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32 CB 1.3 GENETIC VARIATION
TRACKING GENES
Predicting Inheritance
Scientists use a grid-like tool called a ‘Punnett square’ to track genes and make predictions about
how offspring may inherit alleles.
The Punnett square shows only the probability of what might occur and not the actual results.
Probability is the chance of something occurring. If you flip a coin 100 times, since there are 2
sides to the coin, you would expect 50 heads and 50 tails. If you flip the coin 100 times, you may
actually get 60 heads and 40 tails. Prediction is one thing - actually getting the predicted result is
another. The Punnett square only shows the chances of what might occur each time the event is
undertaken.
bb
BB
b b
B
B Bb
The alleles from the parents are
split (simulating meiosis) and put
into their own box.
Alleles from each parent are
then combined (simulating
fertilisation) to give one
possible outcome.
1. (a) Complete the Punnett square above to show the other three possible outcomes.
(b) What is the probability (%) that these two rats will produce a white offspring?
2. (a) Complete the Punnett square below to show the possible results of a cross between
two white rats.
1
2
White male
Alleles =
White female
Alleles =
(b) What is the probability (%) that these two rats will produce a black offspring?
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