1. Recall: several hypotheses about inheritance
What made Mendel successful?
3 key points to any successful biological experiment:
Choosing the appropriate organism to study
Designing and performing the experiment correctly
Analyzing the data properly

2. Pisum sativum – The common pea plant
What organism did he study???
Pisum sativum – The common pea plant
Why the pea plant?
Commercially available across Europe
Easy to grow and matured quickly
Sexual organs entirely enclosed in flower (self-pollinate)
Control which plants reproduced
Different traits could be observed easily

3. 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

4. Mendel’s First Experiment: Monohybrid Crosses
P generation (parent generation)
Crossed a true-breeding TALL pea plant with a true-breeding SHORT pea plant

5. 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?

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

7. 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

8. 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

9. When individuals with contrasting traits are crossed, the offspring
Every time: one trait was always
dominant over the other
Therefore...
Mendel formulated
the Principle of Dominance:
When individuals with contrasting
traits are crossed, the offspring
will express only the dominant
trait.

10. Law of Segregation Next step: Breeding the F1 generation
Offspring was the 2nd filial generation (F2 generation)
Discussion:
What trait did the offspring of the F2 generation express?

11. Answer: The same result occurred with all 7 traits 3:1 ratio =
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

13. 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

14. Modern day’s terminology: “factors” = genes Genes 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

15. Homozygous vs. Heterozygous
HOMOZYGOUS: the alleles are the same
HETEROZYGOUS: alleles are different
Ex. Trait: Tall and short
T = tall t = short
TT = homozygous dominant tall
tt = homozygous recessive short
Tt / tT = heterozygous dominant tall
(Demonstrate generations on board)

16. Punnett Squares T T
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

17. Example: Applying Mendel’s first law: a monohybrid cross
Problem: A plant grown purebred homozygous tall seed crossed with a homozygous short seed
Show the possible gametes from each parent (possible offspring)
Show the possible gamete combinations at fertilization
What are the possible genotypes of F1
What are the possible phenotypes of F1

18. 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)
Show the possible gametes produced from each parent
Show the possible gamete combinations
What are the possible genotypes?
What are the possible phenotypes?

19. Example 3:
In sheep, the allele for white wool (W) is dominant over the allele for black wool (w). If two white sheep mated,
What are the possible genotypes of the two parent?
What is the probability that an offspring of the two white sheep will be black? (Create Punnet squares for each possible parent combinations)

20. 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

21. Male
Female
Affected Male (homozygous)
Affected Female (homozygous)
Known Heterozygous Female
Known Heterozygous Male
Mating
Parents
Siblings