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A-Level Biology October/November 2024 Q4(b): A theoretical dihybrid cross involves two genes located on different autosomes. Each ge…
A-Level Biology · Paper 9700/42 · October/November 2024 · Question 4(b) · [3 marks]
A theoretical dihybrid cross involves two genes located on different autosomes. Each gene has two alleles, one dominant and one recessive. A parent, homozygous dominant for both genes, is crossed with a parent that is homozygous recessive for both genes. This produces F1 individuals that are then crossed to produce the F2 generation. State the phenotypic ratio of this dihybrid F2 generation and explain why some of these offspring phenotypes are different from the original parental phenotypes.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
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The phenotypic ratio of the F2 generation is 9:3:3:1.
New phenotypes, which are different from the original homozygous parental phenotypes, are formed because of independent assortment. During metaphase I of meiosis in the F1 individuals, homologous chromosomes align randomly at the equator. As the two genes are on different chromosomes, the orientation of one homologous pair does not affect the orientation of the other. This results in new combinations of alleles in the gametes, leading to the recombinant phenotypes (e.g., dominant for one trait, recessive for the other) in the F2 generation upon fertilisation.
How the marks are awarded
- B1 — Correctly stating the classic dihybrid phenotypic ratio of 9:3:3:1.
- B1 — Identifying 'independent assortment' as the genetic principle responsible for the new combinations.
- B1 — Specifying that it is the 'homologous chromosomes' that assort independently. An alternative mark could be awarded for stating the stage, 'metaphase I of meiosis'.
Common mistakes
- Stating an incorrect ratio, such as the monohybrid phenotypic ratio (3:1) or the dihybrid genotypic ratio.
- Confusing independent assortment with segregation. Segregation refers to the separation of alleles for a single gene, not the inheritance pattern of different genes relative to each other.
- Explaining the variation by referencing crossing over, which is incorrect here as the genes are on different chromosomes, making independent assortment the relevant mechanism.
- Giving a vague answer like 'variation occurs during meiosis' without specifying the precise mechanism (independent assortment) and the stage (metaphase I).
Examiner tip: For questions on inheritance, always link the observed phenotypic ratios to the specific meiotic events, such as independent assortment of homologous chromosomes in metaphase I, that cause them.
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