Unit 2: Genetic Processes
Lesson 5: Abnormal Meiosis
Karotypes: During stages of cell division, chromosomes can be stained, viewed and photographed under a
microscope. Doing this allows scientists to count, compare and arranged the chromosomes according to
their size and type. A picture of chromosomes that have been arranged according to number, size, shape
or some other characteristic is called a karyotype.
This karyotype, shows a normal
male. Chromosomes 1 to 22 are
the autosomes, which are the
non-sex chromosomes. The sex
chromosomes are the last pair,
showing an X chromosome and Y
chromosome. If there were two
X chromosomes, it would be a
female. Even though the X and
Y chromosomes differ greatly
in size, parts of the X and Y
chromosomes contain matching
regions that enable them to undergo synapsis and behave as homologous pairs during meiosis. Karotypes
are important tools to help scientists identify genetic disorders.
Abnormal Meiosis: Non-disjunction
Factors that interfere with meiosis and/or fertilization can contribute to genetic disorders and
infertility. On rare occasions, during meiosis, entire chromosomes are lost or gained from cells. This is
called:
Non-disjunction: the failure of homologous chromosomes to move to opposite pose of the cell
during meiosis; results in an abnormal number of chromosomes in the daughter cells.
Non-disjunction could occur during meiosis I or meiosis II. Non-disjunction leads to daughter
cells, or gametes, having either
extra chromosomes ore missing
chromosomes. If the nondisjunction occurs during meiosis
II, there will still be potential
for some of the gametes to be
normal.
For humans, non-disjunctions
means that you could potentially
have sperm or eggs that have
either 24 or 22 chromosomes.
This means that if these
gametes were to fertilize with a normal, haploid gamete, the resulting zygote would have the wrong
number of chromosomes.
Trisomy: a chromosomal abnormality in which
there are three homologous chromosomes in
place of a homologous pair. A trisomy occurs
because a gamete containing 24 chromosomes (n
+ 1), instead of 23 chromosomes, fertilizes with
a normal gamete (n).
n + (n+1) = 2n + 1
Non-disjunction Disorders:
Homework:
Monosomy: a chromosomal abnormality in which
there is a single chromosome in place of a
homologous pair. A monosomy occurs because a
gamete containing 22 chromosomes (n – 1),
instead of 23 chromosomes, fertilizes with a
normal gamete (n).
n + (n – 1) = 2n – 1
1. Sex-determination systems are extremely diverse. Research the ways in which sex-determination
occurs for chickens, turtles, crocodiles, fish, bees, ants, wasps, and earthworms.
2. What is non-disjunction and how can it lead to both trisomy and monosomy?
3. Why do you think there are more types of trisomy disorders than monosomy disorders?
Solutions:
1.
Answers will vary. Some examples listed below.
2. & 3.