WEB TUTORIAL 12.2
Mistakes in Meiosis
Text Sections
Section 12.6 Incorrect Chromosome Number: Aneuploidy, p. 194
Introduction
Most of the time, meiosis proceeds normally, producing four haploid daughter cells
from each diploid parent cell. But errors do occasionally occur, resulting in
gametes with too many or too few chromosomes. This condition is called aneuploidy. How do such mistakes happen? In this tutorial, you'll learn about an error
called nondisjunction and about some of its consequences.
Learning Objectives
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Understand the consequences of nondisjunction during meiosis.
Know the difference between an aneuploid and a polyploid condition.
Narration
Nondisjunction in Meiosis I
A diploid cell that undergoes meiosis typically produces four haploid cells, which
have precisely half the genetic material of the parent cell. The diploid cell has two
complete sets of chromosomes, and each of the haploid cells has a single complete
set of chromosomes.
Nondisjunction refers to an error in cell division. In some cases homologous chromosomes do not separate from each other during cell division. In other cases, the
problem occurs when sister chromatids do not separate from each other.
Nondisjunction can occur in mitosis or during meiosis I or II.
If nondisjunction occurs in meiosis I, both chromosomes of a homologous pair
migrate to the same pole, leaving one daughter cell without a chromosome. The
cells go through meiosis II normally, producing four gametes with unusual numbers of chromosomes. Two gametes have one complete set of chromosomes plus
one extra chromosome, indicated by n + 1. The other two gametes lack a chromosome and are designated n – 1.
Exercise 1
What do you think the gametes would look like if all pairs of homologous chromosomes went to the same pole during meiosis I? Drag the correct chromosome
arrangements shown on the right to fill the gametes on the left.
Results from Exercise I
Now let's see the steps leading to the formation of abnormal diploid gametes.
Nondisjunction occurs in meiosis I as all pairs of homologous chromosomes
migrate to just one of the poles of the cell. The cell divides. In the cell containing
the chromosomes, meiosis II is normal.
The sister chromatids separate and the cell divides to form two diploid, rather than
the normal haploid, gametes.
Nondisjunction in Meiosis II
The results are different when nondisjunction occurs in meiosis II rather than meiosis I.
Here, meiosis I proceeds normally, with each chromosome of a pair migrating to
opposite poles.
If nondisjunction occurs in meiosis II, both sister chromatids of a chromosome
migrate to the same pole of the cell. Only one chromosome separates abnormally,
but this results in two abnormal cells.
Two of the gametes are normal, having a haploid number of chromosomes. The
other two are abnormal: one has an extra chromosome, indicated by n + 1, and the
other has one chromosome too few, indicated by n – 1.
Consequences of Nondisjunction
The abnormal gametes produced by meiotic errors have major consequences in the
next generation. If a gamete with n + 1 chromosomes fuses with a normal gamete,
the result is a diploid zygote with a third copy of a chromosome. This chromosome
abnormality is called trisomy.
In humans, a few of the smaller chromosomes can be tolerated in three copies, such
as chromosome 21, which results in Down syndrome. However, most trisomic conditions are lethal.
When a normal haploid gamete and a gamete having n – 1 chromosomes fuse, a
monosomic zygote results. In monosomy, one chromosome is missing, so the cell
has only one copy of a particular chromosome.
In humans, most monosomies are lethal. However, a female can survive with just
one X chromosome, but she will have Turner syndrome.
When a diploid gamete and a haploid gamete fuse, the result is a triploid zygote,
with three sets of chromosomes. Triploidy in humans is lethal.
However, plant species can tolerate extra sets of chromosomes. A number of
healthy plant species have more than 8 sets of chromosomes.
Cases in which the chromosome number differs from normal by just a few are
called aneuploid conditions. Cases in which extra complete sets of chromosomes
exist are polyploid conditions.
You should now be able to…
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Predict the gametes that result if nondisjunction occurs during meiosis I or
meiosis II.
Explain how a trisomic or monosomic zygote could be formed.
Give two examples of non-lethal aneuploid conditions.