SME
Grade 7 Science · Life Science
Genes, alleles and Punnett squares
Use this for “Two Tt pea plants are crossed. What is the chance a seedling is tall?”
Diagram
T
t
T
TT
Tt
t
Tt
tt
Tt × Tt: 3 of 4 boxes have a T, so 3 in 4 show the trait.
1 in 4 do not.
This is an example. Choose your own values to start a new problem.
a
First parent
example
b
Second parent
example
r
Boxes with tt (short)
worked out (not typed)
t
Boxes with a T (tall)
worked out (not typed)
P
Chance of tall
worked out (not typed) · %
Q
Chance of short
worked out (not typed) · %
Formulas
r
=
small
t
’s
in
a
×
small
t
’s
in
b
(boxes
with
tt
(short)
=
small
t’s
in
first
parent
×
small
t’s
in
second
parent)
1
=
small
t
’s
in
Tt
×
small
t
’s
in
Tt
t
=
4
−
r
(boxes
with
a
T
(tall)
=
4
−
boxes
with
tt
(short))
3 = 4 − 1
Q
=
r
×
25
(chance
of
short
=
boxes
with
tt
(short)
×
25)
25 = 1 × 25
P
=
t
×
25
(chance
of
tall
=
boxes
with
a
T
(tall)
×
25)
75 = 3 × 25
Assumptions
•
Each parent has two alleles for a gene and passes one, at random, to each child.
•
A capital letter is the dominant allele: one is enough to show the trait. The recessive trait shows only with two small letters.
•
Here T is tall and t is short in pea plants: TT and Tt are tall, tt is short.
•
The 4 boxes are the 4 equally likely combinations; a percent is a chance, not a promise, for any one child.
Step-by-step
WE KNOW
a: Tt, b: Tt
FIND
boxes with tt (short) (r), boxes with a T (tall) (t), chance of short (Q), chance of tall (P)
Step 1 · Find boxes with tt (short): r
Use r = small t’s in a × small t’s in b (boxes with tt (short) = small t’s in first parent × small t’s in second parent)
tt
needs
a
small
t
from
each
parent:
count
each
parent’s
small
t
’s
(Tt
has
1,
tt
has
2)
and
multiply.
r
=
1
Step 2 · Find boxes with a T (tall): t
Use t = 4 − r (boxes with a T (tall) = 4 − boxes with tt (short))
The other boxes have at least one capital letter, so they show the dominant trait.
t
=
4
−
1
t
=
3
(3
tall
to
1
short)
Step 3 · Find chance of short (Q)
Use Q = r × 25 (chance of short = boxes with tt (short) × 25)
Each of the 4 boxes is equally likely: 25% each.
Q
=
1
×
25
Q
=
25%
Step 4 · Find chance of tall (P)
Use P = t × 25 (chance of tall = boxes with a T (tall) × 25)
Each of the 4 boxes is equally likely: 25% each.
P
=
3
×
25
P
=
75%
Check
Put the numbers back in. Both sides should match.
1 = 1 × 1 ✓
3 = 4 − 1 ✓
25 = 1 × 25 ✓
75 = 3 × 25 ✓
Topic check
See what you know: 5 questions on this topic
New questions each time. Pass to mark it complete.
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