Fundamentals of Genetics
• List four characteristics that make
Pisum sativum a good subject for
• Outline the three major steps of Gregor
Mendel’s garden pea experiments.
• Explain how Mendel derived ratios from
• Compare and contrast Mendel’s two
laws of heredity.
• Began experiments in
• Austrian monk
• Studied genetics of peas
• Father of genetics
• Failed his math/science
• Attended Univ. of Vienna
Why the Garden Pea?
• Many varieties – 32 to start – used
• More than one form of the same
• Small, easy to grow, matures
quickly, large numbers of
• Ability to cross-pollinate and self-
• Step 1 – allow self-pollination to
produce plants that are true-
breeding. P generation.
• Step 2 – cross-pollination. F1
• Step 3 – self-pollination of F1
generation. F2 generatoin.
Two ratios observed
• Dominant trait – expressed trait in F1
• Recessive trait – not expressed in F1
• Self-pollination of F1 yields a 3:1
phenotypic ratio, but a 1:2:1 genotypic
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
• 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
• Capital letters= dominant alleles
• Lower case letters = recessive alleles
• Two alleles for each trait
• Homozygous dominant = DD
• Heterozygous dominant = Dd (hybrid)
• Homozygous recessive - dd
• The likelihood that an event will take
• = number of one kind of possible outcome
total number of possible outcomes
• Provide information about one pair of
• Involve two pairs of contrasting traits.
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
5 Factors that Influence
#1 Incomplete Dominance
• A trait is displayed that
is intermediate between
• Flower color in
– Red + White = Pink
• Two dominant alleles are
• 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
horse…resulting in a
horse with a coat of red
and white hair.
#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
– When A and B are present…they are
#4 Continuous Variation
• Several genes influence a trait
• Examples) height and weight
• A variety of phenotypes exist from one
extreme to another
#5 Environmental Influences
• Phenotypes being influenced by the
• Example) the coat color of the arctic
fox in winter
1. What are dominant and recessive
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.