Biology · Class 9-10 · CBSE

Heredity: Mendel’s Laws, Punnett Squares and Sex Determination

Master heredity: inherited vs acquired traits, Mendel’s monohybrid cross (3:1 phenotype, 1:2:1 genotype), dihybrid cross (9:3:3:1 ratio), Law of Segregation and Independent Assortment, and the XX/XY chromosomal mechanism of sex determination in humans.

1Heredity and Mendel’s Pea Plant Experiments

Heredityis the transmission of genetically determined characteristics from parents to offspring. Gregor Johann Mendel (Father of Genetics) performed hybridization experiments on the Garden Pea (Pisum sativum) because it possessed distinct contrasting pairs of traits, was self-pollinating, and had a short life cycle.

Inherited Traits vs Acquired Traits

  • Inherited Traits

    Source of Change
    Genetically coded in parental DNA
    DNA Alteration in Germ Cells?
    Yes, involves germ cell DNA sequence
    Transmission to Progeny
    Transmitted to subsequent generations (e.g. eye colour, blood group, earlobe shape)

  • Acquired Traits

    Source of Change
    Developed during lifetime due to environment/lifestyle
    DNA Alteration in Germ Cells?
    No, confined to non-reproductive somatic cells
    Transmission to Progeny
    Never passed down to progeny (e.g. muscular body, scar, language, amputated tail in mice)

2Monohybrid Cross and Mendel’s First Two Laws

Mendel’s Monohybrid Cross (Plant Height: TT × tt)

  1. 1

    1. Parental Generation (P): Pure tall plant (TT) crossed with pure dwarf plant (tt).

  2. 2

    2. First Filial Generation (F1): All plants are Tall with heterozygous genotype (Tt). Demonstrates Law of Dominance (Tall allele T masks dwarf allele t).

  3. 3

    3. Self-pollination of F1 (Tt × Tt): Alleles segregate independently into gametes (50% T, 50% t).

  4. 4

    4. Second Filial Generation (F2): Yields 1 TT (pure tall), 2 Tt (hybrid tall), and 1 tt (pure dwarf).

  5. 5

    5. F2 Phenotypic Ratio = 3 Tall : 1 Dwarf; F2 Genotypic Ratio = 1 TT : 2 Tt : 1 tt (1:2:1).

Summary of Mendel’s Core Laws of Inheritance

  • Law of Dominance

    Genetic Principle
    In a heterozygous pair of contrasting alleles, only one expresses itself (dominant), while the other remains unexpressed (recessive)
    Empirical Demonstration
    All F1 plants of TT × tt cross are Tall

  • Law of Segregation (Purity of Gametes)

    Genetic Principle
    During gamete formation, the two alleles of a gene separate so each gamete carries only one allele with pure identity
    Empirical Demonstration
    Dwarf trait reappears in F2 generation (1 tt)

  • Law of Independent Assortment

    Genetic Principle
    When two pairs of traits are combined in a hybrid, segregation of one pair of characters is completely independent of the other pair
    Empirical Demonstration
    Dihybrid cross yields new recombinant phenotypes (9:3:3:1 ratio)

3Chromosomal Mechanism of Sex Determination in Humans

Human cells contain23 pairs of chromosomes(46 chromosomes): 22 pairs are matchingAutosomes, and the 23rd pair constitutes theSex Chromosomes(Allosomes) that determine biological sex.

Sex Determination Cross in Humans

  1. 1

    1. Human female genotype: 44 autosomes + XX sex chromosomes. All female ova contain 22 + X chromosomes (homogametic).

  2. 2

    2. Human male genotype: 44 autosomes + XY sex chromosomes. Males produce two equal types of sperm: 50% carry (22 + X) and 50% carry (22 + Y) (heterogametic).

  3. 3

    3. If an X-carrying sperm fertilizes the ovum: Zygote is XX → develops into a Female baby (girl).

  4. 4

    4. If a Y-carrying sperm fertilizes the ovum: Zygote is XY → develops into a Male baby (boy).

  5. 5

    5. Conclusion: Biological sex is determined genetically solely by the father’s fertilizing sperm. The probability of having a boy or a girl is always exactly 50% (1:1 ratio).

4Exam Corner: Tips, Mistakes & Board Questions

Critical Board Revision Takeaways

  • Genotype is the genetic makeup of an organism (e.g. TT, Tt, tt); Phenotype is the observable physical appearance (e.g. Tall, Dwarf).
  • Dihybrid cross between Round Yellow (RRYY) and Wrinkled Green (rryy) produces an F2 phenotypic ratio of 9 Round-Yellow : 3 Round-Green : 3 Wrinkled-Yellow : 1 Wrinkled-Green.
  • Mothers are never responsible for the sex of a child, because female ova always provide an X chromosome. It is the male sperm (providing either X or Y) that determines the child’s sex.

Exam question · CBSE Class 10 Board

What is the probability of a human couple having a female child?
  1. A.25%
  2. B.50%
  3. C.75%
  4. D.100%

Remember

  • Mendeldiscovered genetic inheritance using garden pea (*Pisum sativum*).
  • Monohybrid cross gives phenotypic ratio3:1and genotypic ratio1:2:1.
  • Dihybrid cross yields the independent assortment ratio9:3:3:1.
  • Sex in humans is determined by the father:XX is femaleandXY is malewith a 50% chance.

Check yourself

  1. Question 1

    A cross between a tall pea plant (TT) and a short pea plant (tt) resulted in progeny that were all tall because:

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