Download Incomplete Dominance Codominance Multiple Alleles

Exceptions
to Mendel’s Principles:
Incomplete Dominance
Codominance
Multiple Alleles
Sex -Linked Disorders
Review: Dominant/Recessive
• One allele is
dominant over the
other (capable of
masking the
recessive allele)
PP = purple
pp = white
Pp = purple
Review Problem:
Dominant/Recessive
• In pea plants, purple flowers (P) are dominant
over white flowers (p) show the cross between
two heterozygous plants.
GENOTYPES:
-25% Homozygous
Dominant (PP)
-50% Heterozygous (Pp)
-25% Homozygous
Recessive (pp)
PHENOTYPES:
-75% Purple
-25% White
P
p
P
PP
Pp
p
Pp
pp
Exceptions to Mendel’s
principles
• Sometimes, there is no dominant or
recessive gene, or the trait is controlled by
many alleles or genes.
Incomplete Dominance
• A third (new) phenotype appears in the
heterozygous condition.
• Flower Color in 4 O’clocks
RR = red
WW = white
RW = pink
Problem: Incomplete
Dominance
• Show the cross between a pink and a
white flower.
GENOTYPES:
-50% Heterozygous
(RW)
-50% Homozygous
(WW)
PHENOTYPES:
-50% Pink
-50% White
R
W
W
RW
WW
W
RW
WW
Incomplete Dominance
•
•
•
•
straight hair, wavy, curly
Straight Hair: SS
Curly Hair: CC
Wavy Hair: SC
Codominance
• The heterozygous condition, both
alleles are expressed equally
• Sickle Cell Anemia in Humans
NN =
normal cells
SS = sickle cells NS = some of
each
Problem: Codominance
• Show the cross between an individual with
sickle-cell anemia and another who is a
carrier but not sick.
N
S
GENOTYPES:
-50 Heterozygous (NS)
-50% Homozygous (SS)
S
NS
SS
PHENOTYPES:
-50% Carrier
-50% Sickle-Cell
S
NS
SS
Codominance
• In codominance, neither allele are
dominant; both are expressed. A cross
between organisms with two different
phenotypes produces offspring with has
both phenotypes of the parental traits
shown.
Multiple Alleles
• There are more than two alleles for
a trait
• Blood type in humans
• Phenotypes:
– Type A, Type B, Type AB, Type O
• Blood Alleles:
– A, B, O
Rules for Blood Type
• A and B are codominant
– AA = Type A
– BB = Type B
– AB = Type AB
• A and B are dominant over O
– AO = type A
– BO = type B
– OO = type O
Problem: Multiple Alleles
• Show the cross between a mother who has
type O blood and a father who has type AB
blood.
O
O
GENOTYPES:
-50% AO
-50% BO
A
AO
AO
PHENOTYPES:
-50% A
-50% B
B
BO
BO
Problem: Multiple Alleles
• Show the cross between a mother who is
heterozygous for type B blood and a father who is
heterozygous for type A blood.
GENOTYPES:
-25% AB -25% BO
-25%AO
-25% OO
A
B
AB
BO
O
AO
OO
PHENOTYPES:
-25% AB
-25% B
-25% A
-25% O
O
Sex Linked Disorders
• Sex-linked genetic diseases are those
where the defective genes are carried on
either the X or Y chromosomes.
• Example: Color Blindness is carried on the
X chromosome. A male only needs the
one X to carry the trait to be affected,
females need both of their X’s.