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Heredity and Evolution

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Class 10 Science - Heredity and Evolution

MP Board

### 1. Important Definitions (महत्वपूर्ण परिभाषाएँ)

  • Heredity (आनुवंशिकता): The transmission of characters/traits from parents to their offspring (progeny).
  • Variation (विविधता): The differences in characters among the individuals of the same species.
  • Gene (जीन): The unit of inheritance; a section of DNA that provides information for one specific protein.
  • Allele (एलील): Alternative forms of a gene that code for a pair of contrasting traits.
  • Dominant Trait (प्रभावी लक्षण): The trait that expresses itself in the presence of a contrasting allele (represented by capital letters, e.g., T).
  • Recessive Trait (अप्रभावी लक्षण): The trait that remains hidden in the presence of a dominant allele and expresses only in homozygous condition (represented by small letters, e.g., t).
  • Genotype (जीनप्रारूप): The genetic constitution/makeup of an organism (e.g., TT, Tt, tt).
  • Phenotype (दृश्यप्रारूप): The external, observable physical appearance of an organism (e.g., Tall or Dwarf).
  • Speciation (जातिउद्भव): The process of formation of new species from existing ones due to various factors like genetic drift, natural selection, etc.
  • Homologous Organs (समजात अंग): Organs that have the same basic structural design and origin, but perform different functions (e.g., forelimbs of humans and wings of birds). Indicates common ancestry.
  • Analogous Organs (समरूप अंग): Organs that have different basic structures and origins, but perform similar functions (e.g., wings of bats and wings of birds). Indicates convergent evolution.
  • Fossils (श्वाश्म / जीवाश्म): Preserved traces or remains of ancient living organisms found in rocks.

### 2. Mendel's Laws of Inheritance

Gregor Johann Mendel is known as the Father of Genetics. He used garden pea (Pisum sativum) for his experiments.

1. Law of Dominance (प्रभाविता का नियम)

  • In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation.
  • The hybrid offspring will exhibit the dominant trait.

2. Law of Segregation / Law of Purity of Gametes (पृथक्करण का नियम)

  • During gamete formation, the alleles for each gene segregate from each other so that each gamete carries only one allele for each gene.

3. Law of Independent Assortment (स्वतंत्र अपव्यूहन का नियम)

  • When two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters (seen in Dihybrid Cross).

### 3. Cross Ratios (संकरण अनुपात)

Monohybrid Cross (एकसंकर संकरण)

  • Parents: Pure Tall (TT) $\times$ Pure Dwarf (tt)
  • $F_1$ Generation: All Tall (Tt)
  • $F_2$ Generation Phenotypic Ratio: 3 : 1 (3 Tall : 1 Dwarf)
  • $F_2$ Generation Genotypic Ratio: 1 : 2 : 1 (TT : Tt : tt)

Dihybrid Cross (द्विसंकर संकरण)

  • $F_2$ Generation Phenotypic Ratio: 9 : 3 : 3 : 1 (9 Round-Yellow : 3 Round-Green : 3 Wrinkled-Yellow : 1 Wrinkled-Green)

### 4. Sex Determination in Humans (मानवों में लिंग निर्धारण)

  • Total chromosomes in humans = 23 pairs (46 chromosomes).
  • Autosomes = 22 pairs (same in males and females).
  • Sex Chromosomes = 1 pair.
    • Females have two X chromosomes (XX).
    • Males have one X and one Y chromosome (XY).
  • Conclusion: The sex of a child is determined by the father (whether he contributes an X or a Y chromosome).

### 5. Mechanisms of Evolution (विकास के सिद्धांत)

  • Acquired Traits (उपार्जित लक्षण): Traits developed during the lifetime of an organism; they cannot be passed on to the next generation (e.g., loss of a tail in mice through surgery).
  • Inherited Traits (आनुवंशिक लक्षण): Traits controlled by genes; they are passed on from parents to offspring (e.g., eye color, height).
  • Darwin's Theory of Natural Selection (डार्विन का प्राकृतिक चयन सिद्धांत): Organisms with traits best suited to their environment are more likely to survive and reproduce ("Survival of the Fittest").
  • Genetic Drift (आनुवंशिक विचलन): Random change in allele frequencies in a population from generation to generation due to chance events.

### 6. Important Board Exam Tips (परीक्षा के लिए विशेष टिप्स)

  1. Always practice drawing the Monohybrid cross and Dihybrid cross checkerboard (Punnett square).
  2. Clearly distinguish between Homologous and Analogous organs with examples, as this is a very frequent question in MP Board exams.
  3. Remember Mendel's scientific name for garden pea: Pisum sativum.