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GREGOR (JOHANN) MENDEL’S PEAS AND PARTICULATE INHERITANCE
Modern views of inheritance date back to Gregor (Johann) Mendel (1822-1884), a retired
Augustinian monk who practiced amateur botany in a quiet garden of his monastery. In 1866 he
published a report of his breeding experiments on the garden pea plant. Only seven (7) years after
Charles Darwin had set forth his Theory of Evolution via Natural Selection, Mendel’s publication
remained obscure until 1900. at that time, three scientists, working independently, rediscovered
Mendel’s Laws. They were Hugo deVries of Holland, Karl Correns of Germany, and Erich von
Tschermak of Austria.
Since then, the science of Genetics has virtually exploded in complexity. Although Mendel
knew nothing of genes, chromosomes, or DNA, his Laws of Heredity provide the backbone of the
modern synthetic approach to the study of Evolution.
Out of Mendel’s studies came his idea of “particulate inheritance” which challenged the
popular nineteenth century notion of “blending inheritance”. Blending inheritance maintained the
traits passed on from the parents through sex cells are blended in the offspring. Darwin and his
colleagues adhered tot his idea, assuming that offspring manifests characteristics that are
intermediate between those possessed by the mother and father. This is illustrated by the analogy
of mixing two colored liquids in a test tube.
Mendel proposed fundamental units of inheritance called “factors” which are distinct
entities that do not blend during fertilization. Thus, the offspring contained the distinct factors or
particles from each parent.
Mendel’s experiments with the edible pea plants are an excellent example of sound
scientific method, even by today’s rigorous standards. He carefully chose the plants whose
characteristics varied in discrete and measurable ways. After testing hundreds of species of
garden plants, he chose the common sweet pea (Scientific name: Pisum sativum). He found that the
pea plant had seven (7) observable characteristics that came in two, and only two, varieties.
Seed shape was either smooth or wrinkled; seed interior was either yellow or green; seed
coat, either gray or white; ripe pods were either inflated or constricted; unripe pods, either green
or yellow; flowers were positioned either axially or terminally on the plant stem; and stems were
either long or short.
Other botanists in Mendel’s day merely described the characteristics of parent and
offspring plants they had bred. Mendel meticulously recorded the number of plants possessing a
given characteristic in each generation. He believed that the ratios of plant varieties in a
generation of offspring would yield clues to the mechanisms of inheritance. Appropriately, he based
his conclusions on large samples from his breeding experiments. Mendel was diligent in his
experimental technique: he examined the characteristics of each plant individually and took the
greatest care to prevent plants from uncontrolled cross-breeding and self-pollination.
Thus, Mendel’s discovery of the Principle of Particulate Inheritance followed from his
carefully controlled experiments. When he crossed plants producing smooth seeds with those
producing wrinkled seeds, the results were always the same…smooth seeds. No plants contained
both smooth and wrinkled seeds, and none of the seeds was just a little bit wrinkled. No blending
had occurred. The same thing was noted for all the characters. As we shall observe on the next
plate, the wrinkled seeds did reappear in later crosses. Mendel discovered that some traits were
dominant and some, recessive. For his seven (7) pea plant characters, one variety was always found
to be DOMINANT. In order to explain how RECESSIVE traits reappeared in later generations,
Mendel proposed that factors contributed by both parents was maintained in the offspring in
discreet units that could be inherited and passed on, regardless of whether they were expressed in
the outward appearance of the offspring. This is Particulate Inheritance.
DISCUSSION QUESTIONS:
1. Name the Augustinian monk who is given credit for his pioneering work in inheritance and is today
referred to as the “Father of Genetics”: ____________________________
2. What organism did this monk use in his breeding experiments?
A. Common Name: _______________________________
B. Scientific Name: ______________________________
3. In what year did he publish his important work…called “Experiments in Plant Hybridization”?
_______________
4. Give one possible reason WHY his book and his work went relatively “unnoticed” until about
1900:____________________________________________________________
5. Out of Mendel’s studies came his idea of___________________ inheritance which challenged
the popular 19th century notion of___________________ inheritance.
6. List the names of the three noted scientists who “rediscovered” Mendel, around 1900
____________________
_______________________
________________________
7. Important to the understanding of Genetics today is the knowledge of how DNA, genes, and
chromosomes work. Did Mendel know of these things? ____________ What were Mendel’s “units”
of inheritance called in 1865? ___________________
8. While studying Pisum sativum Mendel found that the pea plant had ______ (#) easily observable
traits that were expressed in only ______ (#) varieties.
9. Complete the chart below by filling-in the blanks:
TRAIT
PHENOTYPES (varieties)
Seed Shape
Seed Interior
Ripe Pods
Unripe Pods
Flower Position
Stem Length
Smooth
Yellow
White
Constricted
Wrinkled
Gray
Green
Axial
Long
10. Mendel was successful, (while others failed) because he…
A. studied individual traits rather than the whole organism…….TRUE or FALSE
B. introduced mathematics into his work (ratios)………………..TRUE or FALSE
C. evaluated results obtained from large amounts of data……TRUE or FALSE
Variation in Human Species: The Genetic Wheel
PURPOSE:
Variability is a basic characteristic of living things. Reserves of genetic variability are what natural
selection acts on as a species meets changes in its environment. Human beings show variation in
many traits that can often be detected easily by their appearance. Some of these traits may
persist partly because natural selection is not even involved. That is, some of the traits leave no
effect on survival today, at least not in any way geneticists can see.
With the exception of identical twins it is highly improbable that any two people will have
the same combination of genetically determined traits. The new characteristics in which you will
work with here represent over 200,000 different combinations.
Part I: Determination of Personal Physical Characteristics & Abilities
TRAIT
1. Ability to taste
Phenylthiocarbamide
2. Hitch Hiker’s
Thumb
3. Mid-Digital Hair
(On any finger)
4. Tongue Rolling
5. Ear Lobes
6. Dimples
7. Freckles (facial)
Dominant
Genotypes
Taster
(TT) (T-)
Straight
(EE) (E-)
Present
(MM) (M-)
Roller
(KK) (K-)
Free
(LL) (Ll)
Present
(PP) (P-)
Present
(JJ) (J-)
Recessive
Genotype
List Your
Phenotype
Non-Taster
(tt)
Bent 45° plus
(ee)
Absent
(mm)
Non-Roller
(kk)
Attached
(ll)
Absent
(pp)
Absent
(jj)
Circle Your
Genotype
T-
tt
E-
ee
M-
mm
K-
kk
L-
ll
P-
pp
J-
jj
Discussion Questions:
1. Define Genotype: _______________________________________________
2. Define Phenotype: ______________________________________________
3. For the above traits, how many are you DOMINANT for? _____ Recessive? ___
4. In what ways would a person with the #73 DIFFER in traits from a person who
is the #56? ____________________________________________________
______________________________________________________________
5. Is there someone in the class like you with respect to the 7 traits above? Let’s
say there is…list 3 additional traits that make you different from that person:
___________________
____________________
_________________