Punnet Squares and
Blood Types
Mrs. McCarthy
March 12, 2016
I. Punnett Square- helpful way of showing results from
any cross
Ex. If a purebred tall pea plant is crossed with a purebred short pea plant, the
Punnett Square will show the probability of the offspring from this cross
A. Probability- chance of how often something will
occur, often written in a percent
B. Ratio- numerical expression showing the
relationship between two occurrences
Ex. 1:2 or 50% or 1/2
3:4 or 75% or 3/4
Note: Like fractions, ratios should ALWAYS be written
in its lowest form.
Ex. 2:6 should be written as 1:3 or 33%
II. Review:
A. Monohybrid cross- a cross that involves
one pair of contrasting traits
B. Dihybrid cross- a cross that involves
two pairs of contrasting traits
III. How a Punnett Square Works
A. Punnett squares are written using the
alleles in the genotype
TT
T
Tt
T
T
t
1. Remember the law of segregation! The alleles separate
during meiosis. Each gamete gets only ONE allele!
IV. Monohybrid Punnett Square
Practice
Monohybrid crosses:
1. In peas, yellow seeds are dominant to green
seeds. Cross a heterozygous yellow plant with a
homozygous recessive (green) plant.
Step 1: Write out genotype and phenotypes:
Y=yellow y=green
Step 2: Write out cross: Yy ● yy
Monohybrid Punnet Square
Practice
Step 3:
Complete a
punnett
square
Y
y
y
Yy
yy
y
Yy
yy
Step 4: List your final results by genotypes and
phenotypes with the proper ratio
Genotype
Phenotype
½ Yy
½ Yellow
½ yy
½ Green
Monohybrid Punnet Square
Practice
2. Cross a heterozygous round seed plant with
another heterozygous round seed plant.
R= round
r= wrinkled
Rr ● Rr
R
r
R
RR
Rr
r
Rr
rr
Genotype
¼ RR
½ Rr
¼ rr
Phenotype
¼ Round
½ Round
¼ wrinkled
Monohybrid Punnet Square
Practice
3. In guinea pigs, black fur is dominant to white fur.
Cross a homozygous dominant parent with a
homozygous recessive parent.
B= Black
b= white
BB ● bb
B
B
b
Bb
Bb
b
Bb
Bb
Genotype
4/4 Bb
Phenotype
All Black
V. Dihybrid Punnett Square
Practice
Dihybrid Cross:
1. Cross a homozygous round, green pea with a
wrinkled, heterozygous yellow pea.
Step 1: Write out genotype and phenotype
R= round
Y= yellow
r= wrinkled y=green
Step 2: Show your cross
RRyy ● rrYy
Step 3: Show the different types of combinations in
gametes.
RRyy
Ry
rrYy
rY, ry
Dihybrid Punnet Square Practice
Step 4: Show your punnet square
rY
Ry
ry
RrYy Rryy
Step 5: Show your genotypes and phenotypes
Genotype
½ RrYy
Phenotype
½ Round, Yellow
½ Rryy
½ Round, green
Dihybrid punnet square practice
2. Cross a heterozygous round, yellow plant with a heterozygous
round, green plant.
Genotype
Phenotype
R= Round Y=Yellow
1/8 RRYy
1/8 Round,
r= wrinkled y= green
Yellow
RrYy ● Rryy
1/8 RRyy
1/8 Round,
Gametes=RY, Ry, ry, rY and Ry, ry
Yellow
RY
Ry
ry
¼ Round,
green
1/8 rryy
1/8 wrinkled,
green
1/8 rrYy
1/8 wrinkled,
yellow
¼ RrYy
¼ Round,
Yellow
rY
Ry RRYy RRyy Rryy RrYy
ry RrYy Rryy rryy
¼ Rryy
rrYy
VI. Blood Types
A. Co-dominance- two dominant alleles are expressed at the
same time
Ex. Type AB blood
B. Multiple Alleles- genes with three or more alleles
Ex. Blood Types
C. Blood Type Alleles:
A= IAIA or IAi
O= ii
B= IBIB or IBi
AB= IAIB
Cross the same as a regular monohybrid cross!
Blood Types
1.
Cross a homozygous type A man with a type
O woman.
IAIA ● ii
IA
IA
i
IAi
I Ai
i
IAi
IAi
Genotype
Phenotype
4/4 IAi
All type A
Blood Types
2. Cross a heterozygous type A man with a
heterozygous type B woman.
IAi ● IBi
Genotype Phenotype
IA
i
¼ IAIB
¼ Type AB
IB
IAIB
i
IAi
IBi
¼ IAi
¼ type A
ii
¼ IBi
¼ type B
¼ type O
¼ ii
Homework
Complete Genetics Worksheet #1