Dominant & Recessive Traits

• Dominant trait — expressed in heterozygous state (Tt shows tall).
• Recessive trait — expressed only when homozygous recessive (tt dwarf).
• Sex determination — humans XY male, XX female; father determines sex.
• Independent assortment (intro) — different traits inherit independently (dihybrid preview).
• Family pedigrees — track trait through generations.

1. Dominant allele — capital letter (T); recessive lowercase (t).
2. Carrier — heterozygous for recessive disease allele (e.g. Tt for hidden trait).
3. Human examples — free earlobe dominant; attached recessive (NCERT examples).
4. Sex chromosomes — 44 + XX female; 44 + XY male.
5. 50% probability — X or Y sperm → equal chance boy or girl.
6. Mendel's Law of Dominance — one allele dominates in heterozygote.
7. Law of Segregation — alleles separate into gametes.
8. Variation — sexual reproduction, crossing over, random fertilisation.
9. Not all traits single-gene — many polygenic (intro only).
10. NCERT — distinguish inherited vs acquired characteristics (Lamarck vs Mendel context).

Why two tall parents can have dwarf child (if both heterozygous)

Parents: both Tt (tall phenotype, carry recessive t).
Gametes: each produces T or t.
Child tt possible: ¼ probability from Tt × Tt cross.
Dwarf only when **both** t alleles inherited.
Conclusion: recessive trait reappears in F₂ when carriers mate.

• Thinking dominant means better (means only expressed in heterozygote).
• Misconception: mother determines baby's sex (father's X or Y sperm).
• Confusing acquired traits inherited (cut tail in mice — not inherited).
• Forgetting recessive needs homozygous expression.
• Using wrong letter conventions inconsistently in crosses.

• When is recessive phenotype visible?
• How is human sex determined?
• What is a carrier of recessive disease?
• Give Mendel's Law of Dominance in one sentence.

Open the Practice tab for graded questions on Dominant & Recessive Traits.