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

duplication – segment repeated

- usually harmful

- usually lethal

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) a = silver (recessive)

AA = Aa = aa =

pop. frequency of AA = “ “ of Aa = “ “ of aa =

A = a =

If you have: A) 2 golds; B) gold & silver; C) 2 silvers

If genotypes are random combinations, should expect:

AA =Aa =aa =

Are we close?

frequency of A = pfrequency of a = q

freq. of AA = p2

freq. of Aa = 2pqfreq. 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 =

Much change?- can test statistically (2 or chi-square test)

If you have: A) 2 golds; B) gold & silver; C) 2 silvers

Now, mate with individual of same phenotype

AA =Aa =aa =

Non-random mating disrupts equilibrium- positive assortative mating reduces freq. of

heterozygotes

If you have: A) 2 golds; B) gold & silver; C) 2 silvers

Now random mating again:AA =Aa =aa =

Only takes one generation of random mating to restore pop, to H-W equilibrium!

If you have: A) 2 golds; B) gold & silver; C) 2 silvers

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