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SCIPAD - BIG IDEAS - BOOK 2
THE BASIS OF HEREDITY
Predicting Inheritance
From the previous exercise, you saw how the probability of the sex of a baby can be predicted by
looking at the possible combinations of chromosomes of the parents. Most traits (characteristics)
are not controlled by whole chromosomes, but by sections of chromosomes called genes.
Different forms of a gene are called ‘alleles’. You inherit one allele from each parent. Therefore,
you have two alleles for each gene. In rats the colour of their fur is controlled by a gene. Two
forms of this gene occur. Black fur colour is controlled by the allele (B) and white fur is controlled
by the allele (b). If a rat’s alleles are BB or Bb it will be black, because the allele ‘B’ is dominant
over the recessive allele (b) for white fur. Dominant alleles are always shown with capital letters.
The Punnett square below shows the possible outcomes of a cross between a black male rat and
a white female rat.
Black male
Alleles = BB
White female
Alleles = bb
1. Define the term allele.
2. Using the rat example above, describe the appearance of a rat with the alleles:
(a) BB (b) Bb (c) bb
3. From the Punnett square above, state the probability the two rats would have:
(a) A black offspring? (b) A white offspring?
4. (a) Using the space provided below, draw a Punnett square to show the possible
outcomes when two white rats are mated.
Alleles of parent 1
Alleles of parent 2
(b) State the probability the two rats would have:
(i) A black offspring? (ii) A white offspring?
BB
B
b b
B
bb
Bb
Bb Bb
Bb
A gene is a section of DNA that
controls a particular feature or trait.
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