ICSE 10 Biology Genetics Short

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    ⚡ Quick Revision: Genetics Fundamentals

    🔹 The Father of Genetics

    • ✔ Gregor Mendel: An Austrian monk who conducted experiments on Garden Peas (Pisum sativum).
    • ✔ Why Pea Plants? Short life cycle, easy to grow, clear contrasting traits, and easy self/cross-pollination.
    • ✔ Contrasting Traits: Mendel studied 7 pairs of traits (e.g., Tall vs. Dwarf, Round vs. Wrinkled seeds).
    Term Card

    Allele: Alternative forms of a gene occupying the same position on homologous chromosomes.

    🔹 Key Genetic Terminology

    Term Description
    Phenotype Physical/observable appearance (e.g., Tall).
    Genotype Genetic makeup of an organism (e.g., TT or Tt).
    Homozygous Two identical alleles (TT or tt).
    Heterozygous Two different alleles (Tt).
    ❌ Don't Confuse:

    Dominant with Recessive. Remember: A Dominant allele expresses itself even in heterozygotes (Tt), while Recessive only expresses in homozygous (tt).

    ⚡ Quick Revision: Mendel's Laws of Inheritance

    🔹 1. Law of Dominance

    • ✔ Concept: In a heterozygous pair (Tt), one allele suppresses the expression of the other.
    • ✔ Observation: All $F_1$ offspring are phenotypically like the dominant parent.

    🔹 2. Law of Segregation (Purity of Gametes)

    • ✔ Concept: The two alleles of a pair separate during gamete formation.
    • ✔ Result: Each gamete receives only one allele for a trait.

    🔹 3. Law of Independent Assortment

    • ✔ Concept: Distribution of one pair of genes is independent of the distribution of another pair.
    • ✔ Application: This law is based on Dihybrid Crosses.
    Figure: Monohybrid Cross and F2 Ratio
    Term Card

    Monohybrid Cross: A cross between two parents to study the inheritance of only one pair of contrasting characters.

    ❌ Don't Confuse:

    3:1 with 1:2:1. Remember: 3:1 is the Phenotypic ratio, while 1:2:1 is the Genotypic ratio in a monohybrid $F_2$ generation.

    ⚡ Quick Revision: Dihybrid Cross & Sex Determination

    🔹 Dihybrid Cross (Two Traits)

    • ✔ Concept: Breeding plants differing in two pairs of contrasting traits (e.g., Round Yellow vs. Wrinkled Green).
    • ✔ The Golden Ratio: The $F_2$ phenotypic ratio is always 9:3:3:1.
    • ✔ Significance: Proves that traits are inherited independently of one another.

    🔹 Sex Determination in Humans

    • ✔ Chromosomes: Humans have 23 pairs; 22 pairs of Autosomes and 1 pair of Sex Chromosomes.
    • ✔ Mechanism: Females are XX (homogametic); Males are XY (heterogametic).
    • ✔ Probability: There is always a 50% chance of a male or female child.
    Term Card

    Genome: The full complement of DNA (including all genes and the intervening regions) of an organism.

    ❌ Don't Confuse:

    The Y chromosome with the X chromosome. Remember: The Y chromosome is much smaller and determines maleness; the X chromosome is larger and carries many essential genes.

    ⚡ Quick Revision: Sex-Linked Inheritance

    🔹 X-Linked Genetic Disorders

    • ✔ Definition: Traits/diseases carried on the X chromosome (e.g., Color Blindness, Haemophilia).
    • ✔ Criss-Cross Inheritance: Passed from father to daughter, and from mother to son.
    • ✔ Gender Bias: Males are more likely to suffer because they have only one X chromosome.

    🔹 Must-Know Disorders

    Disorder Nature Key Symptom
    Haemophilia X-linked Recessive Blood fails to clot (Bleeder's Disease).
    Color Blindness X-linked Recessive Inability to distinguish Red from Green.
    Term Card

    Carrier: A heterozygous female ($X^CX$) who carries the recessive gene but does not exhibit the trait herself.

    ❌ Don't Confuse:

    Carrier females with Afflicted females. Females are only color blind or haemophilic if they are homozygous recessive ($X^cX^c$).

    ⚡ Quick Revision: Mutation & Variation

    🔹 Mutation: The Sudden Change

    • ✔ Definition: A sudden, permanent change in the chemical structure of a gene or in the number of chromosomes.
    • ✔ Types: Can be Gene Mutations (e.g., Sickle cell anemia) or Chromosomal Mutations (e.g., Down’s Syndrome).
    • ✔ Mutagens: Agents like X-rays, UV radiation, and certain chemicals that cause mutations.

    🔹 Variation: The Spice of Life

    • ✔ Inherited Variation: Differences passed from parents to offspring (e.g., eye color, earlobe type).
    • ✔ Importance: Provides raw material for Evolution and helps organisms adapt to changing environments.
    Term Card

    Pedigree Chart: A diagram showing the inheritance of a particular trait over several generations of a family.

    ❌ Don't Confuse:

    Acquired traits with Inherited traits. Remember: Acquired traits (like muscle building) do not affect DNA and are not passed to offspring.

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