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166 SCIPAD - BIG IDEAS - BOOK 2
THE BASIS OF HEREDITY
(a) What is the probability of these two rats having a black offspring?
(b) What is the probability of these two rats having a white offspring?
(c) If these rats had 16 offspring, how many would you expect to be
(i) Black? (ii) White?
2. (a) A purebred white (bb) rat is mated with a non-purebred black rat (Bb).
Expected percentage of black offspring:
Expected percentage of white offspring:
Expected percentage of black offspring:
Expected percentage of white offspring:
(b) A purebred black rat is crossed with a non-purebred black rat. They produced 20
offspring.
Pure Breeding
Organisms that have the same alleles (e.g. BB or bb) are said to be purebred. Organisms with
different alleles (e.g. Bb) are not purebred.
1. A rat breeder mates two black rats, but these animals are not purebred. Complete the
Punnett square, below, to show the possible alleles of their offspring.
Black male
Alleles = Bb
Black female
Alleles = Bb
B
B
b
b
Bb
Bb
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