2. Fundamentals of Genetics
• List four characteristics that make
Pisum sativum a good subject for
genetic studies.
• Outline the three major steps of Gregor
Mendel’s garden pea experiments.
• Explain how Mendel derived ratios from
his observations.
• Compare and contrast Mendel’s two
laws of heredity.
3. Gregor Mendel
• Began experiments in
1857
• Austrian monk
• Studied genetics of peas
• Father of genetics
• Failed his math/science
teacher exam
• Attended Univ. of Vienna
- experimentation
4. Why the Garden Pea?
• Many varieties – 32 to start – used
7
• More than one form of the same
trait
• Small, easy to grow, matures
quickly, large numbers of
offspring…
• Ability to cross-pollinate and self-
pollinate
5. Experimental Design
• Step 1 – allow self-pollination to
produce plants that are true-
breeding. P generation.
• Step 2 – cross-pollination. F1
generation.
• Step 3 – self-pollination of F1
generation. F2 generatoin.
6. Two ratios observed
• Dominant trait – expressed trait in F1
generation.
• Recessive trait – not expressed in F1
generation.
• Self-pollination of F1 yields a 3:1
phenotypic ratio, but a 1:2:1 genotypic
ratio.
7. Mendel’s proposed Theory
• Parent’s pass factors(genes) to their
offspring that produce traits.
• Two alleles for each trait –
heterozygous (Tt) or homozygous (TT, tt)
• Phenotype = physical appearance
• Genotype = set of alleles
• Dominant is expressed over recessive
8. Mendel’s Laws
• Law of segregation – member of each
pair of alleles are separated when
gametes are formed.
• Law of Independent Assortment –
pairs of alleles separate independently
of one another during gamete
formation.
9. Interpreting Mendel’s
Model
• Capital letters= dominant alleles
• Lower case letters = recessive alleles
• Two alleles for each trait
• Homozygous dominant = DD
• Heterozygous dominant = Dd (hybrid)
• Homozygous recessive - dd
10. Probability
• The likelihood that an event will take
place.
• = number of one kind of possible outcome
total number of possible outcomes
14. Review Questions:
1. Write the formula used to determine the
likelihood that an event will occur. How is
probability used in genetics?
2. Construct a Punnett Square to predict the
outcome of a monohybrid cross between two
heterozygous tall pea plants. What are the
expected phenotypic and genotypic ratios?
3. What is a dihybrid cross? What is the
expected phenotypic ratio for a heterozygous
dihybrid cross?
15. 5 Factors that Influence
Heredity
#1 Incomplete Dominance
• A trait is displayed that
is intermediate between
two parents.
• Flower color in
snapdragons
– Red + White = Pink
16. #2 Codominance
• Two dominant alleles are
expressed simultaneously
• Different from incomplete
dominance because both
traits are displayed
• Example) roan coat in
horses is a result of a
cross between a
homozygous red and
homozygous white
horse…resulting in a
horse with a coat of red
and white hair.
17. #3 Multiple Alleles
• Genes with more than one allele
• Example) three alleles can determine
• blood type (A, B, and O)
– A and B are dominant over O
– Neither A nor B are dominant over each
other
– When A and B are present…they are
codominant
18. #4 Continuous Variation
• Several genes influence a trait
• Examples) height and weight
• A variety of phenotypes exist from one
extreme to another
19. #5 Environmental Influences
• Phenotypes being influenced by the
environment
• Example) the coat color of the arctic
fox in winter
20. Review Questions
1. What are dominant and recessive
alleles?
2. Why were true-breeding plants
important in Mendel’s experiments?
3. What is a Punnett Square?
4. What is the difference between
Phenotype and Genotype?
5. Describe two inheritance patterns
besides simple dominance.
