Dichotomous key in microbial taxonomy

1. Dichotomous key
Construction of a dichotomous key- simplest
way to identifying organism
consist of structural features about the
specimen to be identified.
There are two alternatives for each features
each of these two alternatives will lead to other
features until the desired group of the specimen is
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2. A. Made up of pairs of statements which have
opposing (opposite) descriptions
B. Each pair of descriptions are numbered starting at 1
C. Each description in a pair is labeled A or B
D. At the end of each description the key will either
identify an object or give directions to another
specific pair of descriptions
Characteristics of a dichotomous key
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3. A. Start at description 1a - if the description is
correct follow the directions
B. If description 1a is incorrect go to 1b - follow
the directions
C. Always read description A first - if it is correct
follow the directions - if it is incorrect go to
description B
D. Continue following directions until object is
identified
How to use a dichotomous key
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4. Example of the use of
dichotomous key
1 With flower Angiosperms
Without flower Go to 2
2 Seedless Go to 3
Seed bearing Gymnosperms
3 Plant body do not differentiate into root,
stem and leaves
Go to 4
Plant body differentiate into root, stem and
leaves
Go to 5
4 Without photosynthetic pigments Fungi
With chlorophyll or other photosynthetic
pigments
Algae
5 No Vascular tissues Mosses/Bryophytes
With vascular tissues Ferns/Pteridophytes
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5. Dichotomous key
 the dichotomous key can also be expressed in a diagrammatic form
Non-flowering plants
Without photsyn-
thetic pigm ent
With c hlorophyll or
other photosynthetic
pigm ent
Algae Fungi
No vasc ula r
tissues
With vasc ular
tissues
Mosses Gym nosperm s
Plant body not
differentiated into
root, stem a nd
leaves
Plant body
differentiated
into stem and
leaves
Seed less Seed -bearing
Ferns
Flowering Plants
(Angiosp erm )
Without flower
With flower
Plants
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6. A. List characteristics of the objects
to be identified
B. Chose one characteristic that is
unique (different) about each
object
C. Use that unique characteristic in
opposing descriptions to identify
How to make a dichotomous key
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7. KKR1116 7

8. Dichotomous key for
identification of
E.coli
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9. KKR1116 9

10. KKR1116 10

11. KKR1116 11

12. KKR1116 12

13. dichotomous key alone cannot be used to identify
specimens, unless the specimens are known in
advance to be included within the key.
comparing the specimen with a more detailed
description is vitally important
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