Chapter 14 — Genetics and Evolution
Key Concepts Quick Recap
| Term | Definition |
|---|
| Allele | Alternative forms of a gene at the same locus |
| Dominant allele | Expressed in heterozygote (masks recessive) |
| Recessive allele | Only expressed in homozygous state |
| Monohybrid cross | Cross involving one pair of contrasting characters |
| Dihybrid cross | Cross involving two pairs of contrasting characters |
| Test cross | Cross of F1 with homozygous recessive; reveals genotype |
| Gene flow | Movement of alleles between populations (migration) |
| Genetic drift | Random change in allele frequencies in small populations |
| Natural selection | Survival and reproduction of individuals with favourable traits |
| Speciation | Formation of new species from existing ones |
I. Choose the Best Answer (MCQs)
1. Down syndrome is caused by: (a) Deletion of chromosome 21 (b) Trisomy 21 (extra copy of chromosome 21) (c) Monosomy X (d) Translocation of chromosome 13
Answer(b) Trisomy 21 (extra copy of chromosome 21) Down syndrome: 47 chromosomes (2n+1 = 47); extra chromosome 21. Cause: non-disjunction during meiosis (usually in mother, risk increases with maternal age). Features: intellectual disability, characteristic facial features, epicanthal folds, flat nasal bridge, single palmar crease, congenital heart defects.
2. Turner syndrome has the karyotype: (a) 47, XYY (b) 47, XXX (c) 45, XO (d) 47, XXY
Answer(c) 45, XO Turner syndrome: 45 chromosomes (one X chromosome missing). Phenotypically female but infertile (streak gonads), short stature, webbing of neck, low hairline, shield-shaped chest. Intelligence normal.
- Klinefelter syndrome: 47, XXY — male, infertile, gynecomastia.
- XYY syndrome: 47, XYY — male, usually tall, fertile.
3. The theory of natural selection was proposed by: (a) Lamarck (b) Hugo de Vries (c) Charles Darwin (d) Gregor Mendel
Answer(c) Charles Darwin Darwin published "On the Origin of Species" (1859). Key concepts: overproduction (more offspring than can survive), variation exists in populations, struggle for existence, survival of the fittest (natural selection), heritable favourable variations accumulate → new species. Wallace independently developed the same idea.
4. According to Hardy-Weinberg principle, the frequency of homozygous dominant genotype (AA) is: (a) p (b) q² (c) 2pq (d) p²
Answer(d) p² Hardy-Weinberg equilibrium: p² + 2pq + q² = 1
where p = frequency of A allele, q = frequency of a allele, and p + q = 1.
- p² = frequency of AA (homozygous dominant)
- 2pq = frequency of Aa (heterozygous)
- q² = frequency of aa (homozygous recessive)
5. Industrial melanism in the peppered moth (Biston betularia) is an example of: (a) Disruptive selection (b) Directional natural selection (c) Stabilising selection (d) Genetic drift
Answer(b) Directional natural selection Before industrial revolution: white moths survived better (camouflaged on white lichen-covered tree bark). After industrial revolution: bark became dark (soot) → dark moths survived (camouflaged), white moths picked off by predators. Allele frequency shifted toward dark form = directional selection. Landmark evidence for evolution by natural selection.
6. The concept of "Survival of the Fittest" was coined by: (a) Charles Darwin (b) Herbert Spencer (c) Alfred Wallace (d) Ernst Mayr
Answer(b) Herbert Spencer Herbert Spencer coined the phrase "survival of the fittest" (1864) after reading Darwin's "On the Origin of Species." Darwin later adopted it as a synonym for natural selection in later editions of his book. Darwin's own term was "natural selection."
7. Which of the following is NOT a factor that disrupts Hardy-Weinberg equilibrium? (a) Mutation (b) Large population size (c) Natural selection (d) Genetic drift
Answer(b) Large population size Hardy-Weinberg equilibrium requires: large population, random mating, no mutation, no migration, no natural selection. Large population size SUPPORTS equilibrium. Small population size → genetic drift (random changes) → disrupts equilibrium.
II. Short Answer Questions
Q1. State and explain Mendel's Laws of Inheritance.
Answer
Law 1 — Law of Dominance: When two different alleles are present in heterozygote (Aa), one allele (dominant, A) is expressed while the other (recessive, a) is suppressed. Only dominant allele determines phenotype in F1.
Law 2 — Law of Segregation (Purity of Gametes): During gamete formation, the two alleles of a gene separate from each other so that each gamete carries only one allele. Gametes are pure (not mixed). Example: Aa → A gametes and a gametes.
Law 3 — Law of Independent Assortment: Genes for different characters (on different chromosomes) segregate independently during gamete formation. Alleles of one gene do not influence the segregation of alleles of another gene.
- Only applies to genes on DIFFERENT (non-homologous) chromosomes.
- Linked genes (same chromosome) do NOT assort independently.
Monohybrid cross results:
- F1: all Aa (tall) — only dominant expressed.
- F2: 3 tall : 1 short (3:1 phenotypic ratio); 1 TT : 2 Tt : 1 tt (1:2:1 genotypic ratio).
Q2. Describe Darwin's theory of natural selection with an example.
Answer
Darwin's observations and inferences:
1. Overproduction: Every species produces more offspring than can survive. 2. Variation: Individuals in a population show variation in traits. 3. Struggle for existence: Competition for limited resources (food, space, mates). 4. Natural selection (survival of the fittest): Individuals with favourable variations survive and reproduce; unfavourable variations are eliminated. 5. Inheritance: Favourable traits are passed to offspring. 6. Over generations: Population changes → new species arise (speciation).
Example — Antibiotic resistance in bacteria:
- Random mutations produce some antibiotic-resistant bacteria.
- Antibiotic exposure = selection pressure.
- Sensitive bacteria die; resistant bacteria survive and multiply.
- Over many generations → population becomes predominantly resistant.
- This is evolution by natural selection (not directed; selection acts on existing variation).
Types of natural selection:
- Directional: Favours one extreme phenotype (peppered moth).
- Stabilising: Favours average (human birth weight — very low or very high weight babies have lower survival).
- Disruptive: Favours both extremes over the intermediate.
Common Mistakes
- Saying Darwin coined "survival of the fittest" — Herbert Spencer coined it; Darwin used "natural selection."
- Confusing Turner (45, XO — female) and Klinefelter (47, XXY — male).
- Saying Down syndrome is caused by monosomy — it is TRISOMY 21 (extra chromosome).
- Forgetting Hardy-Weinberg requires a LARGE population; small populations are subject to genetic drift.
- Saying Mendel's Law of Independent Assortment applies to ALL genes — only to genes on DIFFERENT chromosomes.
Exam Checklist
- Mendel's Laws: Dominance, Segregation, Independent Assortment.
- F2 monohybrid: 3:1 (phenotypic); 1:2:1 (genotypic).
- Down syndrome: trisomy 21 (47); Turner: 45, XO (female); Klinefelter: 47, XXY (male).
- Hardy-Weinberg: p² + 2pq + q² = 1; disrupted by mutation, drift, migration, selection, non-random mating.
- Darwin: natural selection; "On the Origin of Species" 1859.
- Industrial melanism: directional selection (Biston betularia).
- "Survival of the fittest": Herbert Spencer (not Darwin).
