Mutations can cause a permanent change in the DNA of an organism. In the 19th century Gregor Mendel determined rules to explain genetic inheritance using pea plants.

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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.

a | a | |
---|---|---|

A | Aa | Aa |

a | aa | 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 BB Bb B BB 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 Cc Cc C Cc 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%.

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