Q1. [3] § 8.2.1 Inherited Traits
The lowest part of the ear called earlobe, is closely attached to the side of the head in some of us (Figure 'X'), and not in others, called free earlobe (Figure 'Y'). Attached and free earlobes are two variants found in human populations. The gene for free earlobe is dominant over attached earlobes.
- (a) A man with attached earlobes marries a woman having free earlobes. 50% of their children have free earlobes and 50% have attached earlobes. Explain the inheritance of this trait and write the trait combinations of the progeny. [2]
- (b) Write the gene combinations of the father and the mother in the above case. [1]
Previously asked in CBSE board exam
2025 31/2/1 Q30
Generated by claude-sonnet-4-6 · 2026-06-15 06:52 · grounding rag
Model Answer
(a) Since free earlobe (F) is dominant over attached earlobe (f), the mother with free earlobes must be heterozygous (Ff) and the father with attached earlobes is homozygous recessive (ff).
Cross: Ff × ff
| | F | f |
|---|---|---|
| f | Ff | ff |
Progeny: 50% Ff (free earlobe) and 50% ff (attached earlobe) — this explains the 1:1 ratio observed.
(b) Father's gene combination: ff (attached earlobes)
Mother's gene combination: Ff (free earlobes)
Source: Chapter 8, Section 8.2.1 – Inherited Traits
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Explanation
- The 50:50 ratio is a classic test cross result (Ff × ff), which tells the examiner the mother must be heterozygous, not homozygous dominant (FF × ff would give 100% free earlobes).
- Always show a Punnett square for 2-mark genetics questions — it earns method marks.
- For part (b), write gene combinations clearly using standard notation (Ff, ff); do not just write "dominant/recessive."
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