🎯 Key Points
- Law of Segregation: two alleles separate during gamete formation; Law of Independent Assortment: genes on DIFFERENT chromosomes are inherited independently
- Monohybrid cross (Tt×Tt) → 3:1 phenotypic ratio; Dihybrid cross → 9:3:3:1 ratio
- DNA replication is semi-conservative (proven by Meselson-Stahl); leading strand continuous, lagging strand made of Okazaki fragments
- Transcription (DNA→mRNA, nucleus) then Translation (mRNA→protein, ribosome); start codon AUG, stop codons UAA/UAG/UGA
- Incomplete dominance = blending (pink snapdragon); Co-dominance = BOTH alleles fully expressed (AB blood group) — different from blending
- Pleiotropy = ONE gene affects MULTIPLE traits (e.g. sickle cell); Polygenic = MULTIPLE genes affect ONE trait (e.g. height) — opposite relationships, easy to confuse
Mendel's Laws
Punnett square for a monohybrid cross between two heterozygous (Tt) parents, showing the 3:1 phenotypic ratio predicted by Mendel's Law of Segregation.
- Law of Segregation: each organism carries two alleles; these separate during gamete formation
- Law of Independent Assortment: genes for different traits are inherited independently (if on different chromosomes)
DNA Structure
- Double helix (Watson and Crick, 1953; X-ray crystallography by Franklin)
- Base pairs: A-T (2 H bonds), G-C (3 H bonds)
- Deoxyribose sugar + phosphate backbone; antiparallel strands (5' to 3')
- Chargaff's rule: A=T, G=C in any DNA
DNA Replication
- Semi-conservative (Meselson and Stahl experiment)
- Helicase unwinds; primase adds RNA primer; DNA polymerase adds nucleotides 5' to 3'
- Leading strand: continuous; lagging strand: Okazaki fragments
Protein Synthesis
- Transcription: DNA → mRNA (in nucleus). RNA polymerase reads template 3'→5', builds mRNA 5'→3'
- Translation: mRNA → protein (at ribosome). tRNA brings amino acids
- Start codon: AUG (methionine); Stop: UAA, UAG, UGA
- Gene expression regulation: promoters, enhancers, lac operon (Jacob-Monod model)
Extensions of Mendelism
- Incomplete dominance: blending (red + white = pink in snapdragon)
- Co-dominance: both alleles expressed (AB blood type)
- Sex-linked traits: X-linked (colour blindness, haemophilia)
- Polygenic inheritance: multiple genes control one trait (skin colour, height)
- Pleiotropy: one gene affects multiple traits (PKU, sickle cell)
Mendel's Laws of Inheritance
- Law of Dominance: characters are controlled by discrete units called factors (genes) that occur in pairs; in a dissimilar pair one factor is dominant (expressed) and the other recessive (masked) — explains the 3:1 F2 ratio
- Law of Segregation (Purity of Gametes): the two alleles of a pair separate during gamete formation so each gamete carries only one allele — universally valid, no exceptions
- Law of Independent Assortment: alleles of one gene assort independently of alleles of another gene (valid only for genes on different chromosomes) — basis of the 9:3:3:1 dihybrid ratio
- Mendel chose garden pea (Pisum sativum): 7 pairs of contrasting traits, easy to grow, self- and cross-pollinable, short life cycle, many offspring
Multiple Alleles and ABO Blood Groups
- Multiple alleles: a gene existing in more than two allelic forms in a population, though any one diploid individual carries only two; the I gene for ABO blood groups has three alleles — IA, IB and i
- IA and IB are codominant to each other; both are dominant over i (recessive)
- IA adds the sugar N-acetylgalactosamine and IB adds galactose to the RBC surface antigen; i adds no sugar
| Blood group (phenotype) | Genotype(s) | Antigen on RBC |
|---|---|---|
| A | IAIA, IAi | A |
| B | IBIB, IBi | B |
| AB | IAIB | A and B (codominance) |
| O | ii | none |
Linkage, Recombination and the Chromosomal Theory
- Chromosomal Theory of Inheritance (Sutton and Boveri): genes are located on chromosomes; the behaviour of chromosomes during meiosis (pairing, segregation) parallels Mendel's factors
- Linkage (T. H. Morgan, in Drosophila): genes located close together on the same chromosome tend to be inherited together and do NOT assort independently, producing fewer recombinants than expected
- Recombination: crossing over during meiosis produces new (non-parental) allele combinations; tightly linked genes recombine rarely, loosely linked genes recombine more often
- Genetic maps: recombination frequency is proportional to the distance between genes (measured in map units / centimorgans) — used by Alfred Sturtevant to map genes on a chromosome
Sex Determination
- XX-XY type (humans, Drosophila): females XX (homogametic), males XY (heterogametic); the male gamete decides the sex of the offspring (50:50 ratio)
- XX-XO type (grasshoppers, many insects): females XX, males XO (single X, no Y); males produce two kinds of sperm (with or without X)
- ZW-ZZ type (birds, some reptiles): females ZW (heterogametic), males ZZ (homogametic) — here the female gamete decides the sex
- Haplodiploidy (honeybees): females (queen, workers) develop from fertilised eggs (diploid), males (drones) from unfertilised eggs (haploid) by parthenogenesis
Mutation
- Mutation: a sudden, heritable change in the DNA sequence or chromosome; a source of variation and raw material for evolution
- Gene (point) mutation: change in a single base pair — e.g. the substitution causing sickle-cell anaemia (GAG→GTG, so Glu→Val at position 6 of the beta-globin chain)
- Frameshift mutations: insertion or deletion of bases shifts the reading frame, altering all downstream codons
- Chromosomal aberrations: deletion, duplication, inversion or translocation of chromosome segments
- Mutagens: physical (X-rays, UV, gamma rays) and chemical (mustard gas, nitrous acid) agents that induce mutations
Genetic Disorders
Mendelian disorders arise from mutation in a single gene, whereas chromosomal disorders arise from the absence, excess or abnormal arrangement of chromosomes (aneuploidy or polyploidy).
| Disorder | Type / cause | Key feature |
|---|---|---|
| Haemophilia | X-linked recessive (Mendelian) | Blood fails to clot; affects mostly males |
| Sickle-cell anaemia | Autosomal recessive (Mendelian); HbS | Glu→Val at position 6 of beta-globin; sickle-shaped RBCs |
| Thalassemia | Autosomal recessive (Mendelian) | Reduced/absent globin-chain synthesis; anaemia |
| Phenylketonuria (PKU) | Autosomal recessive (Mendelian) | Enzyme lacking; phenylalanine accumulates → mental retardation |
| Down syndrome | Trisomy 21 (chromosomal) | Extra copy of chromosome 21; short stature, characteristic features |
| Klinefelter syndrome | 47, XXY (chromosomal) | Sterile male with an extra X; some feminine features |
| Turner syndrome | 45, X0 (chromosomal) | Sterile female with one X missing; short stature |
Note: the HbS allele shows codominance in the heterozygote (HbA HbS = sickle-cell trait / carrier), while sickle-cell anaemia is also a classic example of both a point mutation and pleiotropy.
Dihybrid Cross and Independent Assortment
- A dihybrid cross follows two traits together — e.g. seed shape (round R dominant, wrinkled r) and seed colour (yellow Y dominant, green y); the parents RRYY x rryy give an all round-yellow F1 (RrYy)
- Selfing the F1 (RrYy x RrYy) gives the classic 9:3:3:1 F2 phenotypic ratio — 9 round-yellow : 3 round-green : 3 wrinkled-yellow : 1 wrinkled-green
- The appearance of the two recombinant (parental-unlike) types, round-green and wrinkled-yellow, shows the two genes assort independently — the basis of Mendel's Law of Independent Assortment
- The F1 dihybrid produces four kinds of gametes (RY, Ry, rY, ry) in equal proportion, which combine to fill the 4 x 4 = 16 boxes of the dihybrid Punnett square
Test Cross and Back Cross
- Back cross: crossing an F1 individual back with either of its parents
- Test cross: a special back cross of an individual showing the dominant phenotype (unknown genotype, TT or Tt) with a homozygous recessive (tt) to reveal that unknown genotype
- If the test-cross offspring are ALL dominant (all tall), the parent was homozygous (TT); a 1:1 ratio of dominant to recessive offspring shows the parent was heterozygous (Tt)
- A dihybrid test cross (RrYy x rryy) gives a 1:1:1:1 ratio, directly revealing the four gamete types and confirming independent assortment
Pedigree Analysis
- A pedigree is a chart of a family tree showing the inheritance of a trait across generations, used where controlled crosses are impossible (as in humans)
- Standard symbols: a square = male, a circle = female, a filled/shaded symbol = an affected individual, a horizontal line joining a square and circle = a mating, and Roman numerals label the generations
- Autosomal recessive traits can skip generations and appear in offspring of unaffected carrier parents (e.g. sickle-cell anaemia); autosomal dominant traits tend to appear in every generation
- X-linked recessive traits (haemophilia, colour blindness) affect males far more often, typically passing from a carrier mother to her sons
- Pedigree analysis reveals whether a trait is dominant or recessive, autosomal or sex-linked, and helps predict the risk of a disorder in future children (used in genetic counselling)
🚀 NEET Advanced Edge
Dihybrid cross ratios and gene linkage: A standard dihybrid cross (AaBb×AaBb) gives 9:3:3:1 ONLY if the two genes are on different chromosomes (independent assortment). If the genes are LINKED (close together on the same chromosome), the ratio skews toward the parental combinations, with recombinant types appearing less frequently — recombination frequency is used to map gene distances.
Test cross to determine genotype: Crossing an organism of unknown genotype (could be TT or Tt) with a homozygous recessive (tt) reveals the unknown genotype from the offspring ratio — all tall offspring means TT, a 1:1 tall:short ratio means Tt. This is the standard method for resolving genotype ambiguity behind an identical phenotype.
Worked problem: In a dihybrid cross for seed shape (round R, wrinkled r) and colour (yellow Y, green y), RrYy × RrYy is selfed. Find the proportion of round, green seeds. Approach: Round (R_) = 3/4; green (yy) = 1/4. Since independently assorted, round-green = 3/4 × 1/4 = 3/16.