Calculations - Using probability, direct proportion and simple ratios
You can express the outcome of a genetic cross using probabilityThe extent to which something is likely to be the case. (percentages), direct proportion or ratios. It is important to remember during the process of fertilisation, the allele combinations created are a random process, and that is why probability is used, as nothing is guaranteed. Each of the four possible offspring combinations is as likely to happen during every fertilisation event.
In this Punnett square the top row shows the female alleles and the left-hand column shows the male alleles.
a
a
A
Aa
Aa
a
aa
aa
A
a
Aa
a
Aa
a
a
aa
a
aa
Direct proportion
Half the offspring have the combination Aa and half have aa.
or
Out of 4 offspring 2 have the combination Aa and 2 have aa.
Probability
The probability of the offspring being Aa is 50%.
The probability of the offspring being aa is 50%.
Ratio
The ratio of Aa to aa is 1:1.
Question
Complete the Punnett Square to show the four possible outcomes. Give your answer as a proportion, probability and ratio.
In this Punnett square the top row shows the female alleles and the left-hand column shows the male alleles.
B
b
B
B
B
B
b
B
B
b
B
b
B
BB
Bb
B
BB
Bb
B
B
BB
b
Bb
B
B
BB
b
Bb
Direct proportion
Half the offspring have the combination BB and half have Bb.
or
Out of 4 offspring 2 have the combination BB and 2 have Bb.
Probability
The probability of the offspring being BB is 50%.
The probability of the offspring being Bb is 50%.
Ratio
The ratio of BB to Bb is 1:1.
Question
Complete the Punnett Square to show the four possible outcomes. Give your answer as a proportion, probability and ratio.
In this Punnett square the top row shows the female alleles and the left-hand column shows the male alleles.
c
c
C
C
C
c
c
C
c
c
c
c
C
Cc
Cc
C
Cc
Cc
C
c
Cc
c
Cc
C
c
Cc
c
Cc
Direct proportion
All the offspring have the combination Cc.
or
Out of 4 offspring 4 have the combination Cc.
Probability
The probability of the offspring being Cc is 100%.