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Textbook sections
12.4
Mistakes in Meiosis (p. 26+)
How Do Mistakes Occur? (p.264)
Why Do Mistakes Occur? (p.26S)
After reading the text material, you should be able to
Identify the chromosomal abnormalities associated with Down syn<lrome,
Klinefelter syndrome, and Turner syndrome.
Explain what is meant by a nondisjunction.
Identify three patterns in the occurrence of errors in meiosis.
After completing this tutorial, you should be able to
Explain what happens when nondisjunction occurs in meiosis I.
Explain what happens when nondisjunction occurs in meiosis II.
Describe the results of nondisjunction in humans and in plants.
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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
ol 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 slsfer chromatids do not separate from each other. Nondisjunction can occur
in mitosis or meiosis.
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 - 7.
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
Now let's
I
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 ll
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
a.e abnormil: one has an extra chromosome, indicated by n + 1, and the other has one
chromosome too few, indicated by n - 1.
Consequences of Nondisiunction
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 inDown syndrome. However, most trisomic conditions are
lethal.
When a normal haploid gamete and a gamete having n - 1 chromosomes fuse, a monosomic zygote resulis. 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 iust one X
chromosome, but she will have Turner syndrome.
When a diploid gamete and a haploid gamete fuse, the result is a
three sets of chromosomes. Triploidy in humans is lethal.
tiltloid
zygote, with
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 ane*
uploid conditions. Cases in which extra complete sets of chromosomes exist ate polyploid
conditions.
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aneuploid Having a chromosome number that is not an exact multiple of the normal
diploid number, with either more or fewer than the normal number of chromosomes in the
cell. Trisomy (having an extra chromosome) and monosomy (missing a chromosome) are
examples.
chromosome A single long molecule of DNA and any associated proteins.
diploid Having two sets of chromosomes. Diploid cells have two alleles of each
one on each of the homologous pairs of chromosomes.
gene-
Down syndrome A chromosomal disorder resulting from trisomy of chromosome 21. The
syndrome is characterized by mental retardation, a high risk for heart problems and
leukemia, and a degenerative brain disorder similar to Alzheimer's disease.
gamete A haploid reproductive cell that can fuse
zygole.
with another haploid cell to form
a
haploid Having a single set of chromosomes. Haploid cells have just one allele of each
gene; they do not contain homologous chromosomes.
homologous chromosomes (homologs) Chromosomes of the same type, with the same
genes
in the same locations.
meiosis Celt division that leads to a halving of the chromosome number. One diploid parent cell produces four haploid reproductive cells.
monosomy A condition in which a diploid cell or organism lacks a chromosome of one
type, producing a chromosome number of 2n - 1.
nondisjunction An error that can occur during meiosis or mitosis in which both homologous chromosomes of a pair move to the same side of the dividing cell. One daughter cell
receives two copies of this chromosome and the other daughter cell receives none.
sets of chromosomes. Triploidy, in which an organism
has three complete sets of chromosomes, is an example.
polyploid Having more than two
sister chromatids The paired strands of a recently replicated chromo-some that has not
yet divided.
triploid Having three
sets of chromosomes.
trisomy A condition in which a diploid cell or organism has an extra chromosome of one
type, producing a chromosome number of 2n + 7.
Turner syndrome A rare chromosomal disorder in females who have only one X chromosome. The syndrome is characterizedby short stature, the lack of sexual development at
puberty, and heart abnormalities.
zygote The diploid cell formed by the union of two haploid gametes. Azygote is capable
of undergoing embryological development to form an adult organism.