Genetics and Evolution

Genetics and Evolution together form the single largest scoring block in the NEET Biology paper, and the two halves are logically linked: genetics explains how variation is generated and transmitted, while evolution explains what happens to that variation in populations over time. Master the numerical/ratio-based reasoning of genetics and the evidence-plus-mechanism logic of evolution, and you cover a large fraction of the Zoology questions.

Mendel's Experiments and the Two Laws of Inheritance

Gregor Mendel worked with garden pea, choosing seven characters that each existed in two sharply contrasting forms (tall/dwarf stem, round/wrinkled seed, violet/white flower, and so on). His success rested on three practical decisions: he used true-breeding lines, he studied one or two characters at a time, and he counted large numbers of offspring instead of describing them qualitatively.

When a tall pea plant was crossed with a dwarf one, every F₁ plant was tall — the dwarf condition had not been destroyed, merely masked. Self-pollinating the F₁ gave an F₂ with tall and dwarf plants in approximately 3 : 1. Mendel concluded that each character is governed by a pair of "factors" (now alleles), that one member of the pair can dominate the expression of the other, and that the pair separates cleanly during gamete formation.

• Law of Dominance: characters are controlled by paired factors; in a heterozygote only the dominant allele expresses itself in the phenotype. This law explains why the F₁ is uniform and why the recessive form reappears in F₂.
• Law of Segregation: the two alleles of a pair do not blend; they separate during gamete formation so that each gamete receives only one of them. Gametes are therefore always pure for a given allele.
• Law of Independent Assortment: derived from dihybrid crosses (e.g. round-yellow × wrinkled-green seeds, giving a 9 : 3 : 3 : 1 F₂), it states that the segregation of one allele pair is independent of the segregation of another pair — valid only for genes on different chromosomes or far apart on the same one.

Two tools recur in problem-solving. The test cross (crossing an individual of unknown genotype with the homozygous recessive) reveals genotype directly: a 1 : 1 ratio indicates a heterozygote, all-dominant offspring indicate a homozygote. A monohybrid F₂ always has a genotypic ratio of 1 : 2 : 1 underlying the 3 : 1 phenotypic ratio, and in the dihybrid F₂ each character separately still shows 3 : 1 — proof that the 9 : 3 : 3 : 1 is simply (3 : 1) × (3 : 1).

Deviations from Mendelian Ratios

Mendel's clean ratios are a special case. Several patterns modify them without violating segregation.

1. Incomplete dominance: the heterozygote shows an intermediate phenotype, as in Antirrhinum (snapdragon) flower colour, where red × white gives pink F₁ and a 1 : 2 : 1 F₂ in which phenotypic and genotypic ratios coincide.
2. Co-dominance: both alleles express fully and independently in the heterozygote. Human ABO blood groups illustrate this: I^A and I^B are co-dominant, producing AB blood.
3. Multiple alleles: more than two alleles exist for one gene in the population, though any diploid individual carries only two. The ABO system (I^A, I^B, i) is the standard example, with six genotypes yielding four phenotypes.
4. Pleiotropy: a single gene influences several apparently unrelated traits — for instance the gene for phenylketonuria affects pigmentation and mental development along with phenylalanine metabolism.
5. Polygenic inheritance: a trait such as human skin colour or height is governed by several genes acting additively, each contributing a small effect, producing a continuous range of phenotypes strongly modified by environment.

Chromosomal Basis, Linkage and Sex Determination

Sutton and Boveri united cytology with breeding data in the chromosomal theory of inheritance, pointing out that chromosomes occur in pairs, segregate at meiosis, and assort independently — exactly the behaviour Mendel had inferred for his factors. Morgan's work on Drosophila then showed that genes located close together on the same chromosome tend to be inherited together, producing far more parental than recombinant offspring; this is linkage. The frequency of recombinants reflects the distance between genes and is the basis of genetic mapping (tightly linked genes give low recombination; loosely linked genes approach independent assortment).

Sex determination differs across groups and is frequently asked:

• XX–XY (humans, Drosophila): female XX, male XY; the male is heterogametic.
• XX–XO (grasshopper, many bugs): female XX, male has a single X; sex depends on whether the egg is fertilised by an X-bearing or an X-lacking sperm.
• ZZ–ZW (birds): the female is heterogametic (ZW), male ZZ.
• Haplodiploidy (honeybee): fertilised eggs (diploid) become females — queen or workers — while unfertilised eggs develop parthenogenetically into haploid drones.

In humans the mother always contributes an X, so the father's gamete decides the child's sex. Sex-linked inheritance follows from this: recessive X-linked conditions such as haemophilia and red–green colour blindness appear far more often in males, who need only one copy, and are typically transmitted from carrier mothers to sons.

Mutations and disorders. Mutation is any change in DNA sequence or chromosome structure/number. Point mutations, such as the single base substitution converting glutamic acid to valine in the β-globin chain, cause sickle-cell anaemia; frameshift mutations arise from insertion or deletion of bases. Chromosomal disorders arise from aneuploidy or polyploidy: Down's syndrome (trisomy of chromosome 21), Klinefelter's syndrome (XXY, male with some feminine features, sterile) and Turner's syndrome (45, XO, sterile female). Mendelian human disorders include haemophilia, sickle-cell anaemia (autosomal recessive), thalassaemia, phenylketonuria and colour blindness.

Molecular Basis of Inheritance in Brief

DNA is the genetic material in nearly all organisms; some viruses use RNA. Griffith's transformation experiment, refined biochemically by Avery, MacLeod and McCarty, and the Hershey–Chase experiment with radioactively labelled bacteriophage, established DNA rather than protein as the hereditary molecule. Watson and Crick's double helix — antiparallel strands, complementary A=T and G≡C pairing, roughly 10 base pairs per turn — immediately suggested a copying mechanism, confirmed as semi-conservative replication by Meselson and Stahl in E. coli.

Information flows as DNA → RNA → protein (the central dogma), with reverse transcription as a known exception in retroviruses. Transcription copies one template strand into mRNA; in eukaryotes the primary transcript is processed by capping, tailing and splicing out of introns. Translation reads the mRNA in triplets: the genetic code is triplet, degenerate (most amino acids have more than one codon), non-overlapping, comma-less and nearly universal, with AUG as initiator and UAA, UAG, UGA as stop codons. tRNA acts as the adaptor molecule, and ribosomes provide the catalytic machinery. Gene expression is regulated — in prokaryotes classically through the lac operon, where lactose acts as an inducer that inactivates the repressor, allowing transcription of the genes for β-galactosidase, permease and transacetylase. The Human Genome Project and DNA fingerprinting (based on repetitive VNTR sequences) are the applied extensions of this molecular understanding.

Evolution: Evidence, Mechanisms and Human Origins

The Earth's life history is reconstructed from several independent lines of evidence, and NEET expects you to name them with examples:

• Palaeontological: fossils in successive sedimentary layers show progressive change and the existence of extinct forms; Archaeopteryx links reptiles and birds.
• Comparative anatomy: homologous organs (forelimbs of whale, bat, cheetah and human) share common ancestry and indicate divergent evolution; analogous organs (wings of insect and bird, eyes of octopus and mammal) indicate convergent evolution from different ancestries.
• Embryological and molecular: similarities in early development and near-identity of fundamental biochemical pathways, DNA and protein sequences across taxa.
• Evolution in action: industrial melanism in the peppered moth, and the rapid spread of antibiotic or pesticide resistance, demonstrate selection over observable timescales.

The mechanism proposed by Darwin and Wallace rests on heritable variation, a struggle for existence in populations that reproduce beyond available resources, and differential survival and reproduction of better-fitted variants — natural selection acting as a gradual, cumulative force. Lamarck's earlier idea of inheritance of acquired characters is not accepted. Selection can be stabilising (favouring the mean), directional (shifting the mean) or disruptive (favouring both extremes). Hugo de Vries, working on evening primrose, emphasised sudden large-scale mutations (saltation).

Modern population genetics quantifies this. The Hardy–Weinberg principle states that allele frequencies in a large, randomly mating population remain constant across generations in the absence of evolutionary forces, with genotype frequencies given by p² + 2pq + q² = 1. Departures from constancy signal evolution, and the recognised agents are gene mutation, genetic recombination, genetic drift (including founder effect and bottlenecks), gene migration or flow, and natural selection. Reproductive isolation of populations followed by divergence leads to speciation; adaptive radiation, as in Darwin's finches on the Galapagos or Australian marsupials, describes divergence of many forms from a single ancestral stock in one geographical area.

In human evolution, the accepted sequence runs from Dryopithecus and Ramapithecus (ape-like forms) through Australopithecus and Homo habilis (the first tool-maker, brain roughly 650–800 cc), to Homo erectus (about 900 cc, used fire), Neanderthal man (about 1400 cc), and finally Homo sapiens, which arose in Africa and spread outward. Key trends are increasing cranial capacity, erect bipedal posture and progressively refined tool use.

Common Mistakes and Exam Traps

1. Confusing incomplete dominance with co-dominance. In incomplete dominance the heterozygote is a new, intermediate phenotype (pink); in co-dominance both parental phenotypes are simultaneously and fully visible in the heterozygote — as in AB blood group, where both A and B antigens are expressed, neither blended nor masked.
2. Mixing up linkage and independent assortment. Genes close together on the same chromosome show linkage — inherited together, giving mostly parental-type offspring; independent assortment applies strictly to genes on different chromosomes (or very far apart on the same one). A dihybrid ratio near 9:3:3:1 signals independent assortment; a ratio strongly skewed toward parental combinations signals linkage.
3. Confusing homologous and analogous organs. Homologous organs (forelimbs of whale, bat, human) share common ancestry despite different functions — divergent evolution; analogous organs (insect and bird wings) share function despite different ancestry — convergent evolution. NEET usually gives an organ pair and expects the correct relationship, not just a similarity judgement.
4. Misreading the Hardy–Weinberg principle as a description of an evolving population. It is a null hypothesis describing a population in genetic equilibrium (no evolution occurring). Any departure of observed genotype frequencies from p² + 2pq + q² = 1 is evidence that one of the five evolutionary agents — mutation, gene flow, genetic drift, non-random mating or natural selection — is acting, not proof that evolution has already been described by the equation itself.