Beyond
Mendel
Beyond Mendel
Incomplete and Co-dominance
 there are patterns of inheritance that do not
follow the same patterns that Mendel
observed.
• they still follow the same rules as laid out
by Mendel’s laws
 Incomplete dominance
• occurs when neither of the two alleles for
the same gene can completely conceal the
presence of the other.
• example: Mirabilis jalapa (Four
o’clock Plant)
Incomplete Dominance
• example: Mirabilis jalapa (Four o’clock Plant)
• a cross between a true breeding red-flowered
plant and a true-breeding white-flower produces
offspring with pink flowers.
• when representing incomplete dominance upper
and lower case letters are not used.
• all upper case letters are used with
subscripts/superscripts to denote the alleles.
R1R1 → red flower
R2R2 → white flower
R1R2 → pink flower
Incomplete dominance
Incomplete dominance
• two human examples of incomplete dominance
are sickle cell anemia and familial
hypercholesterolemia
Sickle Cell Anemia
HbA HbA → normal red blood cells
Hbs Hbs → sickle shaped red blood cells
HbA Hbs → have the sickle trait
• this is called heterozygous advantage because if
you have one copy of the you don’t have the
disease and you are resistant to malaria.
Incomplete dominance

Familial Hypercholesterolemia
• a genetic condition that prevents the tissues
from removing low-density lipoproteins
(bad cholesterol) from the blood.
• if you are homozygous for the trait you have
six times the amount of cholesterol in your
blood.
• most have a heart attack by the age 2
• heterozygous individuals have about twice as
much cholesterol in their blood and may have
a heart attack by the age 35.
Co-Dominance
• occurs when both alleles are fully expressed.
• example: Blue Roan Horses
• a heterozygous animal where both the base
colour and white are expressed.
• both black and white hairs grow on the
body creating a blue appearance
Co-dominance

In humans a single gene determines a person’s ABO
blood type.
• this gene determines the type of antigen, if any, that is
attached to the cell membrane or red blood cells.
• the gene is designated as I and has three common
alleles:
IA
IB
i
• the different combinations of the three alleles
produce four phenotypes known as A, B, AB and O
Co-dominance
Co-dominance





the IA allele is responsible for the A antigen
the IB allele is responsible for the B antigen
the i allele causes there to be no antigen
the IA and IB heterozygous mix are co-dominant with
each other.
Sometimes called “intermediate” inheritance