell having an extra chromosome (trisomy), and the other daughter cell having a missing chromosome (monosomy).

Question 11

Variation in chromosome numbers: Aneuploidy: 6

Answer 11

• Monosomics (2n-1)
• Trisomics (2n+1)
• Double monosomics (2n-1-1)
• Tetrasomics (2n+2)
• Nullisomics (2n-2)
• In general, plant species are more tolerant of variation in chromosome numbers due to frequent rounds of polyploidy

Question 12

Aneuploidy: consequences = 5

Answer 12

• In true diploids, aneuploidy is normally lethal (most mammals, some plants)
• Monosomics and trisomics are most commonly tolerated, usually with reduced vigour, and fertility problems
• Aneuploidyisfoundmostcommonlyinpolyploid species - the extra genomes act as buffers
• Aneuploid plants are useful in gene mapping – wheat, maize, tomato, cotton
• sugarcane cultivars are all aneuploid – their chromosomes sets are known as ‘swarms’

Question 13

Variation in human chromosome number: aneuploidy = 3.

Answer 13

• In humans, loss of whole autosomes (monosomy) is not tolerated
• Trisomy of certain chromosomes is tolerated, e.g. Trisomy 21: Down’s syndrome
• The frequency of occurrence dramatically rises when giving birth after the age of 35-40

Question 14

Human sex chromosome aneuploidy: 2 examples

Answer 14

• Klinefelter’s syndrome
  – XXY, XXXY, XXXXY
  – phenotypically male, usually sterile
  – also related to maternal age, accounts for 1 in 600 births
• Turner’s syndrome
  – XO monosomic
  – phenotypically female, always sterile
  – 1 in 4000 births (>99% of foetuses do not go to term)
  – it is the most common chromosomal aberration seen in spontaneously aborted foetuses

Question 15

Macromutations – large-scale chromosomal changes

Answer 15

***Six major types of Macromutations in two categories are recognized:

• Changes in Chromosome number
  5.Polyploidy - Addition of whole sets of chromosomes
  6. Aneuploidy - Addition or subtraction of chromosomes, but not whole sets
• Change within or between chromosomes
  1. Deletions - Loss of chromosome sections
  2. Duplications - Duplication of chromosome sections
  3. Inversions - Flipping of parts of chromosomes
  4. Translocations - Movement of one part of a chromosome to another part

Question 16

Abnormal mitosis and meiosis can result in euploidy and aneuploidy chromosome number changes. Please explain.

Answer 16

Abnormalities in both mitosis and meiosis can lead to changes in chromosome number, resulting in two main categories: euploidy and aneuploidy.

1. Euploidy:
   - Euploidy refers to a normal or balanced chromosome complement in an organism. It occurs when the individual has the correct number of complete chromosome sets.
   - In humans, euploidy typically refers to organisms with 46 chromosomes (diploid) or organisms with multiples of 23 chromosomes (triploid, tetraploid, etc.).
   - Euploidy can arise through mechanisms such as normal cell division during mitosis or the fusion of gametes during fertilization.
   - Euploidy is generally compatible with normal development and functioning, as the correct number of chromosomes allows for proper gene dosage and balanced gene expression.
2. Aneuploidy:
   - Aneuploidy refers to an abnormal or unbalanced chromosome complement in an organism. It occurs when there is a gain or loss of one or more chromosomes within a set.
   - Aneuploidy can result from errors during cell division in both mitosis and meiosis.
   - In mitosis, errors such as nondisjunction (failure of chromosomes to separate) or chromosome breakage and rearrangement can lead to aneuploidy in somatic cells.
   - In meiosis, errors in chromosome segregation during either the first or second division can result in the production of aneuploid gametes.
   - Aneuploidy can have significant effects on development and functioning, as it disrupts the normal balance of gene dosage and gene expression.
   - Examples of aneuploidy in humans include conditions like Down syndrome (trisomy 21), Turner syndrome (monosomy X), and Klinefelter syndrome (XXY).

Overall, normal mitosis and meiosis ensure the proper distribution of chromosomes to daughter cells, maintaining euploidy. However, errors during these processes can lead to aneuploidy, which can have profound effects on the individual’s phenotype and overall health.

Question 17

Please explain the following terms: autopolyploidy, allopolyploidy, euploidy, and aneuploidy.

Answer 17

1. Autopolyploidy:
   - Autopolyploidy refers to a condition in which an organism has multiple sets of chromosomes derived from the same species.
   - It occurs due to errors in cell division, such as failures in chromosome separation during mitosis or meiosis, resulting in the doubling or multiplication of the entire chromosome set within an individual.
   - Autopolyploids can arise from the duplication of chromosomes within a single species, leading to organisms with 3n (triploid), 4n (tetraploid), or higher multiples of the haploid chromosome number.
   - Autopolyploidy can occur spontaneously or through artificial induction using certain chemicals or laboratory techniques.
   - Autopolyploidy often results in larger and more robust individuals compared to their diploid counterparts, as the extra sets of chromosomes provide additional genetic material.
2. Allopolyploidy:
   - Allopolyploidy refers to a condition in which an organism has multiple sets of chromosomes derived from different species through hybridization.
   - It occurs when two different species interbreed and produce offspring with chromosomes from both parent species.
   - The hybrid offspring may have an uneven number of chromosomes, but through errors in cell division, the chromosome sets can be doubled, resulting in a balanced allopolyploid.
   - Allopolyploidy commonly occurs in plants and contributes to speciation and the formation of new plant species.
   - Examples of allopolyploidy include wheat, which originated from the hybridization of multiple ancestral species, resulting in hexaploid wheat with six sets of chromosomes.
3. Euploidy:
   - Euploidy refers to the normal or balanced chromosome complement in an organism.
   - It occurs when an individual has the correct number of complete chromosome sets.
   - In humans, euploidy typically refers to individuals with 46 chromosomes (diploid), where each chromosome is present in two copies (one from each parent).
   - Euploidy can also refer to organisms with multiples of the haploid chromosome number (e.g., triploid, tetraploid) if the entire set of chromosomes is present in multiples.
   - Euploidy is generally compatible with normal development and functioning, as the correct number of chromosomes allows for proper gene dosage and balanced gene expression.
4. Aneuploidy:
   - Aneuploidy refers to an abnormal or unbalanced chromosome complement in an organism.
   - It occurs when there is a gain or loss of one or more chromosomes within a set.
   - Aneuploidy can result from errors during cell division, such as nondisjunction (failure of chromosomes to separate) or chromosome breakage and rearrangement.
   - Examples of aneuploidy in humans include conditions like Down syndrome (trisomy 21), Turner syndrome (monosomy X), and Klinefelter syndrome (XXY).
   - Aneuploidy disrupts the normal balance of gene dosage and gene expression, which can have significant effects on development and functioning.

These terms describe different conditions related to the number of chromosomes in an organism, ranging from balanced multiples of chromosomes (euploidy) to unbalanced chromosome numbers (aneuploidy) and variations resulting from within-species duplication (autopolyploidy) or hybridization between different species (allopolyploidy).