PATTERNS OF INHERITANCE
THE MANNER IN WHICH A GENE IS TRANSMITTED
5 PATTERNS OF INHERITANCE
• Incomplete Dominance
• Codominance
• Autosomal Dominant
• Autosomal Recessive
• Autosomal = On a chromosome that is NOT sex chromosome
(X or Y)
• Sex-linked
BEYOND MENDELIAN GENETICS:
INCOMPLETE DOMINANCE
Mendel was lucky!
Traits he chose in the
pea plant showed up
very clearly…
One allele was dominant over another, so phenotypes
were easy to recognize.
But sometimes phenotypes are not very obvious…
INCOMPLETE DOMINANCE – NEITHER TRAIT EXPRESSED
Snapdragon flowers come in many colors.
If you cross a red snapdragon (RR)
with a white snapdragon (rr)
You get PINK flowers (Rr)!
Heterozygous genotype results in
an Intermediate Phenotype
(blended trait)
RR

Rr
rr
INCOMPLETE DOMINANCE
When F1 generation (all pink flowers) is self
pollinated, the F2 generation is 1:2:1
red, pink, white
R
r
R
r
R R
Rr
Rr
rr
Examples
What happens if you cross
a pink with a white?

Rr
r
r
rr
R
r
R r
rr
Rr
rr
A pink with a red?

RR
R
R
r
Rr
R
R R
RR
Rr
Rr
ANOTHER VERSION OF
INCOMPLETE DOMINANCE
• Both alleles contribute to
phenotype
• Mixing of parental traits
• Ex. A gene for hair texture:
Curly hair allele from one parent
and Straight hair allele from other
= Wavy hair
CODOMINANCE
• Both alleles expressed,
NOT mixed
• Each trait is retained
CODOMINANCE
• Both traits expressed.
• Ex. An animal with allele for white hair
and an allele for red hair produce a roancolored coat (both white hairs and red
hairs, not blended).
EXAMPLE OF CODOMINANCE
CODOMINANCE WITH MULTIPLE ALLELES:
BLOOD TYPE
• Multiple alleles control the ABO
blood groups in humans.
• The A and B alleles are codominant
to each other, and the O allele is
recessive
• A Type x B Type = A and B type
• A, B, O alleles A, B, AB, O blood
BLOOD TYPE
POLYGENIC INHERITANCE (MULTIFACTORIAL INHERITANCE)
• More than one gene involved in
determining a particular
characteristic, e.g. height or skin
colour.
• Not to be confused with multiple
alleles
• Example: Human height
SEX LINKED INHERITANCE
• Genes that are carried by
either sex chromosome
EXAMPLE: COLOR BLINDNESS
EXAMPLE: HEMOPHILIA
• Blood-clotting disorder caused by a
mutation in a clotting factor
• Queen Victoria of England was a
carrier of the gene for
hemophilia.
PEDIGREE ANALYSIS
• Used to study trait inheritance in humans and predict how a
disease is transmitted in families
• The trait must cause a phenotype
• Females are circles
• Males are squares
• A filled in symbol indicates that individual has the trait of interest.
• Partially Filled = Carrier (Heterozygote)