1. Mendel and Genetics

2. What is genetics?
Genetics, a branch of biology, is the scientific study of heredity and variation of organisms.
Gregor Mendel, an Austrian monk who studied botany. He made important progress in understanding heredity.
Mendel studied the traits of ordinary garden peas.
During Mendel’s time, the belief was that the traits of the parents blended.

3. Gregor Mendel’s work with plants
Most plants have female and male parts on the same flower.
**Male part of the flower produces pollen (sperm).
**Female part of the flower produces ova (eggs)

4. Gregor Mendel’s work with plants
Plants reproduce sexually.
Pollen (male gamete) and ovum (female gamete) join making a new diploid cell.
Fertilization produces a new diploid cell.
Two options for the same flower:
Self-pollinating: pollen and ovum come from the same flower.
Cross-pollinating: ovum on one flower and pollen from flower on a different plant..

5. Mendel’s peas Mendel had pea plants that were true breeding.
Seed from purple flowered plants had only purple flowered offspring.
True breeding plants are ones that produce offspring like themselves.
Mendel experimented with cross pollination between different parents.

6. Gregor Mendel’s work with peas
A trait is a specific observable characteristic that varies from one individual to another.
Seven traits of peas studied by Gregor Mendel: dominant version of trait is on the left, recessive on the right.

7. Crossing different pea plants
P = parental generation
F1 = first filial generation (offspring or children)
F2 = second filial generation (grandchildren)
Mendel’s Cross pollination
P two different parents, example: trait = height; alleles = tall or short
F1 offspring are all heterozygous plants. He used F1 as parents
Heterozygotes have different alleles.
Homozygotes have the same alleles.
F2 ‘grandchildren’ have ratios of 3 out of 4 dominant trait or 75% and 1 out of 4 recessive trait or 25%.

8. Mendel—Father of Genetics
Mendel’s application of scientific method and mathematics to heredity in peas led to his discovery of ‘factors’.
Mendel concluded that biological inheritance is determined by factors passed from one generation to the next.
Today, scientists call the factors that determine traits genes.

9. Mendel—Father of Genetics
Each trait that Mendel studied was controlled by one gene that occurred in two contrasting forms.
The different forms of a gene are called alleles
Example—trait is flower color
Gene controls flower color
Two forms are purple and white
Alleles are P-purple, p-white

10. Mendel and his work Mendel predicted: The role of genes in heredity
Alleles for a gene occur in pairs for an organism
One allele for a gene is in each gamete
Mendel had three key concepts:
Dominance
Segregation
Independent Assortment (Day 3)

11. Two of Mendel’s Principles
1. Law of Dominance:
One allele does the ‘talking’. One allele is expressed. One allele was dominant over the other in the F1 generation.
2. Principle of Segregation:
When gametes are formed, the pairs of hereditary factors (alleles) become separated. Each sex cell (egg/sperm) receives only one version of the gene.
3. Principle of Independent Assortment:
Day 3 (Hint: multiple chromosomes divide in meiosis)

12. Gregor Mendel’s ideas
Mendel observed that some traits appeared to dominate.
Dominant allele of gene is always expressed in classic Mendelian genetics.
Other traits skipped a generation. Recessive trait.
A trait is an observable characteristic that varies between individuals.
Mendel counted the results of cross pollination. He calculated the probability of outcomes from his hypothesis.
Probability is how likely something will happen. Mendel assumed the forms of a trait happened as random events with equally likely probabilities.
Consider flipping a coin. Two equally likely outcomes.

13. Using a Punnett Square
STEPS: 1. determine the genotypes of the parent organisms 2. write down your "cross" (mating) 3. draw a Punnett square
Parents are tall and short. Parent genotypes:
TT and t t
Cross
T T  t t

14. Punnett square T T t T T  t t T t
4. "split" the letters of the genotype for each parent & put them "outside" the p-square
5. determine the possible genotypes of the offspring by filling in the p-square
6. summarize results (genotypes & phenotypes of offspring)
T T
T T  t t
T t t
Genotypes:
100% T t
Phenotypes:
100% Tall plants

15. Monohybrid cross: F2 generation
If you let the F1 generation self-fertilize, the next monohybrid cross would be:
T t  T t
(tall) (tall)
Genotypes:
1 TT= Tall
2 Tt = Tall
1 tt = dwarf
Genotypic ratio= 1:2:1
Label your ratio TT:Tt:tt
T t
T T
T t
t t T t
Phenotype:
3 Tall
1 dwarf
Phenotypic ratio= 3:1
Label your ratios tall: short

16. Secret of the Punnett Square
Key to the Punnett Square:
Determine the gametes of each parent…
How? By “splitting” the genotypes of each parent:
If this is your cross
T T  t t
The gametes are: T T t t

17. Once you have the gametes… t t t t
T t T T

18. Another example: Flower color
For example, flower color:
P = purple (dominant)
p = white (recessive)
If you cross a homozygous Purple (PP) with a homozygous white (pp): 
P P p p
ALL PURPLE (Pp)
P p

19. Cross the F1 generation:
P p P p 
Genotypes: PP Pp
1 pp P p
P P
P p
p p P
Phenotypes:
3 Purple
1 White p

20. Stations Each group will visit all stations. Station 1—Vocabulary A
Station 2—Punnett Squares
Station 3—Analysis of Punnett Square for Parents
Station 4—Vocabulary B
Station 5—Segregation
Station 6—Phenotype/Genotype
Use your handout to record your work at each station.

21. Resources
Dr. Dennis O’Neill, 2013, Basic Patterns of Genetics, Palomar College
Paul Anderson, 2013, Mendelian Genetics, Bozeman Science,
CrashCourse, 2013, Heredity, Crash Course in Biology,
Staff, 2013, Glossary of Terms, Palomar College,