CAIE IGCSE Biology 17.4.13 Using a Test Cross
Practise using a homozygous recessive partner and offspring phenotypes to identify an unknown dominant genotype.
- Syllabus
- 2026–2028
- Course
- Biology 0610
Exam points
- cross the unknown dominant-phenotype individual with a homozygous recessive partner
- infer heterozygosity if any recessive-phenotype offspring are produced
- infer likely homozygous dominance from many all-dominant offspring while noting sample size
17.4.13—To use a test cross to identify question 1
The garden pea, Pisum sativum, is a plant which has flowers that have both male and female parts. P. sativum is naturally self-pollinating.
Question (a)
(a)
Gregor Mendel studied inheritance in the garden pea, P. sativum.
The flowers of P. sativum that he studied were either purple or white. The gene that controls flower colour has two alleles, B and b.
When Mendel crossed purple-flowered plants with white-flowered plants all the plants in the next generation had purple flowers.
[ 5 ]
Question (i)
(i)
Test crosses can be used to determine the genotype of a plant with purple flowers.
The genetic diagrams show test crosses for purple-flowered plants with two different genotypes.
Complete the genetic diagrams for test cross 1 and test cross 2.
offspring
genotypes
offspring
phenotypes
test cross 2
purple flowers × white flowers
BB ×
[ 5 ]
Question (b)
(b)
Pickerel weed, Pontederia cordata, is a plant that grows in shallow water on the edges of ponds and lakes in North America.
A few seedlings of these plants are white. The white seedlings cannot make chlorophyll.
Researchers carried out several crosses using pickerel weed plants.
Their results are shown in Table 4.2.
Table 4.2
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Question (i)
(i)
It is not possible to carry out a test cross with pickerel weed plants.
Suggest why.
[ 2 ]
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