Mendelian Genetics:
The Laws of Inheritance
By Lisa Marie Meffert, PhD
Morpho plelides
Mark T. Hark, MD
BioEd Online
Blending Model of Inheritance vs.
Particulate Model of Inheritance
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Blending model:
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Offspring are a blend of the parents.
The blend of characteristics is passed on to the next
generation.
Variation is washed out over time.
Particulate model:
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Offspring are a combination of the parents.
Parental contributions are passed to the next
generation as separate entities.
Variation is maintained over time.
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Mendel’s Research on Particulate
Inheritance
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Made pure breeding (parental) lines for
different phenotypes of pea plants:
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Gregor Mendel
Crossed the pure lines to get F1 generation
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Plant height (tall vs. short)
Pea color (green vs. yellow)
Pea texture (round vs. wrinkled), etc.
All of the F1 individuals looked the same
Crossed F1 lines to get F2 generation
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¾ of the F2 individuals looked like one of
the parental lines
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¼ of the F2 individuals looked like the other
parental line
Pisum sativum
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Working out Mendelian Genetics
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A Punnet square is bookkeeping tool.
The allele is the specific form of
hereditary particle passed on through
male or female gametes.
Allele capital letter T confers tallness.
In this case, the tall parental line has
all T’s.
Allele lower case letter t confers
shortness. The short parental line has
all t’s.
Cross a tall male with a short female.
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Crossing the Parental Lines:
Male Contribution
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First, the male passes
on his alleles, one per
gamete (reproductive
cell).
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Crossing the Parental Lines:
Female Contribution
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Then, the female passes
on her alleles, one per
egg.
Crossing these two
parents yields all Tt
offspring.
Since T is dominant to t,
all of the offspring are tall
pea plants.
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Crossing the F1 Generation
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Crossing two homozygous
individuals, TT and tt, yields
all heterozygous offspring, Tt.
This generation is referred to
as F1.
Now, cross two individuals
from the F1 generation
(Tt x Tt).
First, the male pea plant
passes on its alleles, one
per gamete.
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Crossing the F1 Generation
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Now, the female passes on her
alleles, one per egg.
Genotypes of the F2
 ¼ of the offspring are TT.
 ½ of the offspring are
heterozygotes (one T and
one t).
 ¼ of the offspring are tt.
Phenotypes of the F2
 ¾ are tall (at least one T).
 ¼ are short (homozygous
for t).
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Moving On to Two Traits at a Time
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Keep T and t for tall and
short plants, respectively.
Add R and r for round and
wrinkled seeds,
respectively.
A double heterozygote
male produces four types of
gametes.
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Cross with a Female Double Heterozygote
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A double
heterozygote female
produces four types
of eggs.
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Crossing the Double Heterozygotes
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The male passes
on his alleles, two
per gamete.
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Crossing the Double Heterozygotes
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The female passes
on her alleles, two
per egg.
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Double Heterozygote Cross:
Assessing the Resultant Genotypes
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At least one T yields tall
stature.
At least one R yield
round seeds.
Thus, there are 9 ways
(gray) to produce tall,
round-seeded offspring.
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Double Heterozygote Cross:
Assessing the Resultant Genotypes
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Again, at least one T yields
tall stature.
And one R is required for
round seeds, so rr must
produce wrinkled seeds.
Thus, there are 3 ways
(lighter gray) to produce
tall, wrinkly seeded
offspring
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The Third Phenotype
in a Double Heterozygote Cross
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Two t alleles yield short
stature.
At least one R produces
round seeds.
Thus, there are 3 ways
(lightest gray) to produce
short offspring with round
seeds.
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The Fourth Phenotype
in a Double Heterozygote Cross
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Two t alleles yield short
stature.
Two r alleles produce
wrinkled seeds.
There is only 1 way
to produce short offspring
with wrinkled seeds.
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