red
pink
white
Precursor
Red pigment
Red pigment forming enzyme
A = Dominant allele makes a normal enzyme
a = Recessive allele makes a defective one
AA
Aa
aa
Pigment forming
enzyme is present in
limiting quantities, so
having more makes the
flowers more red.
Six possible genotypes can occur because there are multiple alleles
1. Fill in the
genotypes left to
right, top to
bottom.
2/3
1/3
2. What the
chance diamond
will be…
?
B =
A =
O =
AB =
1
Another case of
multiple alleles at
a single gene,
which show a
dominance series.
A = agouti, is the
wild type allele.
Dominance is only
relevant in pairs
of alleles.
Ay-yellow
A -agouti
X
P
F1
2
:
1
P
F1
Genetic Explanation
Sickle-Cell Anemia
• Hemoglobin is responsible for
transporting O2
• A variant of hemoglobin, HbS,
results in the aggregation of
hemoglobin causing the cell to
change shape.
• Recessive for cell shape, HbS/
HbS
Since gel electrophoresis can
separate the two protein
products of the two alleles in a
heterozygote, the phenotype is
codominant.
Genotype/Phenotype 2nd lecture (Chapter 3)
tan
gray
brown
Observation of 9:3:3:1 ratio (4 phenotypic classes) indicates 2
genes! Can tell which alleles are recessive by looking at F2 ratios.
A molecular explanation for two genes one phenotype.
Green is the ground state without any pigment
(except chlorophyll which is in all plant parts)
Precursor
Tan pigment
Tan pigment forming enzyme
Precursor
Gray pigment
Gray pigment forming enzyme
A = Dominant allele makes a normal enzyme B = Dominant allele makes a normal enzyme
a = Recessive allele makes a defective one b = Recessive allele makes a defective one
Tan pigment + Gray pigment = Brown colored seeds
X
P
AABB
F1
F2
aabb
A-B-
AB AB
AABB
Ab
AABb aB AaBB
ab
AaBb
Ab
aB
ab
9 A-B-: 3 aaB-: 3 A-bb: 1 aabb)
• Whenever a dominant allele of either gene is present,
trait is expressed.
Molecular explanation = two similar flower color genes both
of which encode a functional pigment forming enzyme.
Either gene can do the same job.
Precursor
Red pigment
A
B
Red pigment forming enzymes from the A and B genes
A = Dominant allele makes a normal enzyme
a = Recessive allele makes a defective one
B = Dominant allele makes a normal enzyme
b = Recessive allele makes a defective one