Wize University Biology Textbook > Modern Concepts of Inheritance
Chromosomal Nondisjunction, Aneuploidy and Polyploidy
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Chromosomal Nondisjunction, Aneuploidy and Polyploidy
Chromosomal Nondisjunction
Nondisjunction (i.e. non-separation) during meiosis can lead to cells containing more than the correct number of homologous chromosomes. Can occur during meiosis I (right) or meiosis II (left).
Aneuploidy and Polyploidy
Numerical mutations are those in which there's a change to the number of chromosomes in a specie's cell. An organism with the appropriate number of chromosomes for their species is called euploid.
- Aneuploidy: change in number of chromosomes (2n + #)
- Generally occurs due to nondisjunction (sister chromatids did not separate) during meiosis.
- This usually results in the abortion of the child but can result in a child being born with genetic disorders. Example: Down's syndrome, also called Trisomy 21.
- Polyploidy: increase in chromosome sets (3n, 4n, 5n, etc...)
- Extremely rare in animals and more commonly found in plants.
- Autopolyploidy: chromosome sets multiply from within the same species (1 parent).
- Allopolyploidy: chromosome sets multiple from 2 or more species (2 distinct parents).
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Example: Genes & Alleles
Consider a plant with a karyotype n=5 and 4n= 20.
How many different alleles of a gene can you find in an individual of this plant?
tetra=4, so up to 4 different alleles.
How are new alleles generated?
Through mutations.
How many double helices would you find in a cell of this plant if that cell was in the G1 phase? How about G2 phase?
20
(During S phase (DNA replication) Each of the 20 chromosomes will be duplicated and will exist as pairs of sister chromatids. Each chromatid is a DNA helix, so there would be 40 in G2).
Practice: Chromosome Error Type
A human child that has 45 total chromosomes must suffer from:
Practice: Nondisjunction Stage
Julia has triple X syndrome, meaning she has three X chromosomes. On one X she has an SNP that she shares with her paternal grandmother, one has an SNP that she shares with her maternal grandfather. When did this aneuploidy happen to cause Julia to have this syndrome?
- Meiosis I in the mother
- Meiosis I in the father
- Meiosis II in the mother
- Meiosis II in the father