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KARYOTYPES
Normal female cells contain 46 chromosomes, 23 received from
the mother via the egg and 23 from the father via the sperm. The 46
chromosomes consist of 22 homologous pairs of autosomes
(chromosomes that do not determine the sex of the organism ) and 2 Xchromosomes that are sex-determining . Normal male cells also
contain 46 chromosomes; the 22 pairs of autosomes and two dissimilar
chromosomes - an X-chromosome and a much smaller Y-chromosome.
The possession of a Y-chromosome determines a human to be a male.
These facts about human chromosomes were discovered relatively
recently in scientific history - until the mid 1950s, humans were thought
to possess only 44 chromosomes. The development of techniques for
karyotyping, or chromosome analysis, in humans became available at
that time. These techniques allow for the detection of normal vs.
abnormal chromosome content in tested cells. Thus, karyotyping has
become an important process in studying actual or potential birth
defects due to chromosome abnormalities.
White blood cells or fetal cells in the amniotic fluid are commonly
used for chromosome analysis. The cells are removed from the patient
and stimulated by chemicals and nutrients to divide rapidly in a test
tube. The cells are fixed, then dropped onto glass slides in such a way
as to spread the chromosomes of any cell that was in mitotic metaphase
at the time of fixation. The slides are stained and the chromosomes are
photographed for later analysis. In the original technique, the primary
analytical tool was scissors!
The pattern of chromosome movements in meiosis was discussed
in the previous lab session. Recall that homologous chromosomes
align side-by-side along the equator in metaphase I and segregate
during anaphase I (see below). In meiosis II, the sister chromatids
separate to yield four gametes, each containing one representative of
each of the chromosomes typical of the species.
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MI
M II
Meiosis sometimes fails to segregate the chromosomes correctly,
and these failures almost always lead to the death of the fetus formed
by the defective gamete. There are a few exceptions, as you will see
below. Consider non-disjunction, the failure of homologous
chromosomes to segregate in meiosis I . In our hypothetical species,
some of the gametes resulting from the non-disjunction will have too
many chromosomes, and some will have too few (see below).
MI
M II
The defective gametes may be fertilized by normal gametes; the
expected outcomes are:
Monosomic
Trisomic
Those cells (embryos) that have only one representative of a
chromosome type are said to be monosomic for that chromosome.
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Those cells that have three representatives of a chromosome type are
said to be trisomic. (The normal condition is disomic.) In humans, the
autosomes are numbered from 1-22 in order of decreasing size. (Your
instructor will show an overhead depicting a typical male chromosome
spread and the resulting ordered karyotype. Notice that in addition to
the 22 pairs of autosomes, the male possesses a large X and a small
Y.) If an individual were to have three chromosomes 13, then the
resulting condition or genetic defect would be termed trisomy-13. An
individual with a single chromosome 20 would have monosomy-20, and
so forth.
Most human monosomics and trisomics spontaneously abort or
are stillborn. The most notable exceptions are those having Down's
syndrome or Trisomy-21. These individuals suffer from numerous
physical and mental defects and tend to experience premature aging (if
surgery corrects their internal physical defects to allow their survival at
all).
How would Down's syndrome arise, i.e., how does a person
become trisomic for chromosome 21?
An interesting correlation exists between the age of the mother
and the likelihood of bearing a child with Down'syndrome. Women
under 35 almost never have children with Down's syndrome, while a few
percent of children born by mothers over 40 have Down's syndrome
(regardless of the age of the father). What conclusion(s) can you draw
from this information?
Sex chromosomes can also undergo non-disjunction. In a female,
this can sometimes lead to the production of eggs containing either 0 or
2 X-chromosomes. In males, the X and Y chromosomes behave as if
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they were homologous, segregating at meiosis I. In some cases,
however, they fail to segregate, leading to gametes containing no sex
chromosomes or both an X and a Y. Fill in the table below to see what
kinds of sex chromosome abnormalities might result from fertilization
involving gametes containing normal and abnormal numbers of sex
chromosomes.
Can you think of any other, rarer abnormalities which might arise?
Recall that the presence of a Y chromosome is sufficient, in the
presence of one or more X chromosomes, to confer maleness. (But
because the X chromosome carries information of vital importance, a Y
alone is fatal.)
Based on this, what are the sexes of the individuals in the table
above?
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A single X, sometimes referred to as XO, is Turner's syndrome.
XXX, or Triplo-X, occurs at about the same frequency as Turner's
syndrome, about 1 in 1000 females. Finally, XXY, or Klinefelter's
syndrome, occurs in about 1 in 1000 males. These conditions, which
usually result in sterility, are described in your text.
Analysis of chromosome spreads
At the supply table you will find xerox copies of banded
chromosome spreads (stained to reveal a bar-code pattern unique to
each chromosome). Your task is to cut out the chromosomes and
arrange them in the usual manner, from autosome pair 1 to autosome
pair 22 in descending order, keeping the sex chromosomes separate.
Then, having arranged the chromosomes, decide what genetic defect, if
any, is demonstrated.
Report your result to your instructor, who might provide a suitable
reward for the correct diagnosis – but be prepared to explain how
the defect arose.
Obviously, karyotyping unbanded chromosomes would be
difficult. To make things easier, scientists devised ways to stain the
chromosomes so that they have characteristic stripes or bands; all
chromosomes #1 have the same banding pattern, so the two copies can
be identified easily. The same applies to chromosomes 2-22 and the X
and Y chromosomes. Today, human geneticists can stain
chromosomes specific colors using a technique known as FISH
(fluorescent in situ hybridization) or chromosome painting to further
reduce the likelihood of error in the diagnosis of chromosome disorders.
Internet Resources
To see examples of FISH, go to
http://www.health.auckland.ac.nz/webpath/genhtml/genetidx.htm.
For more information about Down's syndrome and other genetic
disorders, see http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=OMIM.
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