Quick Revision Notes: Class 12 Biology

Chapter: Principles of Inheritance and Variation

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.

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).

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 ($1:2:1:2:4:2:1:2:1$)

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.

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.

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.

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.

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

Hemophilia (हीमोफीलिया): Sex-linked recessive disease. Blood does not clot properly.
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.
Phenylketonuria (फेनिलकीटोनूरिया): Inborn error of metabolism (autosomal recessive). Lack of enzyme phenylalanine hydroxylase. Leads to mental retardation.

Down’s Syndrome (डाउन सिंड्रोम): Trisomy of chromosome 21 ($47$ chromosomes).
- Symptoms: Mentally retarded, furrowed tongue, broad palm, stunted growth.
Klinefelter’s Syndrome (क्लाइनफेल्टर सिंड्रोम): Presence of an extra $X$ chromosome ($44 + XXY$, total $47$).
- Symptoms: Masculine development with feminine characters (gynecomastia), sterile males.
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.