In a plant, gene A (alleles A and a) and gene B (alleles B and b) lie on the same chromosome…
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In a plant, gene A (alleles A and a) and gene B (alleles B and b) lie on the same chromosome. A plant that received A and B from one parent and a and b from the other is test crossed with a plant aabb. Among the offspring, show both dominant characters, show both recessive characters, show the dominant character of gene A only, and show the dominant character of gene B only. Take one map unit for each per cent of recombination.
- What is the recombination frequency between A and B?
- How far apart are A and B on the chromosome, in map units?
- In a larger test cross of the same kind with offspring, how many recombinant offspring are expected?
- A third gene, C, lies on the same chromosome between A and B, map units from A. What recombination frequency is expected between C and B?
The worked solution
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Parental and recombinant. The plant got A with B from one parent and a with b from the other. So AB and ab are the parental combinations ( offspring), and Ab and aB are the recombinants (). The aabb parent adds only a and b, so each offspring shows the combination in the gamete it got from the test-crossed plant.
(a) Recombination frequency. . The parental types are far more common than the recombinants, as Morgan saw for linked genes.
(b) Distance. One map unit for each per cent of recombination: map units.
(c) Larger cross. of is recombinant offspring.
(d) Gene C. C lies between A and B, map units from A. So it is map units from B, and the expected recombination frequency between C and B is .
Answer: (a) . (b) map units. (c) offspring. (d) .
Check. With recombination, the test-crossed plant makes Ab and aB gametes at about each and AB and ab at about each. Out of offspring that is about and of each, close to the counts given.
Watch out. The recombinants are not always the "mixed" classes. If the plant had received A with b and a with B, the classes Ab and aB would be parental, and AB and ab recombinant. Always start from the parents.
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