Terms and Concepts - character, trait, , locus Mendelian Genetics - homozygous/heterozygous - / Reading: Chap. 13, pp. 265-276, 282-286 - P, F1, F2 I. Intro - dominant/recessive A. Motivating question - law of segregation B. Mendel - law of independent assortment II. Mendel’s findings -Testcross A. Mendel’s experiments - Rules of probability B. Law of segregation of alleles - Incomplete C. Law of independent assortment of traits - codominance III. Complications - Quantitative characteristics

Blending vs. particulate inheritance? What Darwin didn’t know:

How did heritability work? What exactly was passed down from parents to offspring? No idea about: Genes, , DNA, mitosis and meiosis

Gregor Mendel 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 Austrian contemporary of Darwin Fig 22.1 Published shortly after Darwin - but work was “buried”

1 What did he do? Mendel didn’t know about chromosomes either! breeding Testing mechanisms of inheritance: blending vs. • Results were buried for ~40 years – not broadly acquired characteristics (e.g., accepted until ~16 years after his . Lamarck) Used many different characters •Early in the 20th century, Sutton and Boveri Published results in 1865 (working independently) formulated the theory of inheritance, which proposes that meiosis causes the patterns of inheritance that Mendel observed.

Why did his experiments succeed? 2. “Either/or” 1. Control over fertilization characters 3. True breeding parents II. What did Mendel find?

A. Mendel’s experiments

B. Law of segregation (of alleles)

C. Law of independent assortment (of traits)

4. Multiple generations: P, F1, F2

A. Mendel’s experiments: Simple cross Mendel tested many traits

P - true breeding parents with different traits for same 3 to 1!!! character.

F1 - Cross two of same generation

F2 - evaluate resulting traits: 3 to 1

2 Mendel’s interpretation Genotype vs. phenotype - one factor from each parent

homozygous vs. heterozygous - dominant vs. recessive

- particulate inheritance: can get pure traits back

B. Law of segregation of alleles 2. Rules of probability 1. The factors controlling the trait of an individual go into different . 1. “Both-and rule” Cross true breeding lines - chance of 2 or more independent events (homozygotes), get all heterozygous offspring both occurring together - multiply probabilities of each event When heterozygous plants produce gametes, the two parental factors 2. “Either-or rule” segregate: half the gametes get one type, - probability of an event when several ways half get the other type. for it to occur - add probabilities of each pathway All possible combinations, random combinations

3. OK, prove it! The testcross 4. What do we know now? Dominant phenotype: what genotype?

Predictions follow from particulate inheritance

3 How does the law of segregation relate to meiosis? C. Law of independent assortment Chromosomes, genes, and alleles What about two or more characters? Are they inherited together or independently?

Alleles segregate on the homologous chromosomes

Homologous chromosomes separate after doubling

Sister chromatids separate

13.9

1. Two traits: an example Together Independent

556 total

Which hypothesis does this support? 13.6a 13.6b

Rules of probability Law of independent assortment From YyRr x YyRr (of characters) Yellow round: YYRR YYRr YyRR YyRr (1/4*1/4) + (2*1/4*1/4)+(2*1/4*1/4)+(4*1/4*1/4) = 9/16 “Independent segregation of each pair of Green round: alleles (i.e., genes coding for each yyRR yyRr (1/4*1/4) + (2*1/4*1/4) = 3/16 character) during formation.”

Yellow wrinkled: YYrr Yyrr (1/4*1/4) + (2*1/4*1/4) = 3/16

Green wrinkled: yyrr (1/4*1/4) = 1/16

4 2. What we know now: What if genes for two traits are Mendel’s independent assortment referred to characters. How does this relate to independent assortment of chromosomes in meiosis? 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 independent.

13.9b

III. Complications Did Mendel get lucky? A. Incomplete dominance

(not that way - he was a monk!)

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

Is this the same as blending?

B. Multiple alleles – co-dominance C. Complications: Quantitative Characters -One trait determined by multiple genes -Could lead to perception of “blending” but that’s not what it is.

Red ? fig. 13.19

5 13.19b 13.20

IV. Summary: KEY CONCEPTS KEY CONCEPTS

Mendel discovered that in garden , individuals Genes are located on chromosomes. have two factors, or versions, representing each trait. - We now know these are alleles - different The separation of homologous chromosomes versions of each gene. during meiosis I explains why alleles of the - Prior to the formation of eggs and sperm, the two same gene segregate to different gametes. alleles of each gene separate. - One is transmitted to each egg or sperm cell.

KEY CONCEPTS KEY CONCEPTS

If genes are located on different Important exceptions exist to the rules that individuals chromosomes, then the alleles of each gene have two alleles of each gene and that alleles of different are transmitted to egg cells and sperm cells genes are transmitted independently. independently of each other. - Genes on the same chromosome are not transmitted independently of each other. - some traits are controlled by more than one gene, or genes exhibit incomplete dominance or are co-dominant.

6