Quick Revision Notes: Class 12 Biology

Chapter: Principles of Inheritance and Variation

MP Board / NCERT

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### 1. Important Definitions (महत्वपूर्ण परिभाषाएँ)

• Genetics (आनुवंशिकी): The branch of biology that deals with the study of heredity and variations.
• Heredity (पैतृकता / वंशागति): Transmission of characters from parents to offspring.
• Variation (विभिन्नता): The degree by which progeny differ from their parents.
• Allele (युग्मविकल्पी): Alternative forms of a gene that code for a pair of contrasting traits.
• Phenotype (समलक्ष्यी): External, observable appearance of an organism.
• Genotype (जीनप्ररूप): Genetic constitution or genetic makeup of an organism.
• Homozygous (समयुग्मजी): Having identical alleles for a trait (e.g., TT or tt).
• Heterozygous (विषमयुग्मजी): Having dissimilar alleles for a trait (e.g., Tt).
• Test Cross (परीक्षण संकरण): Crossing an organism of dominant phenotype with a recessive parent to determine its genotype.

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### 2. Mendel’s Laws of Inheritance (मेंडल के वंशागति के नियम)

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

• 1. Law of Dominance (प्रभाविता का नियम):

  • Characters are controlled by discrete units called factors (genes).
  • Factors occur in pairs.
  • In a dissimilar pair of factors, one member dominates (dominant) and the other is masked (recessive).

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

  • Alleles do not show any blending and that both the characters are recovered as such in the $F_2$ generation though one of these is not seen at the $F_1$ stage.
  • Gametes are pure for a trait.

• 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 (applicable to dihybrid cross).

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### 3. Cross Ratios at a Glance (संकरण अनुपात)

Type of Cross	Phenotypic Ratio (लक्षणप्ररुपी अनुपात)	Genotypic Ratio (जीनप्ररुपी अनुपात)
Monohybrid Cross (एकसंकर संकरण)	$3 : 1$	$1 : 2 : 1$
Dihybrid Cross (द्विसंकर संकरण)	$9 : 3 : 3 : 1$	Complex ($1:2:1:2:4:2:1:2:1$)

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### 4. Post-Mendelian Discoveries / Deviations (मेंडलवाद के अपवाद)

• Incomplete Dominance (अपूर्ण प्रभाविता):

  • $F_1$ generation does not resemble either of the parents and shows a phenotype intermediate between the two.
  • Example: Flower color in Mirabilis jalapa (Four o'clock plant) or Snapdragon (Antirrhinum majus).
  • Phenotypic and Genotypic Ratio in $F_2$: $1 : 2 : 1$.

• Co-dominance (सह-प्रभाविता):

  • Both alleles are expressed equally in the $F_1$ generation.
  • Example: ABO blood grouping in humans, coat color in cattle.

• Multiple Alleles (बहुविकल्पी):

  • Existence of more than two alleles for the same gene.
  • Example: ABO blood groups in humans (controlled by gene $I$ with alleles $I^A, I^B,$ and $i$).

• Pleiotropy (बहुप्रभाविता):

  • A single gene exhibits multiple phenotypic expressions.
  • Example: Phenylketonuria (PKU) in humans, starch grain size and seed shape in pea plants.

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### 5. Chromosomal Theory of Inheritance (वंशागति का गुणसूत्र सिद्धांत)

• Proposed by Walter Sutton and Theodor Boveri (1902).
• Parallels between behavior of chromosomes and genes:
  • Occur in pairs.
  • Segregate during gamete formation such that only one of each pair is transmitted to a gamete.
  • Independent pairs segregate independently of each other.

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### 6. Linkage and Recombination (सहलगनता और पुनرसंयोजन)

• Linkage (सहलगनता): Physical association of two or more genes on a chromosome. Discovered by T.H. Morgan in Drosophila.
• Recombination (पुनर्सयोजन): Generation of non-parental gene combinations due to crossing over during meiosis.
• Note: Strength of linkage is inversely proportional to the distance between genes.

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### 7. Sex Determination (लिंग निर्धारण)

• XX - XY Type: Male is heterogametic ($XY$) and female is homogametic ($XX$). Example: Humans, Drosophila.
• XX - XO Type: Females have two $X$ chromosomes, males have only one $X$ and no $Y$. Example: Grasshopper.
• ZZ - ZW Type: Female is heterogametic ($ZW$) and male is homogametic ($ZZ$). Example: Birds, Butterflies.

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### 8. Genetic Disorders (आनुवंशिक विकार)

A. Pedigree Analysis (वंशावली विश्लेषण)

• Analysis of family history over generations to trace the inheritance of a particular trait. Useful for genetic counseling.

B. Mendelian Disorders (मेंडेलियन विकार) - Single gene mutation

1. Hemophilia (हीमोफीलिया): Sex-linked recessive disease. Blood does not clot properly.
2. Sickle-cell Anemia (सिक्ल-सेल एनीमिया): Autosomal recessive trait. Glutamic acid ($Glu$) is replaced by Valine ($Val$) at the 6th position of the $\beta$-globin chain of hemoglobin. RBCs become sickle-shaped under low oxygen.
3. Phenylketonuria (फेनिलकीटोनूरिया): Inborn error of metabolism (autosomal recessive). Lack of enzyme phenylalanine hydroxylase. Leads to mental retardation.

C. Chromosomal Disorders (गुणसूत्रीय विकार) - Absence, excess, or abnormal arrangement of one or more chromosomes

1. Down’s Syndrome (डाउन सिंड्रोम): Trisomy of chromosome 21 ($47$ chromosomes).
   • Symptoms: Mentally retarded, furrowed tongue, broad palm, stunted growth.
2. Klinefelter’s Syndrome (क्लाइनफेल्टर सिंड्रोम): Presence of an extra $X$ chromosome ($44 + XXY$, total $47$).
   • Symptoms: Masculine development with feminine characters (gynecomastia), sterile males.
3. Turner’s Syndrome (टर्नर सिंड्रोम): Absence of one $X$ chromosome ($44 + XO$, total $45$).
   • Symptoms: Sterile females, rudimentary ovaries, short stature, lack of secondary sexual characters.