Genetic Evolution: Comprehensive NEET Biology Notes

1. Principles of Inheritance and Variation

1.1 Mendel’s Laws of Inheritance

Gregor Mendel, the father of genetics, conducted hybridization experiments on garden peas between 1856 and 1863. He formulated the laws of inheritance based on his observations. Mendel's work was pioneering in applying statistical analysis and mathematical logic to biological problems. He proposed that inheritance involves factors (now known as genes) that are passed unchanged from parent to offspring through gametes over successive generations.

Key Points:

• True Breeding: A line that, when self-pollinated, shows the same traits over generations.
• Monohybrid Cross: A cross between two plants differing in one trait.
• Law of Dominance: In a pair of contrasting traits, one is dominant and the other is recessive.
• Law of Segregation: Alleles segregate randomly during gamete formation.

Example: Crossing tall (TT) and dwarf (tt) plants:

• F1 Generation: All tall (Tt)
• F2 Generation: 3:1 ratio of tall to dwarf

Punnett Square:

1.2 Inheritance of One Gene

Mendel's monohybrid cross experiments led to the discovery of the segregation of alleles. For instance, when he crossed pure-breeding tall (TT) and dwarf (tt) plants, all F1 hybrids were tall (Tt). Upon self-pollination, the F2 generation exhibited a 3:1 ratio of tall to dwarf plants.

Key Points:

• Genotype: Genetic makeup (e.g., TT, Tt, tt)
• Phenotype: Physical appearance (e.g., tall, dwarf)
• Dominant and Recessive Traits: Dominant traits mask recessive ones in heterozygous conditions.

1.3 Inheritance of Two Genes

Mendel's dihybrid cross experiments with pea plants that differed in two traits (e.g., seed color and shape) showed that traits assort independently during gamete formation. This led to the formulation of the Law of Independent Assortment.

Key Points:

• Dihybrid Cross: Cross between two organisms differing in two traits.
• F2 Generation: 9:3:3:1 ratio of phenotypes.

Example: Crossing plants with round yellow seeds (RRYY) with plants with wrinkled green seeds (rryy):

• F1 Generation: All round yellow seeds (RrYy)
• F2 Generation: 9 round yellow: 3 round green: 3 wrinkled yellow: 1 wrinkled green

Punnett Square:

1.4 Sex Determination

Sex determination mechanisms vary among different species. In humans, sex is determined by the presence of XX (female) or XY (male) chromosomes.

Key Points:

• XY System: Found in humans and Drosophila; males have XY and females have XX.
• XO System: Found in some insects; males have XO and females have XX.
• ZW System: Found in birds; males have ZZ and females have ZW.

Example: Human sex determination:

• Male: XY chromosomes
• Female: XX chromosomes

1.5 Mutation

Mutations are changes in the DNA sequence that can result in altered phenotypes. They can occur due to various factors and lead to genetic diversity.

Key Points:

• Point Mutation: Change in a single nucleotide (e.g., sickle cell anemia).
• Chromosomal Mutations: Involves changes in chromosome structure or number (e.g., Down’s syndrome, Turner’s syndrome).

Types of Mutations:

• Insertion/Deletion: Addition or loss of DNA segments.
• Substitution: Replacement of one nucleotide with another.

Example: Sickle cell anemia results from a point mutation in the β-globin gene, causing the substitution of valine for glutamic acid.

1.6 Genetic Disorders

Genetic disorders can be broadly classified into Mendelian disorders and chromosomal disorders.

Mendelian Disorders:

• Haemophilia: X-linked recessive disorder affecting blood clotting.
• Sickle Cell Anemia: Autosomal recessive disorder causing abnormal hemoglobin.
• Phenylketonuria: Autosomal recessive disorder leading to mental retardation due to the inability to metabolize phenylalanine.

Chromosomal Disorders:

• Down’s Syndrome: Trisomy of chromosome 21.
• Turner’s Syndrome: Monosomy of the X chromosome (XO).
• Klinefelter’s Syndrome: XXY condition in males.

Pedigree Analysis:

• A tool used to track the inheritance of traits in humans over generations.
• Helps identify carriers and predict genetic risks.

Quick Recap

• Mendel’s Laws: Dominance, Segregation, Independent Assortment.
• Genotype vs. Phenotype: Genetic makeup vs. physical appearance.
• Sex Determination: XY, XO, ZW systems.
• Mutations: Point mutations, chromosomal mutations.
• Genetic Disorders: Mendelian (single-gene) and chromosomal disorders.

Practice Questions

1. Explain the Law of Segregation with an example.
2. Describe the chromosomal theory of inheritance.
3. How do point mutations differ from chromosomal mutations? Provide examples.
4. What are sex-linked disorders? Give examples.
5. How is sex determined in birds compared to humans?

Concept Connection

Link to Chemistry:

The study of genetic mutations involves understanding the chemical structure of DNA and how changes at the molecular level can impact genetic information.

NEET Exam Strategy

• Time Management: Allocate specific time slots for each topic to ensure comprehensive coverage.
• Practice Questions: Regularly solve NEET questions on genetics and related processes.
• Diagrams and Charts: Use diagrams to visualize genetic crosses and mutations for better understanding.

Comprehensive Quick Reference Guide and Glossary

Quick Reference Guide

• Mendel’s Laws: Principles of inheritance.
• Genotype: Genetic makeup (e.g., TT, Tt, tt).
• Phenotype: Physical appearance (e.g., tall, dwarf).
• Sex Determination: Mechanisms in different species.
• Mutation: Changes in DNA sequence.
• Genetic Disorders: Mendelian and chromosomal disorders.

Glossary

• Allele: Different forms of a gene.
• Dihybrid Cross: Cross between two organisms differing in two traits.
• Homozygous: Having two identical alleles of a gene.
• Heterozygous: Having two different alleles of a gene.
• Mutation: A change in the DNA sequence.
• Pedigree Analysis: Study of family history to track genetic traits.