PATTERNS OF HEREDITY
CAN BE PREDICTED
• PUNNETT SQUARES SHOW POSSIBLE
OUTCOMES FOR INHERITANCE
• RATIOS AND PERCENTAGES CAN
EXPRESS THE PROBABLITY OF
OUTCOMES.
PUNNETT SQUARE
• Shows how the parent’s
alleles might combine in
offspring
• Top of square shows one
parent’s alleles for a trait
(D,D for dominant tall);
while side shows other
parent’s alleles for the
trait (d,d for recessive
short)
• Each box shows way the
alleles from each parent
would combine in
offspring. (In this case,
each child would be Dd,
one dominant and one
recessive, so they would
all be tall.
Ratios can Express Probability
• RATIO: compares the relationship between 2
quantities. 4:4 is four out of four
• PROBABILITY: the chance of a specific outcome in
relation to the total number of possible outcomes.
Percentages can Express Probability
• PERCENTAGE: is a
ratio that compares a
number to 100. (the
chance 2 hybrid
black guinea pigs
Bb, will have a
recessive brown pig
bb, is 25% or 25 out
of 100. Punnett
square shows it as
1:4.
• Punnett squares &
probability do not
guarantee the
outcome, only
predict an outcome.
Calculating Probability
• Human females have 2 X-chromosomes (X,X) &
males are (X,y). The punnett square shows only 2
possible outcomes (male-Xy or female-XX).
• Punnett squares & probability only predict the
likelihood of different outcomes, the actual
experimental results may not match predicted
outcomes.