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Genetic Variation: The Source of
Evolutionary Diversity
The five major vertebrate classes exist due to evolutionary
change. This change is, in turn, caused by deterministic
and stochastic factors according to the process of natural
selection.
Natural selection can be summarized in 3 basic steps:
1. Variation
2. Selection
3. Reproduction
The source of variation is genetics. The source of this
genetic
variation is our present focus.
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Genetic Variation: The Source of
Evolutionary Diversity
The evolution of diverse species within and among
the vertebrate classes is due, originally, to genetic
variation.
What are the sources of genetic variation?
I. Mutation in DNA.
II. Sexual reproduction (meiosis and recombination of
alleles as shown on Punnett squares).
III. Unique inheritance patterns (as shown on Pedigrees).
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I. Mutation in DNA. A lot goes into creating a living
organism. Mutations are mistakes in DNA. Mistakes can
happen at any of the steps listed below (and more!).
gene: codes for a trait
allele: form of a gene (A or a)
genotype: genetic make-up (AA, Aa, or aa)
phenotype: trait
dominant: A
recessive: a
heterozygous: Aa
homozygous: AA or aa
gamete: reproductive cell (contains half of alleles)
meiosis: how gametes form
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Variation source:
New alleles can form when a mutation occurs
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II. Sexual reproduction: Meiosis: how gametes form.
R r
u u
Z Z
6 genes (12 alleles)
P p
t t
H h
4 chromosomes (2 pairs)
MEIOSIS:
Sex cells duplicate and
divide twice to form
daughter cells.
R
u
Z
Only half of alleles present in each
cell. These cells can combine with
other gametes to form the new cell
(zygote) of a new organism.
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R
u
Z
P
t
H
r
u
Z
P
t
H
r
u
Z
p
t
h
p
t
h
Variation source: Offspring contain alleles from BOTH parents
--not exactly like either parent
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II. Sexual reproduction: Punnett squares .
Possible alleles from male
gamete (sperm)
Possible
alleles from
female gamete
(egg)
R r
R
r
r
r
r
r
Any one of
these four
combinations
could result!
Variation source:
Many possible combinations of alleles form genetic traits of offspring
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II. Sexual reproduction: Dihybrid Punnett squares .
Parental (P
generation)
genotypes
Possible
alleles
from
female
gamete
(egg)
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Possible alleles
from male
gamete
(sperm)
Possible
offspring (F1
generation)
Variation source:
Even more possible combinations of alleles to form traits of new offspring
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III. Unique inheritance patterns
In addition to the classic recessive/ dominant inheritance
pattern, traits can be passed on in other ways. These include:
Incomplete dominance
Codominance
Polygenic traits
Multiple alleles
Sex linked traits
Further details can be found in Biology (Miller, 2000), pp.
209-216 and 230-239.
Inheritance patterns can be tracked on pedigrees.
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Variation source:
Alleles interact in various ways to produce genetic traits
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III. Unique inheritance patterns
Incomplete dominance
Codominance
Normal cell
Sickled cell (diseased: Sickle
Cell Anemia)
Heterozygous individuals
have enough sickled cells to
be malaria-resistant, but not
enough to have Sickle Cell
Aniemia.
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Variation source:
Alleles interact in various ways to produce genetic traits
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III. Unique inheritance patterns
Polygenic traits
More than one gene makes the trait
Sex
linked
traits
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Multiple alleles
More than one allele makes the trait
Can you see the
numbers in each
circle? If not, you
are probably male.
(See next slide)
Variation source:
Alleles interact in various ways to produce genetic traits
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III. Unique inheritance patterns: Sex-linkage.
Possible
alleles
from
female
gamete
(egg)
Possible alleles from male
gamete (sperm)
The Y chromosome codes
only for “maleness”. So
the linked alleles are
always 0 and never show
dominance (even over
recessive alleles such as h).
Dominant
phenotype
Dominant
phenotype
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Dominant
phenotype
Recessive
phenotype
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III. Unique inheritance patterns: Mapping with Pedigrees
A pedigree is a multi-generational family tree that displays
gender, mates, offspring, and one or more genetic traits.
Parents
Children
Grandchildren
n=normal allele (Dominant)
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p=affected allele (recessive)
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