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MENDEL’S THEORY
Introduction to Genetics!
Terms
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


Heredity – the passing of characters from parents
to offspring
Cross: Mating or breeding of two individuals
Genetics : the branch of biology that focuses on
heredity
Terms
3

Artificial fertilization

Hybrid
 Offspring
formed from parents having different forms
of a trait

Cross pollination
 Crossing
two contrasting traits during pollination
Gregor Mendel
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


Austrian Monk
Father of Genetics
Crossed Pisum sativum
(garden pea) and
developed the
patterns of genetics
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
People thought that the traits just blended
 Example:
tall + short = medium
 Example: red + white = pink
 Red + Blue = purple
Why the Pea Plant?
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1.

Characteristics exist in two clearly different forms
Example:
 white
or purple flower, yellow or green pod, round or
wrinkled seed etc….
Why the Pea Plant?
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2.
3.
Male and female reproductive parts are enclosed
in the same flower
They possess good characteristics for breeding
a)
b)
c)
d)
Small
Grow easily
Mature quickly
Produce many offspring
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THE EXPERIMENT
The Experiment
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
Mendel’s first experiments were monohybrid
crosses
(crosses that include one pair of contrasting traits)
 Flower color

Steps
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1.
Mendel allowed each variety of garden pea to
self pollinate
This insured true-breeding or a purebred
P generation (parental)
a)
b)
I.
These are the first two individuals that are crossed in a
breeding experiment
Steps
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2.
Cross pollination
Crossed 2 P generations that had contrasting traits
a)
I.
b)
c)
Example: one white and one purple
Result: Showed only one trait. The other disappeared
F1 generation (first filial)
Steps
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3.
Self pollination between F1 generations
a)
b)
c)
d)
Results: F2 generations
The trait reappeared
705 to 224
3:1 ratio
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MENDEL’S HYPOTHESIS
Mendel’s Hypothesis
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1.
2.
For each inherited character, an individual has
two copies of the gene – one from each parent.
There are alternative versions of genes
Alleles: different versions of genes
a)
I.
II.
Example: purple or white
Offspring receive one allele from each parent
Mendel’s Hypothesis
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3.
When two different alleles occur together, one of
them may be completely expressed, while the
other may have no observable effect on the
organism’s appearance.
a)
b)
Dominant: The expressed form of the character
Recessive: The trait that is not expressed when the
dominant form is present
Brown is a dominant eye
color.
Mendel’s Hypothesis
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4.
When gametes are formed, the alleles for each
gene in an individual separate independently of
one another.
a)
b)
Gametes carry only one allele for each inherited
character
During fertilization, each gamete contributes one
allele
Called Law of Segregation
Terms
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


Dominate alleles are Capitalized (G)
Recessive alleles are lower case (g)
Homozygous: two alleles of a particular gene
present in an individual are the same
 (GG,

gg)
Heterozygous: alleles of a particular gene present
in an individual are different
 (Gg)
Example
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If you use B for your alleles then…

Homozygous Dominant (or pure dominant) =
BB

Heterozygous =
Bb

Homozygous Recessive (or pure recessive) =
bb
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
Genotype: Genetic makeup of an organism
 What
it is made of (genes)
 The set of alleles that an individual has for a character
 Ex

BB, Bb, bb
Phenotype: Physical appearance of a character
 What
it looks like
 Example: blue eyes, red hair, tall, short