Recall: several hypotheses about inheritanceWhat made Mendel successful?3 key points to any successful biological

experiment:1. Choosing the appropriate organism to study2. Designing and performing the experiment correctly3. Analyzing the data properly

What organism did he study???

Pisum sativum – The common pea plant

Why the pea plant?1. Commercially available across Europe2. Easy to grow and matured quickly3. Sexual organs entirely enclosed in flower

(self-pollinate) Control which plants reproduced

4. Different traits could be observed easily

To setup his experiment, Mendel obtained purebreed plants Purebreed – decended from ancestors of a distinct type

or breed Ie. All tall vs. All short plants

Thus, Mendel produced plants that were true breeding – only producing offspring that grew either tall or short

Mendel repeated this several times for 7 different traits:

- Stem length, flower position, seed shape, seed colour, pod shape, pod colour, flower colour

Mendel’s First Experiment: Monohybrid Crosses

P generation (parent generation) Crossed a true-breeding TALL pea plant with a true-

breeding SHORT pea plant

The offspring were the 1st filial generation (F1 generation) This generation consisted of hybrid plants since they

were the result of a cross between 2 purebred plants Monohybrid cross when only 1 trait is involved

Discussion: What trait did the F1 generation

demonstrate?Tall or short or medium?...or

something we’ve never even heard about?

Answer:100% in the F1 generation were tall!!!

Mendel concluded that the “TALL” trait is dominant and the “SHORT” trait is recessive

Dominant trait – characteristic that is always expressed in an individual

Recessive trait – characteristic that is latent and therefore not usually expressed in an individual

***May be expressed if it is the only trait present

He did this several times using true breeding plants for 7 traits that he chose to study

Stem length Flower position Seed shape Seed colour Pod shape Pod colour Flower colour

Every time: one trait was always dominant over the other

Therefore...Mendel formulatedthe Principle of Dominance:When individuals with contrastingtraits are crossed, the offspring will express only the dominant trait.

Law of Segregation

Next step:Breeding the F1 generationOffspring was the 2nd filial generation (F2

generation)

Discussion:What trait did the offspring of the F2

generation express?

Answer:3 out of 4 plants in the F2 generation were tall and 1 was short

The same result occurred with all 7 traits

75% of the time – expressed dominant 25% of the time – expressed recessive

3:1 ratio = Mendelian ratio

Mendel’s Law of Segregation

Thus, Mendel drew the following conclusions:In the F1 generation, each parent starts with

2 heredity “factors” – one dominant, one recessive

The factors segregate and only one factor from each parent is given to the F2 generation offspring; therefore, the offspring has 1 factor from each parent

If the dominant factor is present it will be expressed (even if the recessive factor is also present)

The recessive factor will only be expressed if only recessive factors are present

Modern day’s terminology:“factors” = genesGenes have:

Dominant alleles Recessive alleles

*Note: When a dominant and recessive allele are together, the dominant allele will be expressed but the recessive allele is still there and still can be passed onto the next generation

Homozygous vs. Heterozygous

HOMOZYGOUS: the alleles are the sameHETEROZYGOUS: alleles are different

Ex. Trait: Tall and shortT = tall t = shortTT = homozygous dominant talltt = homozygous recessive short Tt / tT = heterozygous dominant tall

(Demonstrate generations on board)

Punnett Squares

Used to calculate probability of inheriting a particular trait.

Allows you to determine the: Genotype: the genetic make-up behind a trait Phenotype: The actual physical appearance of the

trait

T

t

t

T

Example:Applying Mendel’s first law: a monohybrid cross

Problem: A plant grown purebred homozygous tall seed crossed with a homozygous short seed

a) Show the possible gametes from each parent (possible offspring)

b) Show the possible gamete combinations at fertilization

c) What are the possible genotypes of F1d) What are the possible phenotypes of F1

Example 2:

Problem: A plant grown from heterozygous round seeds is crossed with a plant grown from wrinkled seeds (Let R = dominant round, r = recessive wrinkled)

a) Show the possible gametes produced from each parent

b) Show the possible gamete combinationsc) What are the possible genotypes?d) What are the possible phenotypes?

Example 3:

In sheep, the allele for white wool (W) is dominant over the allele for black wool (w). If two white sheep mated,

a) What are the possible genotypes of the two parent?

b) What is the probability that an offspring of the two white sheep will be black? (Create Punnet squares for each possible parent combinations)

Pedigrees

How can we apply this to us?Pedigrees are diagrams that illustrate the

genetic relationships among a group of related individuals

It organizes medical, historical, and family records extending across several generations

Pedigrees been able to show simple dominant traits (only 2 possibilities – dominant, recessive) Ie. Widow’s peak, tongue-rolling, freckles, long

eyelashes, unattached ear lobes, etc

Male

Female

Affected Male (homozygous)

Affected Female (homozygous)

Known Heterozygous Female

Known Heterozygous Male

Mating

Parents

Siblings