Heredity and Evolution — Study Notes

Overview

Heredity and Evolution is a core Biology topic in Railway Group D that explains how traits pass from parents to offspring and how species change over time. The exam typically tests your understanding of Mendel's laws of inheritance, the structure and function of DNA as the genetic material, and Darwin's theory of evolution by natural selection. This topic bridges classical genetics with molecular biology and evolutionary concepts.

Questions appear as direct fact-recall (what is a gene, define DNA) or application-based (predict offspring ratios in a cross, identify evolutionary evidence). Students must grasp the difference between genotype and phenotype, understand dominant-recessive relationships, recognize DNA's double-helix structure, and explain natural selection with examples. Mastering 8–10 key definitions and 2–3 worked Punnett square problems will cover most exam scenarios.

The topic connects to Cell Biology (chromosomes, cell division) and carries 2–3 questions in the General Science section. Focus on Mendel's pea-plant experiments, the structure of DNA discovered by Watson and Crick, and Darwin's observations on the Galápagos Islands as these form the conceptual anchors for typical Railway Group D questions.

Key Concepts

• **Heredity** is the transmission of genetic characteristics from parents to offspring through genes located on chromosomes in the cell nucleus.
• **Genetics** is the study of heredity and variation; Gregor Mendel is called the "Father of Genetics" for his pioneering pea-plant experiments (1860s).
• A **gene** is a segment of DNA that codes for a specific trait; **alleles** are different versions of the same gene (e.g., tall vs short).
• **Dominant alleles** (represented by capital letters, T) mask recessive alleles (lowercase, t) in the phenotype; a recessive trait appears only in homozygous recessive individuals (tt).
• **Genotype** is the genetic makeup (TT, Tt, tt), while **phenotype** is the observable trait (tall or short plant).
• **DNA (deoxyribonucleic acid)** is the hereditary material in all living organisms, structured as a double helix with sugar-phosphate backbones and nitrogenous base pairs (A-T, G-C).
• **Evolution** is the gradual change in species over generations; Darwin's theory of natural selection states that organisms with advantageous traits survive and reproduce more successfully.
• Evidence for evolution includes fossils (preserved remains), comparative anatomy (homologous and analogous structures), embryology (similar embryonic stages), and molecular biology (DNA similarities across species).

Formulas / Key Facts

• **Mendel's Law of Dominance**: In a heterozygote (Tt), the dominant allele's trait is expressed in the phenotype.
• **Mendel's Law of Segregation**: Each parent contributes one allele per gene; allele pairs separate during gamete formation.
• **Mendel's Law of Independent Assortment**: Genes for different traits assort independently during gamete formation (applies to genes on different chromosomes).
• **Monohybrid cross ratio**: F₂ generation shows 3:1 phenotypic ratio (dominant:recessive) and 1:2:1 genotypic ratio (TT:Tt:tt).
• **DNA structure**: Double helix discovered by Watson and Crick (1953); consists of nucleotides with deoxyribose sugar, phosphate group, and one of four bases—Adenine, Thymine, Guanine, Cytosine.
• **Base pairing rules**: Adenine pairs with Thymine (A-T); Guanine pairs with Cytosine (G-C) via hydrogen bonds.
• **Chromosome**: Thread-like structure in the nucleus made of DNA and protein; humans have 23 pairs (46 total).
• **Natural Selection**: Variation → Struggle for existence → Survival of the fittest → Evolution.
• **Homologous structures**: Same basic structure, different functions (human arm, whale flipper) indicate common ancestry.
• **Analogous structures**: Different structure, same function (bird wing, insect wing) result from convergent evolution, not common ancestry.

Worked Examples

**Example 1: Monohybrid Cross** A tall pea plant (TT) is crossed with a short pea plant (tt). Predict the F₁ and F₂ generations.

*Solution:*

• **F₁ cross**: TT × tt → All gametes from TT are T; all from tt are t.

Offspring: 100% Tt (heterozygous tall, because T is dominant). **F₁ phenotype**: All tall plants.

• **F₂ cross**: Tt × Tt

Gametes: Each parent produces T and t. Punnett square: ``` T t T TT Tt t Tt tt ``` Genotypic ratio: 1 TT : 2 Tt : 1 tt Phenotypic ratio: 3 tall : 1 short **F₂ result**: 75% tall, 25% short.

**Example 2: DNA Base Pairing** One strand of DNA has the sequence ATGC. What is the complementary strand?

*Solution:* Apply base-pairing rules (A-T, G-C):

• A pairs with T
• T pairs with A
• G pairs with C
• C pairs with G

**Complementary strand**: TACG

**Example 3: Identifying Evidence of Evolution** Which type of evidence is provided by similar bone structure in the forelimbs of humans, whales, and bats?

*Solution:* These are **homologous structures**—same underlying bone pattern (humerus, radius, ulna) adapted for different functions (grasping, swimming, flying). This indicates **common ancestry** and supports evolution through divergent evolution.

Common Mistakes

• **Confusing genotype with phenotype**: Genotype is the allele combination (Tt); phenotype is the visible trait (tall). A Tt and TT plant both appear tall but have different genotypes.

**Fix**: Always distinguish genetic makeup from observable characteristics.

• **Miscounting Punnett square ratios**: Students often write 4:0 instead of 3:1 for monohybrid F₂.

**Fix**: Count each box separately—three boxes show dominant phenotype, one shows recessive, yielding 3:1.

• **Reversing dominant-recessive notation**: Writing "t" for dominant and "T" for recessive.

**Fix**: Uppercase letter = dominant allele; lowercase = recessive allele (standard convention).

• **Mixing up homologous and analogous structures**: Saying bird and insect wings are homologous because both fly.

**Fix**: Homologous = same structure/origin (even if different function); analogous = same function but different structure/origin.

• **Believing acquired traits are inherited**: Thinking a bodybuilder's muscles pass to children (Lamarckism).

**Fix**: Only genetic changes in gametes (sex cells) are heritable; traits developed in a lifetime (acquired characteristics) are not inherited.

Quick Reference

• **Father of Genetics**: Gregor Mendel; model organism: garden pea (Pisum sativum).
• **Monohybrid F₂ ratio**: 3:1 phenotypic; 1:2:1 genotypic.
• **DNA base pairs**: A-T (2 hydrogen bonds), G-C (3 hydrogen bonds).
• **Watson & Crick**: Discovered DNA double helix structure in 1953.
• **Natural selection**: Variation + Competition → Survival of fittest → Evolution.
• **Homologous structures**: Common origin, different function (evidence of common ancestry).
• **Fossil**: Preserved remains or impressions of organisms from the past; key evidence for evolution.
• **Human chromosomes**: 23 pairs (46 total); 22 pairs autosomes + 1 pair sex chromosomes (XX or XY).