Basics of Genetics for NEET:Dive into the fundamentals of genetics, from Mendel’s laws to DNA replication, tailored for NEET 2025 preparation. Explore inheritance patterns, genetic disorders, and molecular biology in this detailed guide.
Basics of Genetics for NEET 2026
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Basics of Genetics for NEET 2025
Introduction to Genetics
Genetics is the branch of biology that explores how traits are passed from one generation to the next. It all begins with the fundamental molecule of life: DNA. Human DNA, if stretched out, measures about 2.2 meters in length.
This incredible length necessitates precise packaging within the tiny confines of a cell nucleus. DNA, or deoxyribonucleic acid, is inherently acidic due to its phosphate groups. To manage this long molecule, cells employ basic proteins called histones, which are rich in amino acids like lysine and arginine.Basics of Genetics
These histones form an octamer—a group of eight proteins—around which DNA winds tightly, creating structures known as nucleosomes. When fully condensed, these nucleosomes form chromosomes, the thread-like structures visible during cell division.
Chromosomes are highly condensed forms of DNA and proteins. They come in pairs: homologous chromosomes share the same gene loci, meaning they carry genes for the same traits at identical positions, often referred to as twin chromosomes. In contrast, non-homologous chromosomes have different gene arrangements.
At the center of each chromosome lies the centromere, a primary constriction that divides the chromosome into two arms: the shorter p-arm above and the longer q-arm below. Some chromosomes feature a secondary constriction, leading to a small rounded structure called satellite DNA, which consists of repeated sequences useful in DNA fingerprinting techniques.Genetic terms like characters and traits are essential. Basics of Genetics
A character is a broad category, such as height, skin color, or hair texture. Traits are the variations within these categories, like tall, short, or medium height. The genotype refers to the genetic makeup, while the phenotype is the observable expression. Alleles are alternative forms of a gene occupying the same locus on homologous chromosomes, and their combinations determine the genotype, ultimately influencing the phenotype.
Gamete Formation and Meiosis
Gametes, or sex cells, arise from germ cells through a process called meiosis. Germ cells are diploid (2n), containing two sets of chromosomes. Meiosis I and II reduce this to haploid (n), ensuring each gamete has one set.
This halving is crucial for maintaining chromosome numbers during sexual reproduction. The number of gamete types an individual can produce is calculated using 2^n, where n is the number of heterozygous gene pairs.
Probability in gamete formation can be determined using the fork-line method, which maps out possible combinations.Gametes fuse during fertilization to restore the diploid state in offspring.Basics of Genetics
Understanding this process is key for grasping inheritance patterns, as it sets the stage for genetic diversity through recombination and independent assortment.
Principles of Mendelian Genetics
Gregor Mendel, hailed as the father of genetics, conducted groundbreaking hybridization experiments from 1856 to 1863. He chose Pisum sativum, the garden pea, as his model organism due to its bisexual flowers, large sample sizes, short generation time, contrasting traits, and easy availability.
Mendel studied seven contrasting traits: seed shape (round/wrinkled), seed color (yellow/green), flower color (violet/white), flower position (axial/terminal), pod shape (inflated/constricted), pod color (green/yellow), and stem height (tall/dwarf).
These experiments laid the foundation for understanding inheritance. Mendel’s work was rediscovered in 1900 by de Vries, Correns, and von Tschermak, leading to the establishment of Mendelian principles.
Monohybrid Cross Explained
A monohybrid cross examines the inheritance of a single trait. Mendel used pure-breeding lines: homozygous dominant (TT for tall) and homozygous recessive (tt for dwarf). Crossing these produced an F1 generation that was all heterozygous (Tt) and phenotypically tall, with a genotypic and phenotypic ratio of 4:0.Self-pollinating the F1 generation yielded the F2, with a genotypic ratio of 1:2:1 (TT:Tt:tt) and phenotypic ratio of 3:1 (tall:dwarf).
From this, Mendel derived two laws for monohybrid crosses. The Law of Dominance states that in a heterozygous state, the dominant allele expresses itself fully, while the recessive is masked. Alleles always occur in pairs, and Mendel referred to them as “factors,” now known as genes.
The Law of Segregation, or the Law of Purity of Gametes, explains that alleles separate during gamete formation, ensuring each gamete receives one allele. This non-blending nature means traits don’t mix but segregate cleanly.Basics of Genetics
Back Cross and Test Cross
A back cross involves crossing the F1 generation with one of the parents. A test cross specifically pairs the F1 with the homozygous recessive parent to reveal the unknown genotype. If all offspring show the dominant trait, the tested individual is homozygous dominant; a 1:1 ratio indicates heterozygosity.
Test crosses are invaluable in breeding and genetic analysis.An outcross, meanwhile, introduces heterozygosity to combat inbreeding depression. Notably, all test crosses are back crosses, but not vice versa.
Dihybrid Cross and Independent Assortment
Dihybrid crosses study two traits simultaneously. Using pure lines like round yellow seeds (RRYY) and wrinkled green (rryy), the F1 is all heterozygous (RrYy) and round yellow. Selfing produces an F2 with a phenotypic ratio of 9:3:3:1 (9 round yellow, 3 round green, 3 wrinkled yellow, 1 wrinkled green).
The genotypic ratio is more complex: 1:2:1:2:4:2:1:2:1.This reveals parental and recombinant combinations, where new trait pairings emerge due to crossing over. Mendel’s third law, the Law of Independent Assortment, asserts that genes for different traits segregate independently, provided they are on different chromosomes.Linkage is an exception to this law.Basics of Genetics
Linked genes on the same chromosome tend to inherit together, reducing recombination. The farther apart genes are, the higher the recombination frequency, which can be mapped to create genetic linkage maps.
Exceptions to Mendelism
Mendelism isn’t absolute; several exceptions exist. Incomplete dominance occurs when the dominant allele partially expresses in heterozygotes, leading to an intermediate phenotype. In snapdragons or four o’clock plants, red and white flowers cross to produce pink F1, with F2 ratios of 1:2:1 for both genotype and phenotype (red:pink:white).Codominance involves both alleles expressing equally in heterozygotes.
Multiple allelism, where more than two alleles control a trait, is seen in human ABO blood groups with alleles IA, IB, and i. This yields six genotypes and four phenotypes: A, B, AB (codominant), and O.Pleiotropy is when one gene affects multiple traits.Basics of Genetics
Phenylketonuria (PKU), an autosomal recessive disorder, exemplifies this: a mutation in the phenylalanine hydroxylase gene leads to phenylalanine accumulation, causing skin pigmentation issues, mental retardation, and more.Polygenic inheritance, the opposite, involves multiple genes controlling one trait, like human skin color or height, resulting in a bell-shaped distribution curve.
Genetic Disorders in Detail
Genetic disorders arise from mutations or chromosomal abnormalities. Sickle cell anemia, an autosomal recessive condition, stems from a point mutation replacing glutamic acid with valine in the beta-globin chain, distorting red blood cells into sickle shapes and impairing oxygen transport.
Chromosomal changes include polyploidy (extra full sets of chromosomes, common in plants) and aneuploidy (gain or loss of one or two chromosomes). Aberrations like deletions, duplications, inversions, or translocations alter gene order.
Other disorders: Hemophilia (X-linked recessive) lacks clotting factors 8 or 9, causing excessive bleeding. Color blindness (X-linked recessive) affects red-green vision. Thalassemia (autosomal recessive) reduces hemoglobin production. Basics of Genetics
Down syndrome (trisomy 21) features intellectual disability and distinct facial traits. Klinefelter syndrome (XXY) results in sterile males with feminine traits. Turner syndrome (XO) causes sterile females with underdeveloped ovaries.
Chromosomal Theory and Sex Determination
The chromosomal theory of inheritance, proposed by Sutton and Boveri, was experimentally validated by Morgan using Drosophila melanogaster. Morgan, the father of modern genetics, demonstrated gene linkage and sex-linked inheritance.
In females, one X chromosome condenses into a Barr body (n-1 formula, where n is X count), inactivating it. Sex determination varies: humans use XX (female, homogametic) and XY (male, heterogametic); grasshoppers XX (female) and XO (male); birds ZW (female) and ZZ (male); bees employ haplodiploidy, with diploid queens and workers, haploid drones via parthenogenesis.Basics of Genetics
Molecular Basis of Inheritance
Frederick Miescher discovered nuclein in 1869, later renamed nucleic acid by Altman. Nucleic acids include DNA (stable, genetic material in most organisms) and RNA (unstable, genetic in some viruses like HIV).DNA components: beta-deoxyribose sugar (lacking oxygen at 2′ position), nitrogenous bases (purines: adenine, guanine; pyrimidines: cytosine, thymine in DNA or uracil in RNA), and phosphate groups conferring negative charge.Basics of Genetics
Nucleosides are base + sugar (glycosidic bond); nucleotides add phosphate (phosphoester bond). Polynucleotides link via phosphodiester bonds.X-ray crystallography by Wilkins and Franklin revealed DNA’s helical structure with 3.4 Å periodicity. Watson and Crick’s double helix model shows antiparallel strands (5′-3′ and 3′-5′), with A-T (two H-bonds) and G-C (three H-bonds).
It’s right-handed.Chargaff’s rules: purines equal pyrimidines (A=T, G=C). DNA types include A, B (common), C, D, E, and left-handed Z-DNA.Central dogma (Crick): DNA → RNA → protein. Reverse transcription (RNA → DNA) occurs in retroviruses via reverse transcriptase.
DNA packaging: winds around histone octamers into nucleosomes, forming “beads on a string” chromatin, condensing into solenoids and chromosomes. Euchromatin is loosely packed, transcriptionally active, lightly stained; heterochromatin is densely packed, inactive, darkly stained.Basics of Genetics
Search for Genetic Material
Griffith’s 1928 transformation experiment with Streptococcus pneumoniae showed a “transforming principle” converting non-virulent R strain to virulent S when mixed with heat-killed S.Avery, MacLeod, and McCarty (1944) biochemically identified it as DNA: only DNA from heat-killed S transformed R.Hershey and Chase’s 1952 blender experiment provided decisive proof. Using P32-labeled DNA and S35-labeled protein in T2 phage infecting E. coli, they found P32 inside bacteria post-infection, confirming DNA as genetic material.Basics of Genetics
RNA World and DNA Replication
The RNA world hypothesis posits RNA as the first biomolecule, acting as both catalyst (ribozymes) and genetic material. DNA evolved for stability: double-stranded, thymine (more stable than uracil), no 2′ oxygen.Genetic material must be stable, express Mendelian traits, allow mutations, and replicate.Basics of Genetics
DNA replication is semi-conservative, proven by Meselson and Stahl using N14/N15 isotopes in E. coli: new DNA has one old and one new strand.Steps: Topoisomerase relaxes supercoils; helicase unwinds; SSBPs stabilize; primase synthesizes RNA primers; DNA polymerase adds nucleotides 5′-3′; leading strand continuous, lagging forms Okazaki fragments joined by ligase.Basics of Genetics
Transcription Process
Transcription copies DNA to RNA. A transcription unit includes promoter, structural gene, terminator. Template strand (3′-5′) is transcribed; coding strand (5′-3′) matches RNA sequence. Eukaryotes have monocistronic genes (one gene, one protein); prokaryotes polycistronic (multiple).