Science and Technology Class 9 Notes Ch 16 Maharashtra Board

Heredity and Variation

1. Introduction to Heredity

Heredity: The process of passing traits (like height, eye color) from parents to their children.

Variation: Differences in traits among individuals of the same species (e.g., some classmates are tall, others short).

Genetics: The branch of biology that studies how traits and genes are transferred across generations.

Reproduction:

Asexual Reproduction: Offspring are almost identical to the parent, showing minor variations.
Sexual Reproduction: Offspring show more variations due to mixing of genes from two parents.

2. Genes and Traits

Genes: Small segments of DNA that carry instructions for specific traits (e.g., hair color).

Found on chromosomes in the cell nucleus.
Control protein synthesis, which affects traits.

Example: Plant height depends on growth hormones. Efficient enzymes (coded by genes) produce more hormones, making plants taller. Less efficient enzymes result in shorter plants.

Inherited Traits: Characteristics passed from parents, like skin color or ear shape.

Phenotype: The visible trait (e.g., tall or short plant).

Genotype: The gene combination responsible for the trait (e.g., TT or tt).

3. Chromosomes

Definition: Thread-like structures in the cell nucleus that carry genes.

Composition: Made of DNA (deoxyribonucleic acid) and proteins.

Structure:

Chromosomes appear dumbbell-shaped during cell division.
Have a centromere (constricted region) dividing them into two arms.

Types of Chromosomes (based on centromere position):

Metacentric: Centromere in the middle, V-shaped, equal arms.
Sub-metacentric: Centromere near the middle, L-shaped, one arm shorter.
Acrocentric: Centromere near one end, j-shaped, one arm much smaller.
Telocentric: Centromere at the end, i-shaped, only one arm.

Chromosome Numbers:

Humans: 46 chromosomes (23 pairs).
Other examples: Pea plant (14), Dog (78), Fruit fly (8).

Types of Chromosomes:

Homologous Chromosomes: Pairs with similar shape and genes.
Heterologous Chromosomes: Pairs with different shapes.
Autosomes: 22 pairs in humans, control body traits.
Sex Chromosomes (Allosomes): 1 pair (XX in females, XY in males), determine sex.

4. DNA (Deoxyribonucleic Acid)

Discovery: Found by Frederick Miescher in 1869, initially called nucleic acid.

Role: Controls cell functions, growth, and reproduction, earning it the title “Master Molecule.”

Structure (Watson and Crick’s Model, 1953):

Double Helix: Looks like a twisted ladder.

Components:

Sugar and Phosphate: Form the ladder’s rails.
Nitrogenous Bases: Form the ladder’s rungs, paired as:
- Adenine (A) with Thymine (T).
- Cytosine (C) with Guanine (G).

Nucleotides: Building blocks of DNA, made of sugar, phosphate, and a base.

Genes: Segments of DNA that code for specific proteins, controlling traits.

DNA Fingerprinting:

Identifies a person’s unique DNA sequence.
Uses: Identifying criminals, determining lineage, paternity testing.

5. RNA (Ribonucleic Acid)

Role: Helps in protein synthesis, working with DNA.

Structure: Made of ribose sugar, phosphate, and bases (Adenine, Guanine, Cytosine, Uracil).

Types of RNA:

Ribosomal RNA (rRNA): Part of ribosomes, where proteins are made.
Messenger RNA (mRNA): Carries genetic instructions from DNA to ribosomes.
Transfer RNA (tRNA): Brings amino acids to ribosomes for protein assembly.

6. Mendel’s Principles of Heredity

Gregor Johann Mendel: Austrian scientist (1822-1884), known as the “Father of Genetics.”

Experiments: Studied pea plants (Pisum sativum) to understand how traits are inherited.

Why Pea Plants?:

Have clear, contrasting traits (e.g., tall vs. dwarf).
Easy to cross-pollinate and grow.

Traits Studied:

Seed shape: Round (dominant) vs. Wrinkled (recessive).
Seed color: Yellow (dominant) vs. Green (recessive).
Flower color: Purple (dominant) vs. White (recessive).
Pod shape: Inflated (dominant) vs. Constricted (recessive).
Pod color: Green (dominant) vs. Yellow (recessive).
Flower position: Axillary (dominant) vs. Terminal (recessive).
Plant height: Tall (dominant) vs. Dwarf (recessive).

Mendel’s Monohybrid Cross

Definition: A cross between two plants differing in one trait (e.g., plant height: tall vs. dwarf).

Key Terms:

Dominant: Trait that appears in offspring (e.g., tall, denoted TT).
Recessive: Trait hidden in offspring (e.g., dwarf, denoted tt).

Experiment:

Parental Generation (P1): Tall (TT) × Dwarf (tt).

Gametes: TT produces T; tt produces t.

First Filial Generation (F1): All tall (Tt, heterozygous).

Selfing F1 (P2): Tt × Tt.

Second Filial Generation (F2):

Phenotypic Ratio: 3 tall (TT, Tt, Tt) : 1 dwarf (tt).
Genotypic Ratio: 1 TT : 2 Tt : 1 tt.

Conclusion:

Genes exist in pairs (e.g., T and t).
During gamete formation, gene pairs separate (Law of Segregation).
Dominant traits mask recessive traits in F1, but recessive traits reappear in F2.

Mendel’s Dihybrid Cross

Definition: A cross between two plants differing in two traits (e.g., seed shape and color).

Experiment:

Parental Generation (P1): Round-Yellow seeds (RRYY) × Wrinkled-Green seeds (rryy).

Gametes: RRYY produces RY; rryy produces ry.

First Filial Generation (F1): All Round-Yellow (RrYy, dihybrid).

Selfing F1 (P2): RrYy × RrYy.

Second Filial Generation (F2):

Phenotypic Ratio: 9 Round-Yellow : 3 Round-Green : 3 Wrinkled-Yellow : 1 Wrinkled-Green.
Genotypic Ratio: Complex, with 9 combinations (e.g., RRYY, RrYy, rryy).

Conclusion:

Traits are inherited independently (Law of Independent Assortment).
Each gene pair segregates independently during gamete formation.

7. Genetic Disorders

Definition: Diseases caused by abnormalities in chromosomes or gene mutations.

Types:

Chromosomal Disorders: Due to changes in chromosome number or structure.
Monogenic Disorders: Caused by a single gene mutation.
Polygenic Disorders: Involve multiple genes and environmental factors.
Mitochondrial Disorders: Due to defective mitochondrial DNA, inherited from the mother.

A. Chromosomal Disorders

Down Syndrome (Trisomy 21):

Cause: Extra 21st chromosome (47 total chromosomes).
Symptoms: Mental retardation, short height, flat nose, short fingers, life expectancy of 16-20 years.

Turner Syndrome (44+X):

Cause: Missing one X chromosome in females.
Symptoms: Sterility, underdeveloped reproductive organs, short stature.

Klinefelter Syndrome (44+XXY):

Cause: Extra X chromosome in males.
Symptoms: Sterility, underdeveloped reproductive organs.

B. Monogenic Disorders

Definition: Caused by mutation in a single gene, affecting protein production.

Examples:

Albinism:

Cause: Lack of melanin pigment due to defective gene.
Symptoms: Pale skin, white hair, pink eyes.

Sickle-Cell Anemia:

Cause: Mutation replaces glutamic acid with valine in hemoglobin, making RBCs sickle-shaped.

Symptoms: Joint pain, low hemoglobin, fatigue, swelling, frequent infections.

Types:

Carrier (AS): One normal, one defective gene; mild or no symptoms.
Sufferer (SS): Two defective genes; severe symptoms.

Diagnosis: Solubility test or electrophoresis.

Prevention: Avoid marriages between carriers; take folic acid tablets.

Note: Common in Maharashtra, affecting over 2.5 lakh people.

C. Mitochondrial Disorders

Cause: Mutations in mitochondrial DNA, inherited only from the mother.
Example: Leber Hereditary Optic Neuropathy (affects vision).

D. Polygenic Disorders

Cause: Mutations in multiple genes, worsened by environmental factors.
Examples: Cleft lip, cleft palate, diabetes, asthma, obesity, heart disorders.
Note: Do not strictly follow Mendel’s principles due to complex interactions.

8. Harmful Effects of Tobacco

Tobacco Use: Smoking (cigarettes, bidis) or chewing tobacco.

Harmful Chemicals: Nicotine, carbon monoxide, ammonia, sulfur dioxide.

Effects:

Causes cancer (lungs, mouth, larynx, bladder) due to uncontrolled cell growth.
Affects digestion, causes cough, sleeplessness, and trembling.
Leads to arteriosclerosis (hardened arteries) and hypertension.
Shortens lifespan and causes visual disorders or tremors.

Prevention: Avoid all forms of tobacco to protect health.