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Genetics
Vocabulary
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Genetics: The study of heredity,
Heredity: the passing on of traits
from an organism to its offspring
Gregor Mendel
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An Austrian monk and
biologist
Worked during the 1860s
Experimented with pea
plants
Called “The Father of
Genetics” for his
discoveries which began
the field of genetics
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Public Domain
Mendel did his experiments in the
monastery gardens.
Public Domain
Why did Mendel choose to work
with peas?
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Peas grow and reproduce quickly so he
could study many generations.
They had a variety of different traits he
could study at the same time.
He could easily breed or cross them
through pollination. Peas normally selfpollinate but can be artificially crosspollinated by transferring pollen from one
plant to another mechanically.
Experiment 1
Short pea plants X short pea plants
All short pea plants
Short plants are true breeders – short
plants always have short plants as
offspring
Experiment 2
Tall pea plants X Tall pea plants
Sometimes
All tall pea plants
(true breeders)
Sometimes
Mostly tall plants
with some short
plants
Experiment 3
P1 (Parent Generation):
True breeding short X
True breeding Tall
Short X Tall P1
Tall
F1
75% Tall & 25% Short
Mendel concluded each plant must contain
two factors for a particular trait – he called
them characters, we call them genes.
F2
Dominant vs. Recessive
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Genes are the units of heredity.
A gene may have different forms that are
expressed as different traits
They may be dominant (the stronger trait)
or recessive (the weaker trait)
If both are present in an organism the
dominant trait will be expressed and the
recessive trait hidden.
The different forms are called alleles.
Seven Traits of Peas that
Mendel Studied
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6.
7.
Plant Height - tall or short
Seed Shape – round or wrinkled
Seed Color – yellow or green
Pod Color – yellow or green
Position of Flowers – axial or terminal
Pod Shape – inflated or pinched
Flower Color – violet-red or white
Dominant vs. Recessive
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Dominant traits are represented by capital
letters and corresponding recessive traits
as the same letter in lowercase
For example: the dominant tall allele is T
and the recessive short allele is t
Mendel’s Experiment 3 Using
Symbols for Alleles
P1
TT X tt
F1
Tt
Tt X Tt
F2
TT
25%
Tt
Tt
50%
Purebred
Hybrid
tt
25%
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By convention, the dominant gene is
written first in a gene pair no matter
which parent it comes from
If an organism has two alike alleles (both
dominant or both recessive) it is
purebred for that trait
If an organism has one dominant allele
and one recessive allele it is hybrid for
that trait
The Law of Segregation
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A law that states that gene pairs separate
during sex cell formation
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This means that each sex cell (egg or sperm)
contains only one allele for each gene
The Law of Independent
Assortment
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A law that states that each gene pair for a
trait is inherited independently of the gene
pairs for all other traits
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Therefore if you have two hybrid traits, Tt and
Rr, the sex cells may have the following
combinations of alleles TR, Tr, tR or tr
Incomplete Dominance
Discovered by German botanist Karl
Correns
 When neither allele in a gene pair
masks the other entirely but their
effects blend to produce a result
different from the purebred of either
allele
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Incomplete Dominance
First example:
Red Four O'clocks X White Four O’clocks
(RR)
(WW)
Pink Four O’clocks
(RW)
Public Domain: http://en.wikipedia.org/wiki/File:Gul-Abas-4-O%27clock_plant.JPG
Incomplete Dominance
Second example:
Creamy-white Horse X Chestnut-brown Horse
(WW)
(BB)
Palomino Horse
(BW)
A palomino is pale golden-brown
with a white mane and tail
Public Domain: http://en.wikipedia.org/wiki/File:Palomino_Horse.jpg
The Principals of Genetics
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Traits, or characteristics, are passed on
from one generation of organisms to the
next generation.
The traits of an organism are controlled
by genes.
Organisms inherit genes in pairs, one
gene from each parent
The Principals of Genetics
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Some genes are dominant, whereas
other genes are recessive
Dominant genes hide recessive genes
when both are inherited by an organism.
Some genes are neither dominant nor
recessive. These genes show incomplete
dominance.