Last day… started covering basics of heredity &
Mendelian genetics
Mendel’s experiments
showed that heredity is
particulate, not blending
Started to talk about how
meiosis leads to new
combinations of alleles…
1) Homologous chromosomes come together as pairs
(tetrad), process called synapsis
- chiasmata form – crossing-over of chromatids
2) Independent assortment of chromosomes
- when chromosomes align during meiosis,
chromosome from same parent may move
into different cells
3) Random fertilization
- combination of any sperm with any egg allows
trillions of possible combinations
New gene combinations detected by Mendel when he
crossed peas that differed in 2 characters
e.g. pea color (yellow or
green) & shape (round
or wrinkled
Original combinations
were found, but also
new ones
How do new combos
arise?
Ultimately, genetic variation must come from mutation
- many ways for this to happen...
Chromosomes may break – fragments may be lost,
or added elsewhere
Errors also
during
crossing-over
deletion – segment removed
- usually lethal
duplication – segment repeated
- usually harmful
inversion – segment in reversed position
reciprocal translocation – fragments switched between
non-homologous chromosomes
- inversions & translocations can change phenotype
because gene expression may depend on position
Point mutations (change in one nucleotide of DNA)
can be caused by chemicals, UV light, errors in
replication, etc.
- most repaired by enzymes
Most mutations are deleterious, some neutral (do not
change fitness), a very few are beneficial
- generally detrimental, but could not have evolution
without them
Chapter 14
(esp. pp. 262-75)
Paradise Tanager
Evolution after Mendel
After Darwin, evolution generally accepted
but skepticism about natural selection
- poor understanding of heredity a problem
Mendel’s paper noticed in 1900, & theory of heredity
rapidly developed
Initially thought Mendelism was in conflict with
natural selection
- emphasis on major mutations causing discrete traits,
not small variations
Eventually, population genetics developed,
& combined with findings from paleontology,
systematics, etc.  ‘The Modern Synthesis’
- built on & clarified Darwin’s theory
Ronald Fisher
J. B. S. Haldane
Sewall Wright
Population genetics important because populations evolve
(individuals do not)
- population: localized group of individuals
belonging to same species
Evolution – change in population’s genetic structure
(frequencies of alleles or genotypes) over generations
Need pop. genetic theory to understand evolution
(quantify & predict), but could be complex
– try to simplify – 1 locus, 2 alleles
What are genotype frequencies?
- if know allele frequency, can calculate random
expectation
A = gold (dominant)
AA =
Aa =
a = silver (recessive)
aa =
If you have: A) 2 golds; B) gold & silver; C) 2 silvers
pop. frequency of AA =
“
“
of Aa =
“
“
of aa =
A=
a=
If genotypes are random combinations, should expect:
AA =
Aa =
aa =
Are we close?
frequency of A = p
frequency of a = q
freq. of AA = p2
freq. of Aa = 2pq
freq. of aa = q2
p2 + 2pq + q2 = 1
This is Hardy-Weinberg theorem
- at equilibrium, genotypes will follow this ratio
Equilibrium – nothing happening to pop.
Effect of sex:
AA =
Aa =
aa =
If you have: A) 2 golds; B) gold & silver; C) 2 silvers
Much change?
- can test statistically (2 or chi-square test)
Now, mate with individual of same phenotype
AA =
Aa =
aa =
If you have: A) 2 golds; B) gold & silver; C) 2 silvers
Non-random mating disrupts equilibrium
- positive assortative mating reduces freq. of
heterozygotes
Now random mating again:
AA =
Aa =
aa =
If you have: A) 2 golds; B) gold & silver; C) 2 silvers
Only takes one generation of random mating to
restore pop, to H-W equilibrium!
A population geneticist studies a population of American
Robins and finds that the allele for the normal form of
alcohol dehydrogenase has a frequency of 0.92 while a
recessive allele that produces a defective form of the
enzyme has a frequency of 0.08. (Robins often eat
fermenting berries, and may get drunk if they have the
defective form.) If this population is in Hardy-Weinberg
equilibrium, what proportion of the population should be
homozygous for the recessive allele? What proportion
should be heterozygotes?
Reminders:
- purchase ‘lab manual’/subscription for ‘EvolutionLab’
online (see web page…)
- read Chapters 3, 4 & 5 from ‘The Beak of the Finch’
on reserve in Science Library
- ‘Natural Selection’ lab assignment due at start of next
lab
- try practice population genetics question
- consider Q&A session in Biology conference room,
Sunday 12-1 pm