Scientific calculations

Using probability, direct proportion and simple ratios

You can express the outcome of a genetic cross using probability (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.

aa
AAaAa
aaaaa

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.

Bb
B
B
Bb
BBBBb
BBBBb

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.

cc
C
C
cc
CCcCc
CCcCc

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%.

Ratio

The ratio of Cc to CC or cc is 1:0.

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