Modern Synthesis concepts from Laboratory Genetics P = G + E Phenotype = Genotype + Environment 1. 2. Environmental effects on phenotype are not inherited 3. Heredity is based on particles (genes). They retain identity and do not blend. They give rise to continuous and discrete variation. 4. Genes mutate at relatively low rates. Recombination is an important aspect of evolution. 5. Environmental factors can influence mutation rates but do not yield specific mutations that are seemingly the best for surviving in a particular environment.

Chromosomes, Genes, and Alleles Traits are determined by genes (primarily) Many different genes (loci) per chromosome Locus A Locus B Locus C Different forms of a gene are called alleles

Genetically “simple,” single locus traits: Mendel’s peas Trait or Charcteristic Seed Shape Seed Color Flower Color Pod Shape Pod Color Flower Position Plant Stature

Dominance / Recessivity Additivity Mode of Gene Action Dominance / Recessivity Additivity Phenotype Phenotype aa Aa AA aa Aa AA Discrete Phenotypes Continuous Phenotypes

Quantitative traits depend on multiple underlying loci one locus + environment two loci + environment one locus four loci + environment many loci + environment

Diploid Adult C E Allele for Brown Coat Allele for Black Eyes

What will the offspring’s genotype be? What if Mate these ? C c E e What will the offspring’s genotype be? Cc Ee

{ { C E c e Hybrid Mouse Genotype Cc Ee Homologous Pair DNA Replication C c E e { Homologous Pair Sister chromatids

Genetic recombination: chromosomal segments are exchanged between homologues during Meiosis I C E E c C e c e

Genetic recombination: New combination of alleles

Separation of DNA G A M E T S C E C E E C Meiosis I Meiosis II c e c e

WHAT GENOTYPES IF MATE TWO HYBRID MICE (Cc / Ee)? C c E e A L P O S I B Egg Gametes C c E e Sperm Gametes Genotype CC/EE Cc/EE CC/Ee Cc/Ee

Cc / Ee x Cc / Ee c C I. Law of Segregation E e II. Law of Independent Assortment CE Ce ce cE All Possible Sperm Gametes All Possible Egg Gametes CCEE CCEe CcEE CcEe CCEe CcEE CcEe CCee Ccee ccEE ccEe ccee

Mendel’s 1st law: Characters are controlled by pairs of genes which separate during the formation of the reproductive cells (meiosis) Mendel’s 2nd law: When two or more pairs of genes segregate simultaneously, they do so independently.

“Exceptions” to Mendel’s Second Law From Thomas Hunt Morgan (1909): 2,839 flies Eye color A: red a: purple Wing length B: normal b: vestigial AABB x aabb AaBb x aabb AaBb Aabb aaBb aabb Exp 710 710 710 710 Obs 1,339 151 154 1,195

Morgan’s explanation   F1: F2: A B a b A a B b A a B b Crossover has taken place

Parental types: AaBb, aabb Recombinants: Aabb, aaBb The proportion of recombinants between the two genes (or characters) is called the recombination fraction between these two genes. It is usually denoted by r or . For Morgan’s traits: r = (151 + 154)/2839 = 0.107 If r < 1/2: two genes are said to be linked. If r = 1/2: independent segregation (Mendel’s second law).

All allele combinations All allele combinations Linked Loci Probability of recombination = 0.3 ab = 0.35 A a Meiosis aB = 0.15 B b All allele combinations in gametes NOT equally probable Ab = 0.15 AB = 0.35 Probability of recombination = 0.1 ab = 0.45 A a B b Meiosis All allele combinations in gametes NOT equally probable aB = 0.05 Ab = 0.05 AB = 0.45

Why important? Concept: The closer two loci are on a chromosome, the lower the probability of recombination. Why important? (1) Allows one to determine the linear order of genes on a chromosome (make a genome map). (2) Maps allow for the localization of genes, mutant phenotypes, and QTL in the genome.