THE CHROMOSOME CONNECTION
QUESTIONS ADDRESSING POPULAR MISCONCEPTIONS
(Use one or more of these to include in the "Test Your Knowledge" set of questions)
Q. The chromosomes of a species are unique to that species, giving no hint of biological relationship to any other
species.
A. False. Chromosomes show the same evidence for evolutionary history as do proteins and DNA molecules.
TUTORIAL
A. THE BIG IDEA : Chromosomes from closely related animals (based on other criteria) show matching
degrees of similarities.
B . BACKGROUND :
Chromosomes are microscopic strands found in the nuclei of the cells of most living things. As you
may have learned in genetics, the genes (DNA code sequences) for an organism’s characteristics are located on
that organism’s chromosomes. Similarities of the characteristics in the members of a particular species (humans
for example) are due to the similarities in the information on their chromosomes.
Similarities between members of different species might also be due to the similarities in the information
on their chromosomes. Comparison of chromosomes is one of the ways we can determine the evolutionary
relationships between organisms of different species. Organisms get their chromosomes from their parents, and
even further back in time, from their ancestors. The theory of evolution predicts that two species having a recent
common ancestor should have chromosomes that are more similar than two species having a common ancestor
further back in time. In other words, species that are closely related should have very similar
chromosomes.
Cell
Nucleus
Chromosome
Cell Diagram
Photograph
of a Stained
Chromosome
Chromosomes for viewing are usually prepared by squashing and staining the cells from an organism.
Then, a camera attached to a microscope is used to take a picture. When looking into the nucleus of a cell through
a microscope, it is difficult even to see the chromosomes, but it is virtually impossible to distinguish one
chromosome from another without staining, which produces banding patterns. When Giemsa stain is used, the
bands produced are called G-bands. With Giemsa stain, areas with a high concentration of cytosine-guanine
(C-G) base pairs turn dark, while areas with a high concentration of adenine-thymine (A-T) base pairs remain
light.
Even with staining, photographs of the chromosomes still appear fuzzy, so scientists apply techniques to
make the banding appear more distinct, observe many chromosomes, and develop chromosome diagrams, as
shown above. NOTE: these bands are NOT genes; there may be thousands of genes in a single band.
In addition, some studies use chromosomes taken from the metaphase stage (where chromosomes are seen
as fairly short); other studies release the chromosomes during their late prophase, when they are much longer, and
showing many more bands (and appearing somewhat different from their metaphase versions) thus enabling more
detailed analysis. The chromosomes used in this activity came from a published study (Yunis, 1982) using the
late prophase stage, showing about 1000 bands for the whole set.
Chromosome similarity implies biological relationship:
a. Numerous studies show that chromosome similarity is a good measure of genetic relatedness. The
visible structure of chromosomes is an extremely complex pattern of bands and lines. The
probability that two different chromosomes would independently have identical banding patterns
by chance is essentially zero (Wallace, 1966).
b. Detailed studies provide clear evidence that any identical chromosomes in two different species indicates
common ancestry just as surely as identical scratch patterns on two bullets indicate that both came
from the same gun (Wallace, 1966).
When comparing chromosomes from different species, only one member of each pair of matching
chromosomes is used (plus an X and a Y), since both members of each pair appear identical (except for the XY
combination in males). In examining the chromosomes, the length of the chromosome, the location of the
centromere (usually constricted, where spindle fibers attach) and the banding patterns are studied very
closely. The similarities and differences are carefully noted.
CHECK QUESTIONS
1. If two different organisms have any extended identical chromosome banding patterns, they... A) must be
related; B) are probably related; C) could be related, but not necessarily so; D) are probably not related.
2. If you found that two sets of chromosomes (one from animal "X" and one from animal "Y") showed a very
close match (in number of chromosomes and several identical banding patterns), what would be the most
probable classification of the two animals? A) in the same species; B) in two closely related species
(same genus or family); C) could be closely related, but not necessarily; D) not at all related.
C. THE ACTIVITY: “”
[Link to the Activity, Screen 1]
D. RESOURCES :
1. ENSIweb is an online resource which provides detailed classroom-tested lesson plans with reproducible
handouts and expected outcomes. The interactive activity presented here is based on one of those lessons,
“Chromosome Comparisons”: <http://www.indiana.edu/~ensiweb/lessons/chromcom.html>, (adapted
from a lesson created by ENSI teacher Beth Kramer), and a similar lesson developed by ENSI teacher
Larry Flammer
BIBLIOGRAPHY
Alberts, Bruce, et al. 1989. Molecular Biology of the Cell. Garland Publishers. Chiasmata, p.848, fig. 15-11.
BSCS. 1985. Biological Science - A Molecular Approach. Heath. Inversion formation, p.312, fig. 14-11.
Flammer, Larry. 1983. “Karyotype Komparison.” Classroom activity.
Klein, Richard G. 1999. The Human Career. University of Chicago Press. Page 71.
Kramer, Beth. 1995. “Comparison of Human and Chimpanzee Chromsomes”. Classroom activity.
Wallace, B. 1966. Chromosomes, Giant Molecules, and Evolution. New York: W.W. Norton & Co., p. 166.
Yunis, J.J. and O. Prakash, 1982. "The origin of man: A chromosomal pictorial legacy". Science, 215, 15251529.
SCREEN 1a
THE ACTIVITY
PART 1: Matching Chromosomes
THE GOAL: To determine if two organisms are related, by comparing their chromosomes.
In the following activity, we will compare chromosome diagrams prepared during scientific studies of the
species involved. It turns out that the chromosomes of organism “C” are very similar to those of organism “H”,
except that organism “C” has 24 pairs, while organism “H” has only 23 pairs. However, two of the “C”
chromosomes, when oriented a certain way, are very similar to the “H” chromosome #2, so we will show those
two “C” chromosomes as a match for the “H” chromosome #2.
Most of the chromosomes from organism “C” have been matched with organism “H” (to save you some
time), but several still remain in the "holding box" for you to find a match. Just click on each of those “C”
chromosomes (in the box) in turn, and drag it to a position to the right of the “H” chromosome it seems to match
most closely. Don't forget to look at the lengths, centromere position, and banding patterns. When it's a
match, it will stay. When it's not a match, it will bounce back to the holding box; try again.
When this first portion of the set of chromosomes has been matched (1-12 + X), you will move on to the
second batch (chromosomes 13-22 + Y) in the next screen.
When all chromosomes have been matched, you will be asked some questions to check your
understanding, then you will move to part 2 of this activity.
[PROGRAMMING NOTE: Make each chromosome in the boxes so that it can be clicked and dragged to a
position next to the matching “H” chromosome in the series on the left. If it’s dropped very close to its proper
spot, it should be accepted and shift into its exact position. If it’s not dropped in the proper place, it should
bounce back to its position in the box, where it has to be clicked and dragged again. See p.6 for key.]
SCREEN 1b:
SCREEN 1c:
CHECK QUESTIONS
3. How would you compare the chromosomes (generally) of these two organisms (H & C)?
A) identical; B) very similar; C) somewhat similar; D) mostly different; E) totally
different
4. Based on the above observation (and the background information), what would you predict
about the relationship between organisms H and C? A) members of same species; B) in
two closely related species (same genus or family); C) in two distantly related species;
D) in two totally unrelated species; E) impossible to say, no basis for correlation.
SCREEN 2
PART 2: Types of Differences
As you look over the matched chromosomes (go BACK to study them, if you like), you may notice that
only one of the chromosomes is perfectly identical to its match. Which one is that? _____. [3]
The remainder show several kinds of differences. One of the most common involves the addition (or
deletion) of a piece of chromosome at one tip end or the other (or both). There are at least 7 in which the only
difference is a single addition/deletion like this. Can you find them? Three more differ by only two tipadditions/deletions. Can you find these? Feel free to go back to look for these differences., then return here.
Another fairly common difference is where a portion of one chromosome is upside down compared to the
otherwise matching one. We call this an “inversion”. In the diagram below, you will see how to visualize this
difference. If you are familiar with this process, you feel free to SKIP the next part, and go directly to see how
the chromosomes of two additional animals compare with these first two.
[PROGRAMMING NOTES:
Provide for option of skipping this look at inversions, going directly to part 4 when SKIP is clicked.
If possible, it would be nice to animate the flipping (inversion) of the inverted piece, and its insertion
back into the strands which perfectly match its counterpart, to show how this makes both chromosomes identical.
The animation sequence should be repeatable if the user desires.]
SCREEN 3:
PART 3: How Inversions Occur
You might wonder how such an inversion could occur in real chromosomes. As it turns out, during the
first part of meiosis (in sex-cell production), chromosomes can be frequently seen in various strange
configurations of loops and twists called “chiasmata”. During this time, crossing over, inversions, and also
additions and deletions can occur. The photograph below shows such chiasmata (from Alberts, et al, 1989).
In the next figure, you can see how the looping, breaking and reattachment occurring in these chiasmata
can produce inversions (from BSCS, 1985).
[PROGRAMMING NOTE: If possible, please animate the process of inversion as depicted in these four steps in
the right hand diagram, and make the animation repeatable at user’s choice.]
SCREEN 4a:
PART 4: A Look at the Chromosomes from Other Similar Animals
You might suspect that the striking similarities between the chromosomes of these two animals are just a
coincidence, so we will look at the chromosomes of two additional animals in that group (G and O), studied at the
same time, with the same techniques, and see how they compare to organisms H and C. For each chromosome
(numbered), the first one is from “H”, the second is from “C”, the third is from “G”, and the fourth is from
organism “O”(these identities are indicated for the first chromosome). Study and compare the chromosomes
carefully, looking for degrees of similarity, and looking for examples of types of differences, e.g. inversions,
additions/deletions.
SCREEN 4b:
[Insert page showing all chromosomes compared for humans and the great apes]
[PROGRAMMING NOTE: Provide for the user to either scroll back up to the previous diagram, OR click
BACK to the previous diagram from the following screens.]
SCREEN 4c:
CHECK QUESTION
5. Organisms C, G, and O are all in different genera in the same taxonomic family. Based on their degrees
of similarity, where should organism H be placed? A) same species; B) same genus; C) same family;
D) same order or higher level
SCREEN 4d:
INFORMATION:
Let’s see who’s chromosomes we’ve been studying...
organism C is Pan troglodytes (chimpanzee)
organism G is Gorilla gorilla (gorilla)
organism O is Pongo pygmaeus (orangutan)
6. Now, just out of curiosity, which of the following would you suspect organism H to be? A) rhesus monkey;
B) howler monkey; C) gibbon; D) bonobo (pygmy chimpanzee); E) human; F) none of these
SCREEN 4e:
7. The mystery is solved! Now that you realize where each set of chromosomes comes from, and you have a real
sense of their striking similarities, would you still agree that humans should be classified in the same
family as the great apes (Pongidae)? A) yes; B) no
SCREEN 5:
PART 5: Chromosome Analysis and Formation of a Cladogram
Let’s see what the scientists say. All the differences in the chromosomes from these four animals were
recorded, counted and analyzed by the scientists doing this study, and from these data, it was possible to create a
kind of cladogram, showing the likely sequence of steps in the evolution of these species relative to each other,
and how closely related they are. (The following diagram is based on a similar one by Yunis et al, 1982). In the
diagram, the position and lengths of each branch are based on the relative number of chromosome differences
between the animals.
A = common ancestor to great apes and humans
G
C
O = orangutan
G = gorilla
O
H
C = chimpanzees
H = humans
A
When we see this cladogram, we find it matches very closely the patterns revealed from comparative
anatomy and molecular comparisons of apes and humans... another example of...MILE:
Multiple Independent Lines of Evidence
SCREEN 6:
CONCLUSIONS
Traditionally, humans have been placed in the same superfamily as all the apes (Hominoidea), but humans
(all modern and fossil humans) have been placed in their own family (Hominidae), separate from the great apes
family (Pongidae).
However, “...biomolecular studies demonstrate unequivocally that the living African pongids - Gorilla
(Gorilla) and [especially] the chimpanzee (Pan) - are actually more closely related to people than either is to the
living Asian pongid, the orangutan (Pongo).” (Klein 1999). Reflecting the growing evidence (in fact Multiple
Independent Lines of Evidence), it has been proposed that all apes and humans be placed into the same family
(Hominidae), and that humans even be placed in the same subfamily as the great apes (orangutans, gorillas, and
chimpanzees): the Homininae. The chromosome connection seems to strengthen that thinking.
Here are the two classification schemes of the hominoids compared side by side:
TRADITIONAL SCHEME
Superfamily: Hominoidea
Family: Hylobatidae (“Lesser Apes”)
Genus: Hylobates (gibbons)
Genus: Symphalangus (siamangs)
Family: Pongidae (“Great Apes”)
Genus: Pongo (orangutan)
Genus: Pan (chimpanzees)
Genus: Gorilla (gorilla)
Family: Hominidae (Humans)
Genus: Australopithecus (early humans)
Genus: Homo (recent humans)
PROPOSED SCHEME
Superfamily: Hominoidea
Family: Hominidae
Subfamily: Hylobatinae (“Lesser Apes”)
Genus: Hylobates (gibbons)
Subfamily: Homininae (“Great Apes & Humans”)
Tribe: Pongini
Genus: Pongo (orangutan)
Tribe: Hominini
Subtribe: Gorillina
Genus: Gorilla (gorilla)
Subtribe: Hominina
Genus: Pan (chimpanzees)
Genus: Australopithecus
Genus: Homo (humans)
In spite of this, the traditional groupings will probably be kept for the foreseeable future. The traditional
classification scheme is so deeply entrenched in the literature, to formally change it at this time would probably
create more confusion than clarity.
===========================================
ANSWERS TO QUESTIONS, WITH APPROPRIATE REPLIES
1. If reply is A, respond: "You’ve got the idea!"
If reply is B or C, respond: "Studies have shown that there is a much higher level of confidence. They
MUST be related if there are identical or very similar chromosome patterns.”
If reply is D, respond: “Read item “a” above; with any identical banding patterns, the chances that this
would be due to coincidence is virtually zero.”
2. If reply is B, respond: “Very good.”
If reply is A, respond: “If they are in the same species, their chromosomes would all be identical, not just
very similar. Try B.”
If reply is C or D, respond: “Better re-read the background material again, more closely! You must have
missed something crucial.”
3. If reply is B, respond: “Very observant!”
If reply is A, C, D or E, respond: “Not quite...look more closely.”
4. If reply is B, respond: “This would be the most reasonable interpretation. Nice job.”
If reply is A, respond: “For this, the chromosomes would all have to be identical.”
If reply is C, D or E, respond: “This answer is not consistent with the results of studies. Review the
background information again.”
5. If reply is C, respond: “That makes a lot of sense, based on the evidence.” [skip to next screen]
If reply is A, respond: “Same species as which one? Remember, none of the sets is completely
IDENTICAL to any other, which is what would be expected if they were the same species.”
If reply is B, respond: “Which genus? Might be Pan, since H is more similar to C than either is to the
others, but it’s a close call. Try again.”
If reply is D, respond: “Not in view of the fact that H is more similar to C than either is to the others, and
the others plus C are all in the same family. Try again.”
6. If reply is E, respond: “You’ve solved the mystery! That’s exactly correct. Either you are very perceptive, or
you’re very good at following clues! Nice going.” [skip to next screen]
If reply is A, respond: “Nope. Monkeys are very different from apes. Try again.”
If reply is B, respond: “Aha! I’ll bet you picked ‘howler’ for the H in its name. Good clue-following, but
monkeys, especially New World monkeys, are very different from apes and humans. Try again.”
If reply is C, respond: “Not bad, since gibbons ARE apes. However, they are “lesser” apes, much
smaller, and otherwise rather different from the great apes. Try again.”
If reply is D, respond: “Probably the most logical choice, since bonobos are another species of
chimpanzee (Pan paniscus), and H and C are so very similar, but sorry, H is not bonobo. Try
again.”
If reply is F, respond: “Oh come now, be brave! One of the other choices IS correct. Try again.”
7. If reply is A, respond: “Very good. You are not afraid to make logical deductions based on the facts.” [skip to
next screen]
If reply is B, respond: “Even if humans and chimps are more like each other than either is like the other
apes? This is really not logical; you might be letting your biases influence your judgment!” [skip
to next screen]