Download bio 11 gene frequency sept 15

Where we’ve been:
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
allele for brown fur
allele for black fur
48% heterozygous
black
16% homozygous black
36% homozygous brown
***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.
allele for brown fur
allele for black fur
48% heterozygous
black
16% homozygous black
36% homozygous brown
Which mice genotypes contain the recessive allele (b)?
the are
Bb inand
bb mice mouse? The homozygous recessive
How many recessive2,
alleles
the heterozygous
mouse?
1, 2
allele for brown fur
allele for black fur
48% heterozygous
black
16% homozygous black
36% homozygous brown
****Frequency of a RECESSIVE allele=
(# homozygous recessive x 2) + (# heterozygous x 1)
(# individuals in population x 2)
allele for brown fur
allele for black fur
48% heterozygous
black
16% homozygous black
36% homozygous brown
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
= 120 = 60%
100+100
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)
40%
(100 +100
=
)=
80
200
Note that the two allele frequencies
add up to ONE.
0.6 + 0.4 =1.0
= 0.4 =
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!
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!