10th Biology Dominant and Recessive Traits Quiz CBSE

📚 Topics covered — comprehensive guide

• Genes, alleles, chromosomes, and loci (the inheritance toolkit): In Class 10 CBSE Biology, a gene is a DNA segment on a chromosome that controls a character (e.g., plant height). Different molecular versions of the same gene are called alleles and occupy the same locus (position) on homologous chromosomes. During meiosis, homologous chromosomes separate, ensuring each gamete receives only one allele of each gene—this is the basis of simple inheritance patterns you study in “Dominant and Recessive Traits.”

  • Exam anchor: Character = general feature (height), trait = specific form (tall/dwarf), gene = DNA that controls a character, allele = variant of that gene.

• Dominant vs recessive — what “masking” really means: A dominant allele expresses its effect in heterozygotes (Tt), so its trait appears in the phenotype even when a recessive allele is present. A recessive allele is visible only when present in two copies (tt). At the molecular level, many dominant alleles are “haplosufficient,” meaning one functional copy produces enough protein to display the trait, while recessive alleles often involve reduced or absent function.

  • Misconception alert: Dominant ≠ common; recessive ≠ weak. These terms describe expression, not frequency or “strength.”
  • Example: Tall (T) is dominant over dwarf (t) in peas; TT and Tt are tall, tt is dwarf.

• Genotype vs phenotype — decoding symbols into traits: Genotype refers to the allele combination (TT, Tt, tt), while phenotype is the observable trait (tall or dwarf). In complete dominance, different genotypes can yield the same phenotype (TT and Tt both tall). Translating words into symbols (T/t) and then into Punnett squares is the fastest, most accurate exam method in CBSE Class 10 Biology.

  • Quick method: Always define symbols first (T = tall, t = dwarf), then write parental genotypes before drawing the grid.

• Mendel’s laws (foundation for Class 10 genetics): The Law of Dominance explains why one allele can mask another in hybrids (T masks t in Tt). The Law of Segregation states that allele pairs separate during gamete formation, so each gamete carries only one allele. These laws produce the predictable inheritance ratios you repeatedly see in exam questions on dominant and recessive traits.

  • Outcome link: Segregation → 1:2:1 genotypic ratio and 3:1 phenotypic ratio in a monohybrid cross (Tt × Tt).

• Monohybrid crosses and classic ratios (how to get 3:1 and 1:2:1): A monohybrid cross follows one gene with two alleles. For Tt × Tt, a 2×2 Punnett square yields genotypes TT, Tt, Tt, tt, giving a 1 TT : 2 Tt : 1 tt genotypic ratio and a 3 tall : 1 dwarf phenotypic ratio. For TT × tt, all F1 are Tt and tall. These are high-frequency CBSE board patterns—practice until automatic.

  • Worked mini-example: Parents Tt × Tt → gametes T, t and T, t → offspring TT (tall), Tt (tall), Tt (tall), tt (dwarf).

• Punnett squares — visualizing probability step by step: A Punnett square cross-tabulates parental gametes to predict offspring genotypes. Label rows/columns with gametes (T or t), fill each cell by combining alleles, then count outcomes. Converting counts to fractions or percentages (e.g., 1/4 = 25%) lets you answer probability questions quickly and accurately in exams.

  • Verification trick: Phenotypic percentages must sum to 100%; if not, recheck the grid and your gamete labels.

• Test cross and backcross — revealing hidden genotypes: A dominant phenotype could be TT or Tt. Crossing the unknown with a homozygous recessive (tt) reveals the truth: TT × tt → all tall (Tt); Tt × tt → 1 tall : 1 dwarf. This simple setup is a staple question type and a must-master tool for determining whether a dominant-looking parent is pure-breeding (TT) or hybrid (Tt).

  • Exam telltale: “Cross with homozygous recessive” is the signal phrase for a test cross question.

• Carriers and recessive traits — why traits can “skip” generations: A carrier is heterozygous (Tt) for an autosomal recessive trait and shows the dominant phenotype, yet can pass the recessive allele to children. Two unaffected carriers can have an affected child (tt) with a 25% probability. This explains how recessive traits reappear even when parents do not display them—an essential reasoning step in pedigree-style questions.

• Autosomal dominant vs autosomal recessive patterns (family clues): Autosomal dominant traits often appear in every generation; an affected child usually has an affected parent. Autosomal recessive traits can appear among siblings with unaffected carrier parents and may “skip” generations. While detailed pedigrees go beyond Class 10 depth, recognizing these patterns strengthens explanations and justifications in short answers.

• Probability in genetics — quick math for quick marks: Use the product rule for independent events (e.g., chance of “t” from parent 1 and “t” from parent 2 is 1/2 × 1/2 = 1/4 = 25%). Translate verbal questions into symbolic probabilities, and always check totals. Marking schemes often award steps for correct working, so show your fractions or percentages clearly.

  • Speed tip: Memorise common outcomes: TT × tt → all Tt (100% dominant); Tt × tt → 1:1 phenotype; Tt × Tt → 3:1 phenotype.

• Linking cell structure to inheritance (nucleus → chromosomes → genes): Genes reside on chromosomes within the nucleus. During meiosis, homologous chromosomes (and thus alleles) segregate into gametes, creating the genotypic possibilities predicted by Punnett squares. This bridge connects the “Cell Structure” theme to “Dominant and Recessive Traits,” integrating CBSE Class 10 Biology concepts across chapters.

  • Concept bridge: Meiosis explains why alleles separate; fertilisation explains why alleles pair again in zygotes.

• Common exam pitfalls — avoid these easy losses: Do not mix up genotype and phenotype; always write what your symbols mean. Do not assume “dominant = more common” in the population. Do not forget that a dominant phenotype can hide a recessive allele (Tt). In test cross questions, confirm you used a homozygous recessive (tt) partner; any other choice will not reliably reveal the unknown genotype.

  • Presentation tip: End with a clear statement of both genotype ratio and phenotype ratio to collect full marks.

• Beyond complete dominance (for enriched understanding): Although your CBSE syllabus stresses complete dominance, biology also includes incomplete dominance (heterozygote shows an intermediate phenotype) and codominance (both alleles expressed). Knowing these extensions helps you contrast and correctly identify when a question explicitly states “complete dominance,” which is the default assumption in Class 10 monohybrid problems.