Modern Synthesis concepts from Laboratory Genetics P = G + E Phenotype = Genotype + Environment 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) Different forms of a gene are called alleles Many different genes (loci) per chromosome Locus A Locus B Locus C

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

Dominance / Recessivity aaAaAA Additivity Phenotype Mode of Gene Action Discrete Phenotypes Continuous Phenotypes

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

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

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

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

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

Genetic recombination: New combination of alleles C C c c E E e e

C C c c E E e e Separation of DNA Meiosis II C C c c E E e e Meiosis I GAMETESGAMETES

C C c c E E e e Sperm Gametes C C c c E E e e ALLPOSSIBLEALLPOSSIBLE Egg Gametes Genotype CC/EE Cc/EE CC/Ee Cc/Ee WHAT GENOTYPES IF MATE TWO HYBRID MICE (Cc / Ee)?

CCEECCEe CcEeCcEeCcEE CcEeCcEe CCee CcEeCcEe CcEeCcEe Ccee ccEE ccEe ccee CECE CECECeCe CeCe cece cece cEcE cEcE All Possible Sperm Gametes All Possible Egg Gametes Cc / Ee x Cc / Ee I. Law of Segregation c C C c E e E e II. Law of Independent Assortment

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

“Exceptions” to Mendel’s Second Law From Thomas Hunt Morgan (1909): 2,839 flies Eye colorA: reda: purple Wing lengthB: normalb: vestigial AABB x aabb AaBb x aabb AaBb Aabb aaBb aabb Exp Obs 1, ,195

Morgan’s explanation A A BB a a b b  F1: A a Bb a a b b  F2: A a Bb a a b b A a bb 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 = ( )/2839 = If r < 1/2: two genes are said to be linked. If r = 1/2: independent segregation (Mendel’s second law).

Aa B b Meiosis Probability of recombination = 0.3 A a B b Meiosis Probability of recombination = 0.1 ABAB AbAb aBaB abab All allele combinations in gametes NOT equally probable = 0.15 = 0.35 ABAB AbAb aBaB abab = 0.05 = 0.45 Linked Loci All allele combinations in gametes NOT equally probable

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.