Understanding
Inheritance
Chapter 5 Lesson 2
Part 2
Modeling Inheritance
A Punnett square is a model used to
predict possible genotypes and
phenotypes of offspring.
Modeling Inheritance
A pedigree shows phenotypes of
genetically related family members.
Variations in Mendel's Theories
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Incomplete dominance
Codominance
Polygenic inheritance
Sex-Linked traits
Variations in Mendel's Theories
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Sometimes traits appear to be blends
of alleles.
Alleles show incomplete dominance
when the offspring’s phenotype is a
blend of the parents’ phenotypes.
Codominance occurs when both
alleles can be observed in a
phenotype.
Incomplete dominance
a type of inheritance in which the alleles
expressing a particular characteristic are
neither dominant or recessive; two traits
combine or blend together to produce a
different trait (a blend of two traits)
(when offspring of two homozygous
parents show an intermediate phenotype)
Incomplete dominance
example: flower color in snapdragons &
four o’clock flowers
Codominance
• In codominance both traits are
expressed; there are no dominant and
recessive
traits
Example:
roan cattle
Codominance
In incomplete dominance neither allele is
fully dominant. This is different from
codominance, in which both alleles are fully
expressed, resulting in organisms that display
the characteristics of both parents.
Codominance
In codominance, both alleles are expressed
independently and are uniquely recognizable.
Codominance
Some traits, such as human ABO
blood type, are determined by more
than one allele.
Polygenic Inheritance
• two or more genes producing a single trait
• occurs when multiple genes determine the
phenotype of a trait
(examples in humans: hair color, skin color, eye
color, height, intelligence, body build, etc.)
Genes and the Environment
An organism’s environment can
affect its phenotype.
 Some examples of environmental
factors that affect phenotype are soil
type that a flower is growing in or
time of year that a butterfly
develops.
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Genes and the Environment
Example: Hydrangea
For most French hydrangeas
(Hydrangea macrophylla), the
flower color indicates the pH of
the soil. In strongly acid soil (pH
below 6), flowers turn blue. In
alkaline soil (pH above 7),
flowers turn pink or even red. In
slightly acid or neutral soil (pH 6
to 7), blooms may be purple or a
mix of blue and pink on a single
shrub