Transcript 16.1 Genes and Variation - Center Grove Elementary School

Where we’ve been:
• Charles Darwin and his travels (Darwin Notes 1)
• Natural Selection (Notes 2)
– Moths and Natural Selection Spoons
• Evidence for Evolution (Notes 3)
– Comparative Anatomy, Biogeography, DNA
– Packet, Video, Webquest, Biochemical Comparisions
• Mechanisms of Evolution (webquest and this)
– Genetic Drift, Sexual Selection, Mutation, Migration,
Natural Selection
16.1 GENES AND VARIATION
I. “Gaps” in Darwin’s Theory
1. How do VARIATIONS arise?
-Variation was one of the key “ingredients” of
NATURAL SELECTION
- Remember: variation, selection, time
- Some variations are better than others, the
environment SELECTS those.
2. How are FAVORABLE variations (traits) passed on
to offspring?
• What scientist’s work could have help Darwin
with these questions?
• MENDEL!!!
• (Remember, he described INHERITANCE of
“factors” in pea plants).
II. Putting it all together!
By the 1930s, Mendel and Darwin’s work were
combined. Molecular biologists had also
discovered that:
1) DNA was the molecule of heredity. DNA
determines phenotype.
2) Sources of variation include MUTATION and
GENE SHUFFLING
• We can now describe evolution in GENETIC
terms. We can be more specific than “change
over time.”
• Evolve: change over time
• Evolution (in genetic terms) is any CHANGE in the
relative FREQUENCY of ALLELES in a population.
OR
AKA: biological evolution or microevolution
A change in genetic composition within a
population over generations.
Genetic Vocabulary Review
• Gene
• Trait
• Genotype
• Allele
• Phenotype
Vocab:
1. A POPULATION is a group of individuals of the
same species that interbreed and reproduce.
2. A GENE POOL is all genes, including different
alleles, that are in a population.
• 3. RELATIVE FREQUENCY is the # of times an
allele appears in a population.
General Equation:
Relative frequency of an allele=
# of the certain allele in the population
# of TOTAL alleles in the population
MAIN IDEA!
• Allele Frequency is about finding
(# of A’s)
(total # of A’s + a’s)
Application
What is the allele
frequency in the
mice population
Which mice genotypes contain the dominant allele for
fur color (B)?
• 2, the BB and Bb mice
How many dominant alleles are in the homozygous dominant
black mouse? The heterozygous mouse?
• 2, 1
48%
heterozygous
black
36%
homozygous
brown
16%
homozygous
black
allele for
brown fur
allele for
black fur
***Relative frequency of a DOMINANT allele
= (# homozygous dominant x 2) + (# heterozygous x 1)
(# individuals in population x 2)
Why multiply the # of individuals by 2?
Each individual has two alleles for a trait, one from mom, one from dad.
48%
heterozygous
black
36%
homozygous
brown
16%
homozygous
black
allele for
brown fur
allele for
black fur
Which mice genotypes contain the recessive allele (b)?
2, the Bb and bb mice
How many recessive alleles are in the heterozygous
mouse? The homozygous recessive mouse?
48%
heterozygous
black
36%
homozygous
brown
16%
homozygous
black
allele for
brown fur
allele for
black fur
1, 2
****Frequency of a RECESSIVE allele=
(# homozygous recessive x 2) + (# heterozygous x 1)
(# individuals in population x 2)
48%
heterozygous
black
36%
homozygous
brown
16%
homozygous
black
allele for
brown fur
allele for
black fur
III. Example: Calculating the relative
frequency
Example 1: Let's consider a gene with only two alleles. In mice,
Black fur color (BB or Bb) is dominant to brown fur color (bb).
In a population of 100 mice, 36 mice are homozygous dominant
(BB), 48 mice are heterozygous (Bb) and 16 are brown (bb).
Relative frequency of B= # of B alleles in the population
# of TOTAL alleles in the population
Relative frequency of a dominant allele:
= (# homozygous dominant x 2) + (# heterozygous x 1)
(# individuals in population x 2)
Relative frequency of B
= (36 x 2) + (48 x 1)= .60 = 60%
(100 x 2) = 200
or
36+36+48
100+100
= 120 = 60%
200
How could we figure out the frequency of “b”
without doing the big equation?
0.6 + 0.4 =1.0
Note that the allele frequencies add up to 1.
*This is a law of population genetics:
The sum of all allele frequencies will always be
equal to 1. This is because 1 represents the
frequency of all possible alleles within the
population.
Frequency of b?
Frequency of b = # of b alleles in the population
# of TOTAL alleles in the population
Frequency of a recessive allele:
= (# homozygous recessive x 2) + (# heterozygous x 1)
(# individuals in population x 2)
Frequency of b=
(16 x 2) + (48 x 1) =
(100 x 2) = 200
80 = 0.4 = 40%
200
or
(16 + 16 + 48) =
(100 +100) =
80 = 0.4 = 40%
200
Note that the two allele frequencies
add up to ONE.
0.6 + 0.4 =1.0
• Let’s say that these mice have several
generations of offspring. We want to know if
EVOLUTION has occurred. How would we
know?
• If the relative frequencies of the alleles
CHANGE, then evolution has occurred!
IV. 2 Sources of Variation: A Review
Clip: Why don't horses have wheels?
1.Mutations are any change in the DNA
sequence (AATAC  AATAT)
May be caused by:
Mistakes during replication
Radiation or chemicals in the environment
Effects: INCREASE Fitness, DECREASE Fitness, or
NO Effect
When would a mutation have no effect?
Ex: Silent mutation, no change in amino acid sequence
• When would a mutation that caused a
phenotypic change have no effect on fitness?
If it were something that did not effect
SURVIVAL and REPRODUCTION. Ex. extra
finger or extra teeth.
2. Sexual Reproduction (Gene Shuffling)
Q: Why don’t you look exactly like your parents?
(what were the sources of genetic variation
you learned in meiosis?)
Meiosis:
• Random assortment of
genes, random
separation of
chromosomes
(the Law of Independent
Assortment).
• Crossing Over: exchanging
parts of homologous
chromosomes
• REMEMBER! Some variations are better than
others, the environment SELECTS those.
• The source of variation is on the DNA level!