This document discusses different forms of complex inheritance that do not follow Mendel's principles of simple dominance and recessiveness. It describes four types of complex inheritance: incomplete dominance where traits show a blend of parental phenotypes; co-dominance where both parental traits are expressed simultaneously in offspring; multiple alleles where a trait is influenced by more than two allele versions of a gene; and polygenic inheritance where a trait is influenced by multiple interacting genes.

1. Objectives
• Discuss why Mendel’s principles cannot be applied to all
situations.
• Compare and contrast different types of complex
inheritance.

2. It’s complicated
• Mendel got lucky
All of the traits that he
studied were only
controlled by two alleles
• Remember: an allele is
a form of one gene
(yellow or green)
All of the traits were
perfectly dominant or
recessive

3. It’s complicated
Unfortunately, most traits
are not so neat and tidy

4. It’s complicated
• There are 4 forms of
complex inheritance you
need to know
Incomplete dominance
• Co-dominance
• Multiple Alleles
• Polygenic inheritance

5. Incomplete Dominance
• Remember, a gene is the
instruction for a protein
Because there are two
alleles for a trait, two
proteins are being
produced
• In Mendel’s studies, one
protein dominated the
other
The dominant protein was
shown

6. Incomplete Dominance
• Incomplete Dominance:
One gene does not
clearly dominate the other
• The phenotype of the
offspring is a blend of the
two parent phenotypes

7. Incomplete Dominance
• Example: A white flower is
mixed with a red flower
In incomplete dominance a
white flower mixed with a
red flower produces PINK
offspring
• The offspring are a blend of
the two parents
Neither parent is seen in
the offspring

8. Co-dominance
Co-dominance: Neither
phenotype dominates and
BOTH are shown

9. Co-dominance
• Example: A red flower is
mixed with a white flower
• In co-dominance a red
parent flower and a white
parent flower produce
offspring with BOTH red
AND white petals

10. Compare
• Incomplete dominance:
Red + White = Pink
• Co-dominance:
Red + White = Both red
and white on the same
flower

11. Multiple Alleles
With all of the variety in the
world it would make sense
that some traits have more
than two forms

12. Multiple Alleles
Multiple Alleles: A trait for
which there are more than
2 forms of a gene

13. Multiple Alleles
• Example: Blood Type
There are three alleles for
blood type
IA
IB
• i

14. Multiple Alleles
• Genotypes for type A blood are
• IA IA or IAi
Genotypes for type B blood are
IB IB or IBi
• Genotype for AB blood is
• IA IB
Genotype for type O blood is
• ii

15. Polygenic Inheritance
• If you have noticed, most
kids are a blend of their
parents traits
• To this point all of the traits
we have talked about have
been controlled by different
forms of ONE gene

16. Polygenic Inheritance
• Polygenic Inheritence
Remember your words
• Poly = many
What do you think
POLYgenic inheritance
means?

17. Polygenic Inheritance
• Polygenic inheritance: A trait
that is controlled by multiple
genes working together
Some good examples
• Skin color
• Eye color
• Height

18. Polygenic Inheritance
• There are many genes that
control skin color, eye color,
or height
• The interaction of those
genes controls the precise
color of your skin, color of
your eyes, or your height