Number Variation

Summary

This video explores changes in chromosome number, focusing on aneuploidy and polyploidy. It explains aneuploidy as variations in the number of individual chromosomes, such as trisomy (three copies) and monosomy (one copy), and their impact on organisms. Polyploidy, a change in the number of chromosome sets, is also discussed, distinguishing between autopolyploidy (within one species) and allopolyploidy (between species). The causes of these changes, including non-disjunction in meiosis and mitosis, are explained, with a special emphasis on how these variations affect fertility, viability, and species evolution, especially in plants and animals.

Takeaways

• 😀 Aneuploidy refers to a change in the number of a single chromosome, either as trisomy (three copies) or monosomy (one copy).
• 😀 Polyploidy refers to a change in the number of sets of chromosomes, such as triploid (three sets) or tetraploid (four sets).
• 😀 Aneuploidy can cause significant genetic issues, with trisomy and monosomy leading to conditions like Down syndrome and Turner syndrome.
• 😀 Non-disjunction is the failure of chromosomes or chromatids to separate properly during meiosis or mitosis, leading to aneuploidy.
• 😀 In non-disjunction during meiosis I, half of the gametes will be aneuploid, with either two or no copies of a chromosome.
• 😀 In non-disjunction during meiosis II, half of the gametes will be abnormal, with one quarter having an extra chromosome and one quarter having none.
• 😀 Mitosis can also experience non-disjunction, but it is less impactful unless it occurs very early in embryonic development.
• 😀 Polyploidy, while rare, is considered euploidy, and organisms with extra sets of chromosomes can survive, as seen in certain species like Xenopus laevis (a tetraploid frog).
• 😀 Triploid individuals (those with three sets of chromosomes) are typically sterile due to problems in chromosome pairing during meiosis.
• 😀 Autopolyploidy occurs within a single species and can lead to individuals with extra sets of chromosomes, such as triploid or tetraploid organisms.
• 😀 Allopolyploidy involves hybridization between different species, and if non-disjunction occurs, it can result in fertile polyploid individuals that can form new species.

Q & A

What are the two main types of chromosome number variation?

The two main types of chromosome number variation are aneuploidy and polyploidy.

What does aneuploidy refer to?

Aneuploidy refers to a change in the number of one chromosome in a set, resulting in either trisomy (three copies of a chromosome) or monosomy (one copy of a chromosome).

What does polyploidy involve?

Polyploidy involves a change in the number of sets of chromosomes, leading to organisms with more than two sets of chromosomes, such as triploid (three sets) or tetraploid (four sets).

How does aneuploidy differ from polyploidy?

Aneuploidy is a change in the number of individual chromosomes, while polyploidy is a change in the number of chromosome sets.

What is the most common form of aneuploidy?

The most common forms of aneuploidy are trisomy (three copies of a chromosome) and monosomy (one copy of a chromosome).

Why is monosomy of autosomal chromosomes usually lethal?

Monosomy of autosomal chromosomes is typically lethal because having only one copy of a chromosome results in an imbalance of gene dosage, which is often not viable.

What causes trisomy and monosomy?

Trisomy and monosomy are caused by a failure of chromosome segregation during cell division, known as nondisjunction, which can occur in either meiosis or mitosis.

How does nondisjunction in meiosis I differ from nondisjunction in meiosis II?

In meiosis I, nondisjunction leads to all gametes being aneuploid (either with two copies or none of a chromosome). In meiosis II, nondisjunction affects only half the gametes, with one-quarter having an abnormal number of chromosomes, and the other half being normal.

What are the consequences of polyploidy in plants and animals?

Polyploidy, particularly in plants, is often tolerated better and can lead to fertile individuals, as seen with tetraploid plants. In animals, polyploidy may be fatal or result in sterility, especially with triploids, as they cannot produce functional gametes due to uneven chromosome distribution during meiosis.

What is the difference between autopolyploidy and allopolyploidy?

Autopolyploidy occurs within a single species, typically due to non-disjunction, leading to multiple sets of chromosomes. Allopolyploidy involves hybridization between two species, and the resulting hybrid can undergo non-disjunction to form a fertile polyploid organism.