### Chapter 3: The Modern Synthesis

```Chapter 3: The Modern
Synthesis
Hardy-Weinberg equilibrium
• If no selection and mating is random (i.e.,
no processes acting to change the
distribution of genotypes), then the
genotypes of F1 (daughter generation)
should be the same as the genotypes of
F0 (parent generation.
Hardy-Weinberg equations
• 1=p+q
(p = dominant, q = recessive)
• 1 = p2 + 2pq + q2 (square both sides)
• 1 = AA + 2Aa + aa
• Genotype ratio of 1:2:1
Figure 03.02
aa = 0.5 X 0.5 = 0.25
0.5
0.5
0.5
Aa = 0.5 X 0.5 = 0.25
aA = 0.5 X 0.5 = 0.25
0.50
0.5
0.5
0.5
AA = 0.5 X 0.5 = 0.25
Example: Eye color
• In a population of 100 we have 25 blue
eyed people
• Since the allele for blue eyes is recessive
then the blue eyed people are “aa” (q2)
• 25% or .25 are “aa”
• The frequency of the allele for blue eyes =
the square root of .25 = .50 (50%)
• Since p + q = 1, the allele for Brown also =
.50 (50%)
Example 2: Eye color
• In a population of 100 we have 16 blue
eyed people
• Since the allele for blue eyes is recessive
then the blue eyed people are “aa” (q2)
• q2 = 16/100 or .16
• The frequency of the allele for blue eyes =
the square root of .16 = .40
• Since p + q = 1, the frequency of the allele
for Brown eyes is .60 (60%)
Figure 03.02
Example 2: Eye color
• The F1 population 300 of which 27 blue eyed
people
• Since the allele for blue eyes is recessive then
the blue eyed people are “aa” (q2)
• q2 = 27/300 = .09
• The frequency of the allele for blue eyes = the
square root of .09 = .30 (30%)
• Since p + q = 1, the allele for Brown = .70 (70%)
• The frequencies of the alleles for eye color in F1
are not the same as in F0 therefore evolution is
taking place (selection or mate choice has
changed the distributions)
Can Look Like Blending
• If the phenotype is result of multiple genes each
• Example: tallness is controlled by 3 different
genes each with 2 alleles (one for tall and one
for short)
• If you get the tall allele in from all 3 genes then
you get 6 tall (++++++) and you are the tallest, if
you get all short you get 6 short (------) and you
are the shortest.
• But if you get half of each you are in the middle
(+-+-+-), you are also in the middle if you get +++--).
Tall
Short
++++++
+++++++++-+++--++---+----------
Since all allele
effects are
expressed), the
you can have
any combination
of 3 tall and 3
short and have
the same
phenotype
Hidden Variation
• Multiple gene effects mean that natural
selection and/or mate choice is not always
favoring or selecting against the same
genes
• Genes can hide in the recessive state
when phenotype is dependent on multiple
genes (loci).
Phenotypic Plasticity
• Soapberry bugs and mate guarding
• Frequency dependent strategy (Oklahoma
population), trait is plastic
• In Florida sex ratios are stable and trait is
canalized
Other terms
• Pleiotrophy = genes effect more than one trait
• Correlated response = phenotype is dependent
on more than one gene, therefore selection for a
trait effects frequency of multiple genes at the
same time