Read the following scenario and answer the questions:
Anjali is studying inheritance of seed colour in peas, where yellow (Y) is dominant over green (y). She crosses a yellow-seeded plant of unknown genotype with a green-seeded plant (yy). She obtains 48 yellow-seeded and 52 green-seeded plants.
(i) What is the likely genotype of the yellow-seeded parent used by Anjali? Explain your reasoning. (1 mark)
(ii) Write the cross in proper notation and show the gametes and offspring genotypes. (1 mark)
(iii) If Anjali had instead used a pure-breeding yellow-seeded plant (YY) in the cross, what would the proportion of yellow-seeded offspring have been? (1 mark)
(iv) What is the name given to a cross between an individual of unknown genotype and a homozygous recessive individual? Why is it useful? (1 mark)
Generated by claude-sonnet-4-6 · 2026-06-26 01:04 · grounding rag
Model Answer
(i) The yellow-seeded parent is likely heterozygous (Yy). The offspring ratio is approximately 48 yellow : 52 green ≈ 1:1, which indicates one parent contributed dominant (Y) and recessive (y) alleles, so the yellow parent must be Yy, not YY.
(ii)
Cross: Yy × yy
Gametes: (Y, y) × (y, y)
Offspring genotypes: Yy (yellow) and yy (green) in equal proportion (1:1).
(iii) If the yellow parent were YY, all gametes would be Y. Cross: YY × yy → all offspring Yy (yellow). Proportion of yellow-seeded offspring = 100%.
(iv) This cross is called a Test Cross. It is useful because the homozygous recessive parent (yy) reveals the unknown genotype — a 1:1 ratio indicates heterozygous (Yy), while all dominant offspring indicate homozygous dominant (YY).
Source: Chapter 8 – Heredity, Section 8.2.2
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Explanation
- Key concept tested: Mendelian inheritance, dominant/recessive traits, test cross.
- The 1:1 ratio in offspring is the classic indicator of a test cross with a heterozygous parent.
- For (ii), always write gametes explicitly — examiners award marks for showing them.
- For (iii), remember YY × yy → all Yy, so 100% yellow; students often write ½ by mistake.
- Test cross is the exact NCERT term — use it, don't paraphrase it as "back cross" (which is slightly different).