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Notes Chapter 11
Introduction to
Genetics
11-1: The Work of Gregor Mendel
Heredity is the passing of
traits from parent to
offspring.
Genetics is the scientific
study of heredity.
Gregor Mendel
Austrian monk born in 1822
in the Czech Republic.
worked with ordinary garden
peas.
fertilization
Gamete – specialized cell
involved in sexual reproduction.
Examples:
-Sperm (pollen)
-Egg
self pollination
 Single parent (true breeding).
 Sperm/pollen fertilize the egg
cells in the same flower.
 The offspring are identical to the
parent.
cross pollination
 Two parents.
 Hybrid- cross of traits from
different parents.
Gregor Mendel prevented self pollination by cutting
off the male parts of the pea flower.
He cross pollinated by dusting the pollen from a
selected flower onto the female part.
Trait
 A specific characteristic, such as
seed color or plant height.
 Gene- Chemical factors that
determine traits.
 Allele- Different, contrasting,
forms of a gene.
Example: The gene for plant height has
two different alleles available…
T= tall
t = short
Principle of Dominance
 Some alleles are dominant and
others are recessive.
(Think of it like a card game… sometimes
one card “trumps” another. They’re both
good cards, but when played together…
one is dominant.)
Dominant & Recessive Practice
Read the chart and determine which allele is dominant for….
-seed shape?
-pod shape?
-plant height?
-seed color?
-pod color?
-seed coat color?
-flower position?
But what happens to the recessive allele?
Does it just “disappear”?
This is what Mendel observed when he
crossed the F1 generation….
• The “short” allele reappeared in the F2 generation!
• The “short” allele must have been present in the F1,
however it was being “trumped out” & hidden by the
dominant “tall” allele.
Segregation of Alleles
(separation of alleles)
11-2: Probability and Punnett
Squares
The likelihood that a particular event
will occur is called probability, by
the “flip-of-a-coin”.
The principles of probability can be
used to predict genetic crosses.
Punnett square
 A diagram that depicts the gene
combinations from a genetic cross.
Alleles from one
parent.
Alleles from
the other
parent.
T
t
T TT Tt
t Tt tt
Possible
combinations for
offspring.
 homozygous – 2 identical
alleles.
(TT, tt)
 heterozygous – 2 different
alleles.
(Tt)
Practice:
1) Are the parents
homozygous or
heterozygous?
2) What percentage of
the offspring will be
homozygous?
Heterozygous?
T t
T TT Tt
t Tt tt
 genotype – genetic make-up (letters, Tt)
 phenotype – physical trait (what you
see, tall)
T
t
T
t
TT Tt
Tt tt
Genotype & Phenotype Practice
T= tall
t = short
1) If the genotype is TT, what will the phenotype be?
2) If the genotype is Tt, what will the phenotype be?
3) If the genotype is tt, what will the phenotype be?
4) If the phenotype is tall, what are the possible
genotypes?
5) If the phenotype is short, what is the genotype?
11-3: Exploring Mendelian
Genetics
Beyond simple
dominance…
- some alleles are neither
dominant nor recessive…
…and many traits are
controlled by multiple alleles
or multiple genes
Incomplete dominance
 One allele is not completely dominant over the
other.
 The phenotypes appear blended.
Codominance
 Both alleles contribute to the phenotype of
the offspring.
 The phenotypes are not blended… they both
appear separately.
multiple alleles
 Genes that have more than 2 alleles.
polygenic traits
 Traits produced by the interaction
of 2 or more genes (“having many
genes”).
Drosophila fly – Thomas Hunt Morg
 Other factors that can
affect the phenotype of
genes.
- climate, soil
conditions, water