Gregor Mendel conducted experiments with pea plants in the 1850s and 1860s. Through his experiments, he discovered the basic principles of heredity and inheritance. He found that traits are passed from parents to offspring through discrete units called genes, and that these genes segregate and assort independently during the formation of gametes. Mendel's work established the fundamental laws of inheritance and provided the foundation for the modern science of genetics.

1. LESSON # 8
If my father is blond, why am I not?

2. MENDELIAN GENETICS
Inheritance

3. Gregor Mendel
 Father of modern genetics
 Experimented with pea plants (Pisum sativum) 1856-1863
 Was a monk in the Czech Republic
 His work passed unrecognized until his paper was discovered
in 1900 “Experiments in plant hybridization.” (1865)
 This paper showed that each organism has physical traits that
correspond to invisible elements (genes) within the cell.
 These invisible elements, which we now call genes, exist in
pairs (alleles).
 Mendel showed that only one member of this genetic pair is
passed on to each progeny.
 Mendel's laws form the theoretical basis of our understanding
of the genetics of inheritance.

4. What he did right
 Picked the pea plant which is easy self or cross pollinate and grows fast.
 Chose to study traits with only two possible phenotypes :
 Pea texture: round or wrinkled seed phenotype
 Pea color: yellow or green seed phenotype
 Flower color: red or white flower phenotype
 Pea plant size: tall or dwarf plant phenotype
 Pea pod color: green or yellow

5. Genetic studies innovations
 Mendel made two innovations to the science of genetics:
 developed pure lines
 Pure Line - a population that breeds true for a particular trait. All individuals of
the population have the same phenotype for a specific trait they are pure for.
When self-crossing or crossing with individuals of the same pure line population,
the offspring shows the same phenotype.
 Counted his results and kept statistical notes

6. Law of Dominance or Uniformity of
Hybrids.
 When two pure line individuals of different phenotypes are crossed, all the
resulting offspring show one unique phenotype (the dominant phenotype).
 All the offspring that result from interbreeding pure line individuals of
different phenotypes, have the same phenotype and the same genotype.
All tall
First Filial (F1) generation
P: Parental generation

7. Representing the cross
 The trait that is maintained in the F1
generation is considered the
Dominant trait.
 The trait the disappears is
considered the Recessive trait.
 The first letter of the dominant
phenotype is used to indicate the
alleles.
 Capital letter for the dominant allele
 Lower case for the recessive allele
P: Parental generation
First Filial (F1) generation

8. How to begin
Gene studied: Pea stem
Possible alleles:
Possible genotypes and corresponding phenotypes:
Tall (T)
Short (t)
Tall > Short
Genotype TT Tt tt
Phenotype Tall stem Tall stem Short stem

9. P: Purebred Tall stem x Purebred Short stem
TT tt
Gametes T T t t
p(T) = 1 p(t) = 1
Punnet square:
Gametes
T
T
t t
Tt Tt
Tt Tt
Gametes
F1
Genotype 100% Tt
Phenotype 100% Tall stem
F1 Genotypes F1 Phenotypes
p (Tt) = 1 100 % Tall stem

10. Solved Exercise 1
A homozygous brown eyed woman and a man with blue eyes want to have
children. What is the probability of the child having blue eyes?
Gene studied: Eye color
Possible alleles:
Possible genotypes and corresponding phenotypes:
Blue (b)
Brown (B)
Brown > Blue
Genotype BB Bb bb
Phenotype Brown eyes Brown eyes Blue eyes

11. P: Purebred Brown eyes x Purebred Blue eyes
BB bb
Punnet square:
Gametes
B
B
b b
Bb Bb
Bb Bb
Gametes
Answer: The possibility of having a child with blue eyes is 0.
F1 Genotypes F1 Phenotypes
p (Bb) = 1 = 100% 100 % Brown eyes
Gametes B B b b
p(B) = 1 p(b) = 1

12. Exercise 2
What is the probability of having of getting a pea plant with wrinkled peas if we
cross two hybrids?
Gene studied: Pea texture
Possible alleles:
Possible genotypes and corresponding phenotypes:
Smooth ___
Wrinkled ___
Smooth > Wrinkled
Genotype
Phenotype

13. Exercise 2
What is the probability of having of getting a pea plant with wrinkled peas if we
cross two hybrids?
Gene studied: Pea texture
Possible alleles:
Possible genotypes and corresponding phenotypes:
Smooth (S)
Wrinkled (s)
Smooth > Wrinkled
Genotype SS Ss ss
Phenotype Smooth peas Smooth peas Wrinkled peas

14. P: _____________________ x ______________________
___ ___
Gametes __ __ ___ __
Punnet square:
Gametes
Gametes
Answer:_________________________________________
F1 Genotypes F1 Phenotypes

15. P: Smooth pea plant x Smooth pea plant
Ss Ss
Gametes S s S s
p (S) = p(s) = ½ p (S) = p(s) = ½
Punnet square:
Gametes
Gametes
SS Ss
Ss ss
S
S
s
s F1 Genotypes F1 Phenotypes
p (SS) = ¼ = 25%
¾ Smooth peas or 75 % Smooth peas
p (Ss) = ½ = 50%
p (ss) = ¼ = 25% ¼ Wrinkled peas or 25% Wrinkled peas
Possible answers:
There is a 25% chance of getting a wrinkled pea plant.
The chance of getting wrinkled pea plants is 1 out of 4
For every 3 smooth pea plants we get, we could get 1 wrinkled pea plant.

16. Developing the
Law of Segregation
 When two hybrids are crossed Mendel observed the
phenotype that disappeared in the F1 generation
reappears in the F2 generation.
 The F2 generation shows 2 different phenotypes in
a 3:1 ratio
 Seeing this he came to the conclusion that two
alleles that the parents carry for a gene segregate
when gametes are formed, and the offspring only
receives one allele from each parent.

17. Law of Segregation
Mendel’s law of Segregation states:
The two members of a gene pair segregate from each other into
the gametes, so that one-half of the gametes carry one member
of the pair and the other one-half of the gametes carry the other
member of the pair.
 We now know that segregation occurs during meiosis I

18. Exercise 3
How can I find out if a black hen is purebred or heterozygous?
Gene studied: Color feathers in hens
Possible alleles:
Possible genotypes and corresponding phenotypes:
Black ___
White ___
Black > Black
Genotype
Phenotype

19. P: Black feathered hen x White feathered rooster
(homozygous recessive)
BB or Bb bb
Gametes ___ ___ or ___ ___ ___ ___
Punnet square:
Gametes
Gametes
Answer: If the black feathered hen is purebred (BB) ________________________________________
Test cross
Gametes
Gametes
or
Answer: If the black feathered hen is hybrid (Bb) _______________________
Therefore, if when we cross a black feathered hen with a white feathered hen, we get a white
feathered hen we know that ___________________
F1 Genotypes F1 Phenotypes
F1 Genotypes F1 Phenotypes
1st Possibility
2nd Possibility
1st Possibility 2nd Possibility
1st Possibility
2nd Possibility