Genetics & Evolution — NEET revision
The full NEET Genetics & Evolution unit in one place: concept notes from Mendel to the molecular basis of inheritance and evolution, differentiate tables, the doubts students get wrong most, quick-recall facts — plus an interactive DNA double-helix you can tap through.
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Tap through the interactive models for this unit — each part explained in plain English with an exam-accurate NEET note.
Key concepts
Unit 7 covers Mendelian and non-Mendelian inheritance, sex determination and genetic disorders; DNA/RNA, replication, transcription, translation and gene regulation; HGP and DNA fingerprinting; and evidence, mechanisms and patterns of evolution including Hardy–Weinberg equilibrium and human evolution.
Mendel's law of segregation
The two alleles of a gene separate during gamete formation, so each gamete receives one allele.
- Monohybrid F₂ phenotype 3:1
- Genotype 1:2:1
Independent assortment
Alleles of different genes assort independently when the genes are unlinked (or far apart enough for recombination).
- Classic dihybrid F₂ phenotype 9:3:3:1
- Not valid for tightly linked genes
Test cross
Crossing an individual with a homozygous recessive tester reveals its unknown genotype.
- Monohybrid heterozygote test cross → 1:1
Incomplete dominance
Neither allele is completely dominant, so the heterozygote has an intermediate phenotype; the F₂ phenotypic ratio becomes 1:2:1.
- Snapdragon flower colour: red : pink : white = 1:2:1
Codominance & ABO blood groups
In codominance both alleles express in the heterozygote. IA and IB are codominant; both dominate i in the ABO system.
- IAIB → blood group AB
- IAi → A · IBi → B · ii → O
Multiple alleles, pleiotropy & polygenic inheritance
A gene may have more than two alleles in a population; one gene can affect multiple traits (pleiotropy); many genes can contribute additively to a trait (polygenic inheritance).
- ABO: multiple alleles
- Skin colour: polygenic example
Chromosome theory of inheritance
Sutton and Boveri linked Mendelian factors with chromosomes: genes occupy loci on chromosomes and chromosomes segregate/assort during meiosis.
- Genes lie on chromosomes
Linkage & crossing over
Genes on the same chromosome tend to be inherited together. Crossing over in prophase I recombines linked genes; recombination frequency increases with map distance.
- Linkage ↓ independent assortment
- Crossing over: pachytene of prophase I
Sex determination
Humans use XX/XY, birds use ZZ/ZW and honey bees use haplodiploidy.
- Humans: female XX, male XY
- Birds: male ZZ, female ZW
- Honey bee: female 2n, male n
Sex-linked inheritance
X-linked recessive traits such as haemophilia and red-green colour blindness are more frequent in males because males have only one X chromosome.
- Haemophilia + colour blindness: X-linked recessive
Mendelian & chromosomal disorders
Thalassemia is a Mendelian disorder affecting globin synthesis. Down syndrome is usually trisomy 21; Turner syndrome 45,X; Klinefelter syndrome 47,XXY.
- Down: trisomy 21
- Turner: 45,X · Klinefelter: 47,XXY
Search for genetic material
Griffith discovered transformation; Avery, MacLeod and McCarty identified DNA as the transforming principle; Hershey and Chase confirmed DNA as genetic material in bacteriophages.
- DNA, not protein, entered bacteria in Hershey–Chase
DNA structure
DNA is an antiparallel double helix of deoxyribonucleotides. A pairs with T by two H bonds; G pairs with C by three.
- A=T · G≡C
- Strands antiparallel 5′→3′ / 3′→5′
RNA structure & types
RNA is usually single-stranded, contains ribose and uracil, and functions as mRNA, tRNA, rRNA and other RNA types.
- RNA: A,U,G,C
- mRNA carries code · tRNA adaptor · rRNA structural/catalytic
DNA packaging
In eukaryotes DNA wraps around histone octamers to form nucleosomes, which fold into chromatin; prokaryotic DNA is organised in the nucleoid.
- Nucleosome = DNA around histone octamer
DNA replication
DNA replication is semiconservative: each daughter DNA has one parental and one newly synthesised strand. DNA polymerases extend only 5′→3′.
- Semiconservative
- Leading continuous · lagging discontinuous with Okazaki fragments
Central dogma
Genetic information generally flows DNA → RNA → protein. Reverse transcription is a known exception in retroviruses.
- DNA → RNA → protein
Transcription
RNA polymerase copies a DNA template strand into RNA. The RNA sequence matches the coding strand except U replaces T.
- Template read 3′→5′
- RNA synthesised 5′→3′
Genetic code
The code is triplet, degenerate, unambiguous and nearly universal. AUG is the usual start codon; UAA, UAG and UGA are stop codons.
- 64 codons · 61 sense + 3 stop
- AUG: start/Met
Translation
Ribosomes read mRNA codons while tRNAs deliver amino acids. Peptide bonds extend the polypeptide until a stop codon is reached.
- mRNA read 5′→3′
- Protein grows N-terminus → C-terminus
Lac operon
In E. coli, the lac operon is an inducible system. Without lactose the repressor blocks transcription; lactose/allolactose inactivates the repressor and permits expression when conditions favour it.
- Lactose present → repression relieved
Human Genome Project
The HGP mapped and sequenced the human genome and identified genes and large non-coding regions, providing a reference for human genetics.
- Human genome ≈ 3.2 billion bp
DNA fingerprinting
DNA fingerprinting distinguishes individuals using highly variable repetitive DNA regions such as VNTR/STR markers.
- High polymorphism → individual-specific pattern
Origin & evidence of evolution
Evolution is supported by fossils, homologous structures, embryological patterns and molecular similarities.
- Homology → divergent evolution
- Analogy → convergent evolution
Darwin & modern synthesis
Darwin proposed natural selection acting on heritable variation. Modern evolutionary theory combines selection with mutation, recombination, gene flow and genetic drift in populations.
- Evolution = change in allele frequencies across generations
Natural selection types
Stabilising selection favours intermediate phenotypes; directional selection shifts the mean; disruptive selection favours extremes.
- Stabilising · directional · disruptive
Hardy–Weinberg equilibrium
In an ideal non-evolving population, allele and genotype frequencies remain constant: p + q = 1 and p² + 2pq + q² = 1.
- Disturbed by mutation, recombination, selection, gene flow, drift
Gene flow & genetic drift
Gene flow moves alleles between populations; genetic drift is random allele-frequency change, strongest in small populations.
- Founder effect/bottleneck = drift examples
Adaptive radiation
One ancestral lineage can diversify into many species adapted to different niches, as in Darwin's finches or Australian marsupials.
- One ancestor → many ecological forms
Human evolution
Human evolution includes a branching sequence of hominin forms rather than a simple straight ladder; Homo sapiens arose in Africa and later dispersed widely.
- Australopithecus → early Homo forms → Homo sapiens (simplified sequence)
Differentiate inheritance and evolutionary mechanisms
Separate allele behaviour in a cross from allele-frequency change in a population.
Sex determination
| Humans | XX female, XY male; male heterogametic |
| Birds | ZZ male, ZW female; female heterogametic |
| Honey bee | Diploid females, haploid males |
Natural selection
| Stabilising | Intermediate favoured; variance tends to fall |
| Directional | One extreme favoured; mean shifts |
| Disruptive | Both extremes favoured |
High-yield distinctions
These are classic NEET traps.
Incomplete dominance vs codominance ★
Incomplete dominance
- Heterozygote intermediate
- Phenotype ratio often 1:2:1
- Snapdragon
Codominance
- Both alleles fully expressed
- No blending
- AB blood group
DNA vs RNA ★
DNA
- Deoxyribose
- Thymine
- Usually double-stranded
RNA
- Ribose
- Uracil
- Usually single-stranded
Template vs coding DNA strand ★
Template
- Read by RNA polymerase
- 3′→5′ relative to transcription
- Complementary to RNA
Coding
- Same sequence as RNA except T/U
- 5′→3′
- Not used as template for that gene
Gene flow vs genetic drift
Gene flow
- Alleles move between populations
- Often reduces population differences
- Migration-mediated
Genetic drift
- Random sampling change
- Strongest in small populations
- Can fix/lose alleles by chance
Common NEET doubts
Does independent assortment apply equally to tightly linked genes?
No. Linked genes on the same chromosome tend to be inherited together; crossing over can produce recombinants.
Is an AB individual showing incomplete dominance?
No. IA and IB are codominant; both are expressed.
Can a male be a 'carrier' for an X-linked recessive allele without showing it?
Usually no. A male has only one X chromosome, so an X-linked recessive allele is expressed if present.
Does Down syndrome mean three copies of every chromosome?
No. It is usually trisomy of chromosome 21 only.
Is DNA replication conservative?
No. It is semiconservative: each daughter double helix contains one old strand and one new strand.
Does RNA polymerase read the coding strand?
No. It reads the template strand; RNA resembles the coding strand except U replaces T.
Does one codon specify several amino acids?
No. The code is unambiguous: a codon specifies one amino acid or stop. Degeneracy means several codons may specify the same amino acid.
Is the lac operon permanently ON when lactose is present?
Lactose relieves repression, but strong expression also depends on the cell's glucose/cAMP state.
Does mutation alone explain all evolution?
No. Evolution reflects mutation and recombination plus selection, gene flow and genetic drift.
Does Hardy–Weinberg equilibrium mean p = q?
No. It only requires allele frequencies to remain constant under ideal assumptions; p and q can have any values that sum to 1.
Quick recall
Q. Monohybrid F₂ genotype ratio?
1:2:1.
Q. Classic dihybrid F₂ phenotype ratio?
9:3:3:1 for unlinked genes with complete dominance.
Q. ABO genotype for blood group AB?
IAIB.
Q. Human, bird and honey-bee sex systems?
XX/XY; ZZ/ZW; haplodiploidy.
Q. Chromosomal formulae for Down, Turner and Klinefelter syndromes?
Trisomy 21; 45,X; 47,XXY.
Q. Base-pairing rules in DNA?
A–T and G–C.
Q. Direction of DNA/RNA synthesis?
5′→3′.
Q. Start and stop codons?
AUG starts; UAA, UAG and UGA stop.
Q. Hardy–Weinberg genotype equation?
p² + 2pq + q² = 1.
Q. Five major forces that disturb Hardy–Weinberg equilibrium?
Mutation, recombination, natural selection, gene flow and genetic drift.
Q. What is adaptive radiation?
Diversification of one ancestral lineage into multiple forms adapted to different niches.
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