Gregor Mendel conducted experiments with pea plants in his monastery garden to study heredity and the transmission of traits from parents to offspring. He observed seven traits in pea plants that each had two contrasting variants. Through controlled pollination experiments over multiple generations, he discovered that traits are transmitted through discrete factors, now known as genes. His work established the laws of segregation and independent assortment, laying the foundation for modern genetics.

1. Fundamentals of Genetics
Chapter 9

3. Mendel’s Legacy
• Genetics is the study of how characteristics
are transmitted from parents to offspring
– Founded with the work of Gregor Mendel, an
Austrian monk

4. Gregor Mendel
• Mendel conducted experiments at the
monastery with pea plants
• He researched heredity – the transmission of
characteristics from parents to offspring

5. Mendel’s Garden Peas
• Mendel observed seven characteristics of pea
plants
• Each characteristic occurred in two contrasting
traits – genetically determined variants of a
characteristic

6. Mendel’s Garden Peas
Characteristic Trait
• Position of flowers along
stem
• Height of plant

7. Mendel’s Garden Peas
Characteristic Trait
• Pod Appearance
• Pod color

8. Mendel’s Garden Peas
Characteristic Trait
• Seed Texture
• Seed color
• Flower color

9. Mendel’s Methods
• Mendel controlled pollination
– Self-pollination – pollen fertilizes egg cells on the
same flower
– Cross-pollination – pollen fertilizes egg cells on
flowers of different plants

10. Mendel’s Experiments
• P generation – parent generation
• F1 (first filial) generation – offspring of the P
generation
• F2 (second filial) generation – offspring of the
F1 generation

11. Mendel’s Experiments
• First he grew true-
breeding plants
– A plant that is true-
breeding, or pure, for a
trait always produces
offspring with that trait
when they self-pollinate

12. Mendel’s Experiments
• Next, he cross-
pollinated true-
breeding parents of
the P generation
• What do you think
the F1 generation
looked like?

13. Mendel’s Experiments
• The F1 generation was
all purple flowers!

14. Mendel’s Experiments
• Next, he let the flowers from the F1 generation
self-pollinate
• What do you think the F2 generation looked
like?

15. Mendel’s Experiments
• The F2 generation came
out with 75% purple
flowers and 25% white
flowers!

16. Mendel’s Results and Conclusions
• Mendel’s observations led him to hypothesize
that something within the pea plants
controlled the characteristics observed
– He called these controls factors
• Because the characteristics had two
alternative forms, he reasoned that a pair of
factors must control each trait

17. Recessive and Dominant Traits
• Dominant – trait that masks the factor for the
other trait
• Recessive – trait that is masked by the factor
for the other trait

18. The Law of Segregation
• The law of segregation states that a pair of
factors is segregated, or separated, during the
formation of gametes

19. The Law of Independent Assortment
• The law of independent
assortment states that
factors separate
independently of one
another during the
formation of gametes

20. Support for Mendel’s Conclusions
• A gene is a segment of a chromosome that
controls a particular hereditary trait
• The different forms of a gene are called alleles
– Capital letters are used to represent dominant
alleles
– Lowercased letters are used to represent recessive
alleles

21. Section 2
GENETIC CROSSES

22. Genotype and Phenotype
• Genotype – an organism’s genetic makeup (PP,
Pp, pp)
• Phenotype – an organism’s physical
appearance (purple flower, white flower)

23. Genotype and Phenotype
• Homozygous – both alleles of a pair are alike
(PP = homozygous dominant and
pp = homozygous recessive)
• Heterozygous – two alleles are different (Pp)

24. Probability
• Probability – the likelihood that a specific
event will occur

25. Predicting Results of Monohybrid
Crosses
• In a monohybrid cross, one characteristic is
tracked
• Biologists use a Punnett square to predict the
probable distribution of inherited traits in the
offspring

26. Ex 1: Homozygous X Homozygous
• TT x tt (T = tall, t =
short)
• Possible Genotypes
– 100% Tt
• Possible Phenotypes
– 100% tall

27. Ex 2: Homozygous X Heterozygous
• BB X Bb (B = black hair,
b = brown hair)
• Possible Genotypes
– 50% BB, 50% Bb
• Possible Phenotypes
– 100% black

28. Ex 3: Heterozygous X Heterozygous
• Bb X Bb(B = black hair,
b = brown hair)
• Possible genotypes
– 25% BB, 50%Bb, 25% bb
• Possible phenotypes
– 75% black hair, 25%
brown hair

29. Ex 4: Testcross
• How could you determine whether a black
guinea pig is homozygous (BB) or
heterozygous (Bb)?
• By completing a testcross – an individual with
an unknown genotype is crossed with a
homozygous recessive individual

30. Ex 5: Incomplete Dominance
• Incomplete dominance occurs when the
phenotype of heterozygous individual is in
between the two homozygous phenotypes

31. Ex 5: Incomplete Dominance
• RW X RW (R = red, W =
white)
• Possible genotypes and
phenotypes
– 25% RR, red
– 50% RW, pink
– 25% WW, white

32. Ex 6: Codominance
• In codominance, both alleles contribute to the
phenotype
• In some varieties of chicken, the allele for
black feathers is codominant with the allele
for white feathers
– Heterozygous chickens are speckled with black
and white feathers

33. Ex 6: Codominance
• F W FW X F B F B
(F = feathers, B = black,
W = white
• Possible genotypes and
phenotypes
– 100% FWFB, speckled

34. Predicting Results of Dihybrid Crosses
• A dihybrid cross is a cross in which two
characteristics are tracked
• More complicated than monohybrid crosses
because more combinations of alleles are
possible

35. Dihybrid Homozygous X Homozygous
• rryy X RRYY (R= round, r= wrinkled, Y= yellow,
y= green)

36. Dihybrid Heterozygous X Heterozygous
• RrYy X RrYy