SME
Grade 9 Science · Problem type
Incomplete dominance
Use this for “Two pink snapdragons are crossed. How many of 4 offspring are expected to be red, pink and white?”
Diagram
1 of 4 red, 2 of 4 pink, 1 of 4 white.
Incomplete dominance: the heterozygote is in between.
This is an example. Choose your own values to start a new problem.
a
Cᴿ alleles in the first parent
example
c
Cᴿ alleles in the second parent
example
R
Boxes of 4 red
worked out (not typed)
P
Boxes of 4 pink
worked out (not typed)
W
Boxes of 4 white
worked out (not typed)
Formulas
R
=
a
×
c
1 = 1 × 1
P
=
a
×
(2
−
c
)
+
c
×
(2
−
a
)
2 = 1 × (2 − 1) + 1 × (2 − 1)
W
=
(2
−
a
)
×
(2
−
c
)
1 = (2 − 1) × (2 − 1)
Assumptions
•
Snapdragon color: CᴿCᴿ is red, CᵂCᵂ white, and CᴿCᵂ pink, a blend of the two.
•
A parent is written by its count of Cᴿ alleles: 2, 1 or 0.
•
Codominance differs: both alleles show in full, as in roan cattle or AB blood, with no blend.
Step-by-step
WE KNOW
a = 1, c = 1
FIND
boxes of 4 red (R), boxes of 4 pink (P), boxes of 4 white (W)
Step 1 · Find boxes of 4 red (R)
Use R = a × c
A box is red with a Cᴿ from each parent: one parent’s Cᴿ times the other’s.
R
=
1
×
1
R
=
1
Step 2 · Find boxes of 4 pink (P)
Use P = a × (2 − c) + c × (2 − a)
A box is pink with Cᴿ from one parent and Cᵂ from the other, either way round.
P
=
1
×
(2
−
1)
+
1
×
(2
−
1)
P
=
2
−
1
+
1
P
=
2
Step 3 · Find boxes of 4 white (W)
Use W = (2 − a) × (2 − c)
A box is white with a Cᵂ from each parent.
W
=
(2
−
1)
×
(2
−
1)
W
=
1
Check
Put the numbers back in. Both sides should match.
1 = 1 × 1 ✓
2 = 1 × (2 − 1) + 1 × (2 − 1) ✓
1 = (2 − 1) × (2 − 1) ✓
Topic check
See what you know: 5 questions on this topic
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Mendelian and non-Mendelian inheritance
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