Gregor Mendel ( ) & the Foundations of Genetics

Early Views of Inheritance The Humunculus - in the egg or sperm? Pangenesis - the mechanism of acquired inheritance – each tissue has its own genes, which migrate to the egg & sperm Blended Inheritance - characters take on characteristics of both parents

Why Mendel Liked Peas Several variable characters with two discrete traits –easy to score (yellow or green) Can control fertilization, including self- fertilization –can produce pure lines Offspring always have one of the parental traits

X 100% F1F1 P

X F1F1 75% F2F2 + 25%

X F1 75% F2 + 25% X aa AA Aa aa 25% AA 50%Aa P

X aabb AABB P X F1 AaBb F2 aaBb aaBB aabb AABB AABb AaBb AaBB AAbb Aabb 6% 1 19% 3 56% 9 19% 3

Mendel’s Inferences Alternate traits caused by alternate forms of genes, the unit of heredity An organism has two genes, one from each parent, for each character –can produce pure lines Offspring always have one of the parental traits Sperm & eggs always have just one allele (gene variant), because they segregate When two alleles are different, one is fully expressed and one is masked (dominant or recessive)

Mendel’s First Conclusion: Law of Segregation All allele pairs randomly segregate during gamete formation Paired condition restored with fusion (fertilization) Aa a A 1:1

Mendel’s Second Conclusion: Law of Independent Assortment Each allele pair segregates independently of all others AaBb aBABabAb 1:11:1:

Chromosomes are the location of genes Chromosomes: long, threadlike associations of genes found in the nucleus consisting of protein & DNA Mendel’s Laws hold for chromosomes, within chromosomes there is some shuffling, called crossing-over Humans: 46 chromosomes - 22 pairs of autosomes plus 2 sex chromosomes (X and Y)

Mendel’s Laws are a powerful source of variation 2 possible combinations of chromosomes to form gametes > 8,000,000 different gametes 23 When two gametes combine (fertilization), there are approximately (8 million) combinations 2 Actual # of possible combinations of zygotes (fertilized eggs) in humans = 70, 368, 744, 177, 664

Somatic vs Germ Cells 2n 4n Somatic (body) vs Germ (reproductive) Cells 2n 4n 2n 4n 2n nn nn Mitosis (no change) Meiosis (change) No shuffling Shuffling

Somatic vs Germ Cells Somatic (body) vs Germ (reproductive) Cells in Humans 2n 4n 2n n 23 2n 23 Mitosis (no change) Meiosis (change) No shuffling Shuffling

Crossing Over in Meiosis Another Way to Generate Variation Genes on the same chromosome are linked - independence of segregation depends on distance and frequency of crossing-over

From DNA to Protein DNA Base Pairs A-C-G-T Triplet Codons for (20) Amino Acids AAA - CAT etc. RNA Intermediaries Protein (polymer of Amino Acids)

What Proteins Do…. Provide structure Catalyze reactions Recognize molecules Transport molecules Regulate gene expression

Change in one base pair - may or may not change amino acid, changed amino acid may or may not change protein conformation Spontaneous, but also increased by radiation, heat, chemical mutagens Rate ‘Infrequent’: one in a billion bases Mutation Point Mutations AATAAGAAAATATGAA

Detectable Genetic Mutations Many amino acid substitutions do not effect a protein’s function - they are silent Non-silent substitutions affect the proteins conformation (shape) or expression (promote or stop) Sometimes silent substitutions become revealed when the environment is changed Many important genetic diseases (e.g. PKU, Sickle- Cell) Frequency: about one in a million amino acids

Three Genetic Mutations Substitution Insertion Deletion AATAAGAAAATAGAA AATAAGAAAATATGAA AATAAGAAAATAAAGAA

Chromosomal Mutations Chromosomes can be duplicated, portions can be translocated to a different chromosome or inverted on the same, or deleted Usually has profound consequences - sterility or worse Common, e.g. Down’s syndrome 1:700 births Major mode of ‘instantaneous’ speciation in self- fertilizing or inbreeding species, especially plants

Only 1/3 more genes than a worm - Genes like components in assembly lines? Many more harmful mutations per generation Much less coding DNA ( rest junk or spacer or ?? ) Human genomes are complex, but ….

Genetic Load For humans, estimated by reduced fertility and increase in birth defects associated with conceptions between relatives 4 recessive lethals per individual, more than one new lethal per generation In women’s eggs, chromosomal defects in eggs increase with age In men’s sperm, DNA sequence changes increase with age In outbred human conceptions –70% of conceptions never come to term –2 per 1000 live births have genetic defects

What Changes Gene Frequencies? Mutation Genetic drift (random change in small pops) Non-random Mating Migration = Gene Flow Natural Selection

Purifying Selection Dominant or Sex-linked (X or Y) deleterious mutant alleles eliminated rapidly by natural selection Recessive autosomal deleterious mutant alleles reduced slowly by selection –heterozygotes ‘protect’ recessive deleterious mutant alleles –never eliminated: a mutation - selection equilibrium is reached

Stabilizing Selection decreases variation, doesn’t shift mean Trait value Frequency Mean Trait value Frequency Old Mean ParentsOffspring

Directional Selection may reduce variation, shifts mean Trait value Frequency Mean Trait value Frequency Old Mean ParentsOffspring

Disruptive Selection increases variation, may shift mean Trait value Frequency Mean ParentsOffspring Trait value Frequency Old Mean

Sexual Selection

Forms of Sexual Selection Intrasexual (usually male-male competition) –Weapons for within-sex competition Intersexual (usually females choosing males) –Ornaments or signals to attract choosy mates –Why are animals choosy: aesthetic preferences (Darwin’s hyp.) or signals indicate mate quality?

Consequences of Sexual Selection Drives species away from the ecological optimum Major cause of sexual dimorphism via disruptive selection: since ornaments are an advantage in only one sex, there is selection for modifiers that lead to expression in one sex only