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CDS Genetics & Heredity

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This page covers CDS Genetics & Heredity with complete concept notes, 4 graded practice MCQs, key points and exam-specific tips. Free to study.

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Concept Notes

Genetics & Heredity— Rules & Concept

1.Core Concept

Read this first — the foundation of the topic

Genetics is the study of how traits (characteristics) pass from parents to children. Heredity means the passing of traits from one generation to the next through genes. A gene is a small unit of DNA that controls one trait. Every person has two copies of each gene—one from mother, one from father. KEY RULES:

1. Dominant traits always show up if present. Recessive traits only show if both copies are recessive. 2. Each parent gives one gene copy to their child randomly.

3. Traits follow predictable patterns that can be calculated.

2.Formula Block

Memorise — at least one formula appears in every paper
Parent 1 genes × Parent 2 genes = Possible combinations in offspring
Ratio = Dominant : Recessive (usually 3:1 or 1:1)

For a cross between two parents:

3.Exam Patterns

What examiners ask — read before attempting PYQs
RRB asks

(1) What is dominant/recessive trait? (2) Predict offspring traits from parent traits. (3) What is the ratio in Punnett square? (4) Which parent contributes which gene? (5) Name of scientist/law

SHORTCUT

Capital letter (A) = Dominant gene. Small letter (a) = Recessive gene. If you see AA or Aa, the dominant trait shows. Only aa shows recessive trait.

4.Worked Example

Solve this step-by-step before moving on
1
Step 1

Write genes. Parent 1 = TT (tall, dominant). Parent 2 = tt (short, recessive).

2
Step 2

Each parent gives one gene. Parent 1 always gives T. Parent 2 always gives t.

3
Step 3

All offspring get Tt (one T, one t).

4
Step 4

Since T is dominant, all offspring are TALL even though they carry the short gene. Answer: 100% tall plants (all Tt).

5.Exam Traps

Common mistakes students make — avoid these

Students think Aa means half dominant, half recessive. Aa always shows the dominant trait (A) because only one dominant gene is needed. Recessive only shows when both are recessive (aa).

Key Points to Remember

  • Gene = unit of DNA that controls one trait; humans inherit two copies of each gene (one from each parent).
  • Dominant trait shows if even one dominant gene is present (AA or Aa); recessive trait shows only with aa.
  • Punnett square is used to predict offspring traits by crossing parental genes in a 4-box grid.
  • Mendel's Law of Segregation: genes separate during reproduction, so each parent gives only one copy to offspring.
  • 3:1 ratio appears in monohybrid cross (Aa × Aa) = 3 dominant : 1 recessive phenotype.
  • Capital letter = dominant allele; lowercase letter = recessive allele; always use this notation in genetics problems.

Exam-Specific Tips

  • Gregor Mendel discovered the laws of inheritance and is called the Father of Genetics.
  • A dominant allele is represented by a capital letter (e.g., A); recessive allele by lowercase (e.g., a).
  • Monohybrid cross involves inheritance of one trait; dihybrid cross involves two traits.
  • In a Punnett square cross Aa × Aa, the genotypic ratio is 1 AA : 2 Aa : 1 aa.
  • Phenotype = observable trait (e.g., tall or short); Genotype = genetic makeup (e.g., TT, Tt, or tt).
  • All offspring of TT × tt cross have genotype Tt and show 100% dominant phenotype.
  • Test cross = crossing an organism with dominant trait with a homozygous recessive (aa) to reveal hidden genes.
Practice MCQs

Genetics & Heredity — Practice Questions

4graded MCQs · easy to hard · full solution & trap analysis

All MCQs →
Practice 1easy

In a monohybrid cross between a homozygous dominant plant (TT) and a homozygous recessive plant (tt), what will be the genotypic ratio of the F₁ generation?

Practice 2medium

In a monohybrid cross between a tall pea plant (TT) and a dwarf pea plant (tt), what will be the phenotype ratio in the F2 generation?

Practice 3medium

In a monohybrid cross between a homozygous dominant plant (TT) and a homozygous recessive plant (tt), what is the phenotypic ratio of the F₁ generation?

Practice 4hard

In a dihybrid cross between two heterozygous pea plants (AaBb × AaBb), where A (tall) is dominant over a (dwarf) and B (yellow seeds) is dominant over b (green seeds), what is the probability of obtaining an offspring that is homozygous recessive for both traits (aabb)?

60-Second Revision — Genetics & Heredity

  • Remember: Dominant gene (capital letter) hides recessive gene (lowercase). Aa always shows dominant trait.
  • Formula for Punnett square: Draw 4 boxes, write one parent's genes on top, other parent's on left side, combine inside each box.
  • 3:1 ratio rule: Aa × Aa ALWAYS gives 3 dominant : 1 recessive in offspring (in phenotype, not genotype).
  • Trap: Don't confuse phenotype (what you see) with genotype (what genes it has). Aa looks tall but carries short gene.
  • Test cross tip: If offspring show both traits in 1:1 ratio, parent with dominant trait was heterozygous (Aa), not homozygous (AA).
  • Key law: Mendel's Law of Segregation—genes separate during reproduction so each parent gives only ONE copy to child.
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