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164 SCIPAD - BIG IDEAS - BOOK 2
THE BASIS OF HEREDITY
Using Punnett Squares
Scientists use a grid-like tool called a Punnett square to make predictions about various inheritance
problems.
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 actually flip the coin 100 times, you
may actually get 60 heads and 40 tails. Prediction is one thing, and actually getting the predicted
results is another.
The Punnett square only shows the chances of what might occur each time the event is undertaken.
1. Complete the Punnett square above to show the other three possible outcomes.
2. A species of fish called Madeupae doesnotexistii has two chromosomes controlling its sex.
Female fish have the chromosomes FF and males are Fj.
(a) Complete the Punnett square below to show the possible sex of their offspring.
(b) The two fish produce 300 offspring.
		 (i) How many of them would you expect to be male? 
		 (ii) How many of them would you expect to be female? 
XX
XY
X X
X
Y XY
The chromosomes from
the parents are split and
put into their own box.
One chromosome from
each parent is then
combined to give one
possible outcome.
Male
Female
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