Punnett Squares
pp. 180-183
• Probability: likelihood an event will happen
and can be expressed as percentages, fractions,
or decimals
• Monohybrid cross: cross between individuals
that looks at one trait
• Punnett square: diagram to predict the
probability of inheritance of certain traits
• Place the parent genotypes in:
Aa x Aa
• Match letters and fill in squares
Parent I
Parent II
A
A
a
AA
Aa
Aa
aa
a
MENDEL’S RATIOS
Genotype ratio1BB: 2Bb: 1bb
1:2:1
Phenotype ratio3 Black: 1 Brown
3:1
Genotype ratio
2:2 or 1: 1
Phenotype ratio
All black
B
Parent I
Parent I
b
b
Bb
b
Bb
B
Bb
Bb
Parent II
Parent II
TEST CROSS
• Cross the dominant phenotype with a homozygous
recessive (bb)
• If you have recessive offspring then the individual
must be heterozygous
B
b
Bb
Bb
bb
bb
b