Mendelian Genetics Reading: Chap. 14 I. Intro II. Mendel’s findings A. Motivating question B. Mendel II. Mendel’s findings A. Law of segregation B. Law of independent assortment III. Complications IV. Examples from human genetics As humans have altered ecosystems and caused species extinctions world-wide, a critical question is the extent to which such biotic modifications influence ecosystem-level processes.

Terms and Concepts - character, trait, alleles Rules of probability - P, F1, F2 - dominant/recessive - law of segregation - law of independent assortment - homozygous/heterozygous - phenotype/genotype - testcross Rules of probability Complications: - complete, incomplete and co- dominance - multiple alleles - pleiotropy - Epistasis - quantitative characters: polygenic inheritance

Motivating question: Radiation of the Galápagos finches Beak sizes Food availability Range overlap Probable ancestors

Galápagos Islands

What Darwin knew (and inferred): Patterns of distribution Mechanism of natural selection heritable traits “struggle for existence” higher fitness --> more offspring Shift in average traits in population

What he didn’t know: How did heritability work? What exactly was passed down from parents to offspring? Blending vs. particulate? No idea about: Genes, chromosomes, DNA, mitosis and meiosis

Gregor Mendel Austrian contemporary of Darwin Darwin’s thoughts are based on many prior thinkers. We will briefly look at a few of those whose prior work made an impression on Darwin. Austrian contemporary of Darwin Published shortly after Darwin - but work was “buried” Fig 22.1

Who was Mendel? - Austrian monk - Background in agriculture (grew up on a farm) - Failed his teacher’s exam - University of Vienna: math, causes of variation in plants - Teaching at the Brünn Modern School

What did he do? Pea breeding Testing mechanisms of inheritance Used many different characters Published results in 1865

Why did his experiments succeed? Control over fertilization Multiple generations: P, F1, F2 True breeding parents “Either/or” characters

II. What did Mendel find? A. Law of segregation (of alleles) B. Law of independent assortment (of traits)

A1. Mendel’s experiments: Simple cross P - true breeding parents with different traits for same character. F1 - Cross two of same generation F2 - evaluate resulting traits: 3 to 1

Mendel tested many traits 3 to 1!!! Did Mendel fudge?

A2. Mendel’s interpretation - one factor from each parent - dominant vs. recessive - particulate inheritance: can get pure traits back

Genotype vs. phenotype homozygous vs. heterozygous

3. Law of segregation When hybrid plants produce gametes, the two parental factors segregate: half the gametes get one type, half get the other type. All possible combinations, random combinations

4. Rules of probability - multiplicity - additivity

OK, prove it! The testcross Dominant phenotype: what genotype? Predictions follow from particulate inheritance

5. What do we know now?

Chromosomes, genes, and alleles segregate on the homologous chromosomes P p

How does the law of segregation relate to meiosis? Homologous chromosomes separate after doubling Sister chromatids separate Fig. 13.6

B. Law of independent assortment What about two or more characters? Are they inherited together or independently?

1. Two traits: an example Together Independent

Law of independent assortment (of characters) “Independent segregation of each pair of alleles (i.e., genes coding for each character) during gamete formation.”

Rules of probability From YyRr x YyRr Yellow round: YYRR YYRr YyRR YyRr (1/4*1/4) + (1/2*1/4)+(1/2*1/4)+(1/2*1/2) = 9/16 Green round: yyRR yyRr (1/4*1/4) + (1/4*1/2) = 3/16 Yellow wrinkled: YYrr Yyrr (1/4*1/4) + (1/2*1/4) = 3/16 Green wrinkled: yyrr (1/4*1/4) = 1/16

2. What we know now: Mendel’s independent assortment referred to characters. fig. 13.9 How does this relate to independent assortment of chromosomes in meiosis?

What if genes for two traits are on the same chromosome? Independent or linked? Linked, except for…? Crossing over Depends how close they are: genes further apart are more likely to behave as indpendent.

Mendel got lucky…twice 1. Genes for traits he studied were either on separate chromosomes, or 2. Far enough apart on the same chromosome that they assorted independently

III. Complications A. Dominance, Incomplete dominance and Codominance

A1. Incomplete dominance in snapdragon - Phenotype is intermediate - NOT blending Fig. 14.9

A2. Codominance - M, N, MN blood groups MM MN BOTH traits expressed NN

B. Complications: Multiple alleles ABO blood groups Dominant Dominant Codominant Recessive fig. 14.10

C. Complications: Pleiotropy - One gene affects many characters - Sickling allele of hemoglobin fig. 14.15

D. Complications: Polygenic Inheritance and Quantitative Characters - One trait determined by multiple genes - Converse of pleiotropy - e.g., skin color: at least 3 genes fig. 14.12

E. Complications: Epistasis - Expression of one gene depends on another - Mouse coat color: B - black coat b - brown coat C - pigment c - no pigment fig. 14.11

IV. Examples from human genetics Several excellent examples in the book. - Simple traits, geneologies - Genetic disorders (Tay-Sach’s disease, Huntington’s disease, cystic fibrosis, etc.) Understand how they work, but don’t need to memorize the details of each. Why might mating between close offspring lead to increased incidence of genetic disorders?

Where do we go from here? Have: Mechanism for natural selection Mechanism for heritability Not yet: Understanding of meiosis, maintenance of genetic variability “Molecular carrier” of heritable information

The modern synthesis Darwin Mendel Population genetics DNA Fig 22.1 Darwin’s thoughts are based on many prior thinkers. We will briefly look at a few of those whose prior work made an impression on Darwin. Fig 22.1