A horticulture student crossed two pea plants. One plant had black seed coats and genotype Bb. The other plant had white seed coats and genotype bb. Black i...
Question 1 Report
A horticulture student crossed two pea plants. One plant had black seed coats and genotype Bb. The other plant had white seed coats and genotype bb. Black is caused by the dominant allele B. Fig. 1 is a partially completed genetic cross used by the student before planting the offspring. The student will count the colours of seeds produced by many plants, because a small number of offspring may not show the expected proportion.
(a) Complete the four boxes in Fig. 1 with the possible genotypes of the offspring. [2]
(b) Use your completed cross to state the probability that one offspring plant has white seed coats. [1]
(c) Describe the expected ratio of black-seeded plants to white-seeded plants in a large sample of offspring. [1]
(d) Explain why a seed with genotype Bb has black seed coats. [2]
Answer Details
(a) The genetic cross is:
| Gamete B | Gamete b | |
|---|---|---|
| Gamete b | Bb | bb |
| Gamete b | Bb | bb |
The two boxes where \(B\) meets \(b\) contain \(Bb\). [1 mark] The two boxes where \(b\) meets \(b\) contain \(bb\). [1 mark]
(b) Two of the four equally likely genotype combinations are \(bb\), which gives white seed coats. Therefore the probability is \(\frac{2}{4}=\frac{1}{2}\), or 50%. [1 mark]
(c) In a large sample, the expected ratio is 1 black : 1 white. [1 mark]
(d) \(B\) is the dominant allele. [1 mark] One copy of \(B\) is sufficient to produce black seed coats, so in \(Bb\) it masks the effect of the recessive \(b\) allele. [1 mark]
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