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Gagal Berpisah / Nondisjunction Kelas 12 | Materi Pola Hereditas
Nadya Media Official
Overview
This video explains nondisjunction, a phenomenon where chromosomes fail to separate properly during meiosis. This error leads to gametes with an abnormal number of chromosomes, either too many (aneuploidy, like trisomy) or too few (monosomy). The video illustrates how nondisjunction can occur in either meiosis I or meiosis II and discusses the resulting genetic conditions in humans, such as Klinefelter syndrome (XXY) and Turner syndrome (X0), due to fertilization with these abnormal gametes. Understanding nondisjunction is crucial for comprehending inheritance patterns and genetic disorders.
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Chapters
- Nondisjunction is the failure of chromosomes to separate correctly during meiosis.
- This occurs because spindle fibers are unable to pull homologous chromosomes to opposite poles.
- It can happen during either Meiosis I or Meiosis II.
Understanding the basic mechanism of nondisjunction is fundamental to grasping how errors in chromosome distribution lead to genetic abnormalities.
Spindle fibers failing to pull homologous chromosomes to opposite poles during cell division.
- In Meiosis I, homologous chromosomes fail to separate.
- This results in two daughter cells with an abnormal number of chromosomes (one with an extra, one with a missing chromosome).
- The subsequent Meiosis II will then produce gametes with an incorrect chromosome count (e.g., n+1 and n-1).
Errors in Meiosis I have a cascading effect, impacting all resulting gametes and increasing the likelihood of aneuploidy.
One pair of homologous chromosomes fails to separate, leading to one daughter cell receiving both and the other receiving none of that pair.
- In Meiosis II, sister chromatids fail to separate.
- This affects only half of the resulting gametes, with some being normal (n) and others abnormal (n+1 or n-1).
- This occurs when the centromeres holding sister chromatids together do not divide properly.
Nondisjunction in Meiosis II shows that errors can occur later in the process, affecting a subset of the final gametes.
Sister chromatids within a chromosome fail to separate and move to opposite poles during Meiosis II.
- Normal gametes have a haploid number of chromosomes (n).
- Nondisjunction leads to aneuploid gametes: n+1 (extra chromosome) or n-1 (missing chromosome).
- In humans, n=23, so abnormal gametes can have 24 (n+1) or 22 (n-1) chromosomes.
The formation of gametes with incorrect chromosome numbers is the direct precursor to genetic disorders in offspring.
A gamete with 24 chromosomes (n+1) is formed instead of the normal 23 (n).
- Fertilization of an abnormal gamete with a normal gamete results in a zygote with an abnormal chromosome number (aneuploidy).
- A zygote with 2n+1 chromosomes is called trisomic (e.g., Trisomy 21 - Down Syndrome).
- A zygote with 2n-1 chromosomes is called monosomic (e.g., Turner Syndrome - X0).
This explains the origin of common genetic conditions like Klinefelter and Turner syndromes, linking meiotic errors to observable human traits.
Fertilization of a normal egg (X) with an abnormal sperm (XY) results in a Klinefelter male (XXY).
Key takeaways
- Nondisjunction is a failure of chromosome separation during meiosis, leading to gametes with incorrect chromosome numbers.
- Errors can occur in either Meiosis I (affecting all resulting gametes) or Meiosis II (affecting half of the resulting gametes).
- Gametes resulting from nondisjunction are either aneuploid (n+1) or monosomic (n-1).
- Fertilization involving aneuploid gametes results in zygotes with abnormal chromosome numbers, causing genetic disorders.
- Conditions like Klinefelter syndrome (XXY) and Turner syndrome (X0) are direct consequences of nondisjunction during gamete formation.
Key terms
NondisjunctionMeiosisChromosomesSpindle FibersHomologous ChromosomesSister ChromatidsGametesAneuploidyMonosomyTrisomyZygoteKlinefelter SyndromeTurner Syndrome
Test your understanding
- What is the fundamental cause of nondisjunction during meiosis?
- How does nondisjunction in Meiosis I differ in its outcome from nondisjunction in Meiosis II?
- What are the two types of abnormal gametes produced by nondisjunction, and what do their chromosome counts represent?
- How does fertilization involving an aneuploid gamete lead to a genetic disorder in the offspring?
- Explain the relationship between nondisjunction and the development of conditions like Turner syndrome.