1. Chapter 9 Part 1
Patterns of Genetic Inheritance

2. Gregor Mendel Deduced Laws of
Inheritance
A blended model of inheritance existed prior to
Mendel
• Before the science of inheritance was fully
understood, breeders understood that parents
contributed equally to offspring, but believed it
was due to blending.
• Ex. When red and white flowers breed, pink
flowers result and when red or white would
reappear later, it was due to genetic instability.
• Gregor Mendel - an Austrian monk that did
extensive pea plant research starting in the
1860s.

3. Mendel designed his experiments
well
• Garden pea
– Easy to cultivate
– Short generation time
– Produce many offspring
– Normally self-fertilize, resulting in individuals
that look like the parent (true breeding)

5. • Mendel’s design
– Removed anthers to prevent self-fertilization
– Dusted carpel with pollen from a plant with a different
characteristic
– Carpel developed into a pod containing peas
– Only one characteristic resulted
– If those were bred together (F2 generation) both
characteristics can be found again
– Mendel chose easy to identify traits
• Flower color, seed color, seed shape

6. Single-trait crosses reveal units of
inheritance and the law of segregation
Mendel’s law of segregation describes how
gametes pass on traits
• Ensured true breeding (offspring always
looked like parents) and then performed
cross-fertilization experiments
• With previous thinking, breeding these two
together should result in an intermediate,
but that is not what happened.

7. • P generation – original parents
• F1 generation – first batch of
offspring
• F2 generation – offspring of F1
parents (self-fertilization)
• When he crossed tall and short
plants, all offspring were tall.
• When he allowed F1 generation
to self-fertilize, 75% were tall
and 25% were short, so
although F1 did not express
that trait (short), it was still
able to pass it on to future
generations.

8. • The results led him to the conclusion that the
parents contained two separate copies of each
hereditary factor, one dominant and one
recessive.
• Monohybrid crosses – examining only one trait
• Law of segregation
– Each individual has two factors for each trait
– The factors segregate (separate) during the formation
of the gametes
– Each gamete contains only one factor from each pair
of factors
– Fertilization gives each new individual two factors for
each trait

9. The units of inheritance are alleles
of genes
• Alleles – alternative forms of
a gene
• Gene locus – location of an
allele on a chromosome
• Dominant allele – masks
expression of the other allele
(capital letter)
• Recessive allele – dominant
allele masks the expression
of this one (lowercase letter)
Dominant does no mean
normal or most frequent
Through meiosis, only one
allele for each trait is in each
gamete

10. • Homozygous – organism has two identical allele
(homozygous dominant or homozygous recessive)
• Heterozygous – two different alleles – dominant
allele expressed
• Genotype – alleles the individual has
• Phenotype – physical appearance
• Linkage group – grouping of alleles on a
chromosome that will be inherited together (comes
into play when looking at inheritance of multiple
traits)

11. Two-trait crosses support the law of
independent assortment
Mendel’s law of independent assortment describes
inheritance at multiple traits
• Dihybrid cross – two traits are examined
• Two possible results when crossing tall with
green pods and short with yellow pods
– If the dominant alleles always stay together and the
recessive alleles always stay together, then 2
phenotypes will result in F2 plants (tall with green pods
and short with yellows pods)
– If the 4 factors segregate, 4 phenotypes will result
• Tall with green pods, tall with yellow pods, short with green
pods, short with yellow pods
• Mendel found 4 phenotypes (phenotypic ratio
9:3:3:1)

12. • Law of independent
assortment
– Each pair of factors
separates (assorts)
independently (without
regard to how the others
separate)
– All possible combinations
of factors can occur in
gametes
• If alleles are not linked,
traits will follow this law

13. • What are the possible genotypes of a tall
pea plant with green pods?

14. Mendel’s results are consistent with
the rules of probability
• Punnett square
– Diagram used to calculate results of a cross
– Can be used to determine probability of offspring
inheriting a specific trait
• In humans, freckles is dominant over no
freckles. A man with freckles reproduces with a
woman with freckles, but their children have no
freckles. What chance did each child have for
freckles?

15. Testcrosses support Mendel’s laws
and indicate the genotype
One trait test cross
• To confirm F1 were
heterozygous, they were
crossed with homozygous
recessive. Half should
express one phenotype, the
other half the other
phenotype (1:1 phenotypic
ratio)
• Test crosses can determine
if genotype is heterozygous
or homozygous dominant

16. Two trait testcross
• To determine genotype when a
dominant trait is expressed, it is
crossed with one that is
homozygous recessive for both
traits.
• If homozygous dominant for both
traits, only the dominant
phenotypes will be expressed for
both when crossed.
• If heterozygous for one or both
traits, recessive phenotypes will be
expressed after cross.