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In genetic crosses, the observed phenotypic ratios obtained in the offspring are often not the same as the expected ratios - AQA - A-Level Biology - Question 6 - 2018 - Paper 1
Question 6
In genetic crosses, the observed phenotypic ratios obtained in the offspring are often not the same as the expected ratios. Suggest two reasons why. What would be ... show full transcript
Worked Solution & Example Answer:In genetic crosses, the observed phenotypic ratios obtained in the offspring are often not the same as the expected ratios - AQA - A-Level Biology - Question 6 - 2018 - Paper 1
Step 1
Suggest two reasons why.
Answer
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Linked Genes: The genes responsible for the traits may be located close together on the same chromosome, causing them to be inherited together more often than expected.
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Environmental Factors: External factors could influence the expression of certain traits, leading to variations in phenotypic ratios that differ from the expected Mendelian ratios.
Step 2
Step 3
Use the information provided to explain the results in Table 3.
Answer
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The presence of fewer tall, mottled and dwarf, normal offspring suggests a case of linked genes. This indicates that the genes affecting height and leaf type are inherited together more frequently.
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The occurrence of fewer recombinant offspring further supports this, as the crossing over between homologous chromosomes does not happen as often.
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Overall, this provides insight into the inheritance patterns seen in the results, highlighting the impact of gene linkage on expected ratios.
Step 4
Complete Table 4 to show the expected ratio of phenotypes.
Answer
In this scenario, if the genes for height of plant and type of leaf are on different homologous pairs of chromosomes, the expected phenotypic ratios would be as follows:
| Phenotype of offspring | Ratio of offspring |
|---|---|
| Tall (plant) and normal (leaves) | 9 |
| Tall (plant) and mottled (leaves) | 3 |
| Dwarf (plant) and normal (leaves) | 3 |
| Dwarf (plant) and mottled (leaves) | 1 |