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