Protochordates are simple marine chordates without a vertebral column or a cranium. They show the three basic chordate characters – notochord, dorsal hollow nerve cord and pharyngeal gill slits – at least at some stage. They are divided into Urochordata and Cephalochordata (Hemichordata is studied with them in many syllabi). This topic covers their characters, examples and the significance of retrogressive metamorphosis.
Key points
Chordate characters: (1) a rod-like notochord on the dorsal side; (2) a dorsal, hollow, tubular nerve cord; (3) pharyngeal gill slits; also a post-anal tail in many.
Protochordates are also called Acraniata (no skull) and lower chordates.
All are marine; most are small.
Hemichordata (Balanoglossus, tongue worm): body has proboscis, collar and trunk. The "stomochord" in the proboscis was once thought to be a notochord, so it was placed in Chordata; now it is usually treated as a separate non-chordate phylum. Larva: tornaria (similar to echinoderm larvae).
Urochordata (Tunicata) (Herdmania, Ascidia, Doliolum, Salpa): notochord only in the tail of the larva. Adult is sac-like, sessile, covered by a tunic made of tunicin (a cellulose-like substance).
Cephalochordata (Branchiostoma, formerly Amphioxus, the lancelet): notochord extends from head to tail and persists throughout life. All three chordate characters are present in the adult.
Branchiostoma is fish-shaped, burrows in sand, and feeds by filter feeding (ciliary feeding).
Herdmania larva (ascidian tadpole) is free-swimming with all chordate characters; it settles and loses notochord, tail and most of the nerve cord. This is retrogressive metamorphosis.
Important terms
Notochord – flexible, rod-like supporting structure of vacuolated cells, lying dorsal to the gut.
Endostyle – ciliated, glandular groove on the pharynx floor that makes mucus to trap food; thought to be homologous to the thyroid gland.
Atrium – cavity around the pharynx into which water passes through gill slits.
Tunic (test) – outer covering of tunicates.
Myotomes – V-shaped (<-shaped) muscle blocks of Branchiostoma.
Retrogressive metamorphosis – change from an advanced larva to a simpler, degenerate adult.
Key facts
Feature
Urochordata
Cephalochordata
Notochord
Only in larval tail
Whole length, throughout life
Nerve cord
Only in larva; adult has a ganglion
Present throughout life
Body covering
Tunic
Thin cuticle and epidermis
Habit
Mostly sessile adults
Free-swimming / burrowing
Example
Herdmania, Salpa
Branchiostoma
Herdmania pallida is a common solitary ascidian on the Indian coast.
5 important questions
Q1. Name the three fundamental characters of chordates.
A dorsal notochord, a dorsal hollow tubular nerve cord, and paired pharyngeal gill slits. They are present at least at some stage of the life cycle.
Q2. What is an endostyle?
The endostyle is a ciliated, mucus-secreting groove on the ventral side of the pharynx in protochordates. It traps food particles in mucus. It is considered the forerunner of the thyroid gland of vertebrates.
Q3. Explain retrogressive metamorphosis in Herdmania.
Larva is tadpole-like, free-swimming with notochord, nerve cord, gill slits and tail.
Larva attaches by adhesive papillae.
Tail with notochord is absorbed; nerve cord reduces to a ganglion.
Pharynx enlarges with many gill slits; body rotates; tunic forms.
Adult is sessile and simpler: an advanced larva becomes a degenerate adult.
Q4. Describe the general characters of Branchiostoma.
Laterally compressed, fish-like, both ends pointed.
Notochord and nerve cord along the whole body.
Many gill slits opening into an atrium; atriopore.
Oral hood with buccal cirri; wheel organ; filter feeding.
Myotomes; no paired fins; no heart (contractile vessels).
Cyclostomes are the most primitive living vertebrates. They have no jaws and no paired fins, and their round mouth works like a sucker. Lampreys (Petromyzon) and hagfishes (Myxine) are the living examples. This topic covers the general characters of Cyclostomata, the external features and habits of Petromyzon, its ammocoete larva, and the systematic position of the group.
Key points
Cyclostomata means "round mouth" (cyclos = circle, stoma = mouth). They belong to the superclass Agnatha (jawless vertebrates).
Body is long and eel-like, soft and without scales; skin is slimy with mucous glands.
No jaws; the mouth is a circular suctorial buccal funnel with horny epidermal teeth. A rasping tongue scrapes flesh.
No paired fins; only median dorsal and caudal fins.
Notochord persists throughout life; vertebrae are only small cartilaginous rudiments (arcualia).
Skeleton is entirely cartilaginous; cranium is incomplete.
Single median nostril (monorhinous) leading to a hypophyseal sac.
Gills in pouches: Petromyzon has 7 pairs of external gill openings.
Heart is two-chambered (one auricle, one ventricle) with a sinus venosus.
Only two semicircular canals (lamprey) or one (hagfish) in each internal ear.
Lampreys are ectoparasites on fishes: they attach with the buccal funnel, rasp the skin and suck blood. Saliva contains an anticoagulant (lamphredin).
Sea lampreys are anadromous: they live in the sea and migrate to rivers to breed, then die.
Important terms
Agnatha – vertebrates without jaws.
Buccal funnel – cup-shaped sucker around the mouth.
Ammocoete larva – blind, filter-feeding larva of Petromyzon living in river mud.
Endostyle – ciliated groove in the pharynx of the ammocoete; it becomes the thyroid gland in the adult.
Anadromous – migrating from sea to fresh water for breeding.
Key facts
Feature
Petromyzon (lamprey)
Myxine (hagfish)
Class
Petromyzontia
Myxini
Habitat
Sea and fresh water
Only marine
Buccal funnel
Present
Absent; tentacles around mouth
Gill openings
7 pairs
1 pair (in Myxine)
Eyes
Well developed
Degenerate
Development
Indirect, ammocoete larva
Direct
The ammocoete looks like Amphioxus: it has a notochord, endostyle, filter feeding with cilia, and no jaws. After 3–7 years it metamorphoses into the adult lamprey.
5 important questions
Q1. Why are cyclostomes called jawless vertebrates?
They have a cranium and vertebral rudiments, so they are vertebrates. But they have no jaws; instead they have a round sucking mouth with horny teeth. So they are placed in Agnatha, the jawless vertebrates.
Q2. What is the ammocoete larva? Why is it important?
It is the blind, worm-like, filter-feeding larva of Petromyzon that lives burrowed in river mud. It resembles Amphioxus, and its endostyle becomes the thyroid gland, which links protochordates and vertebrates.
Q3. Give the general characters and classification of Cyclostomata.
Jawless, eel-like, scaleless body.
Suctorial mouth with horny teeth; no paired fins.
Persistent notochord; cartilaginous skeleton.
Single nostril; 1–16 pairs of gill pouches.
Two-chambered heart; two or one semicircular canals.
Classes Petromyzontia and Myxini with examples.
Q4. Describe the external features and parasitic habits of Petromyzon.
Size, shape and divisions of body (head, trunk, tail).
Fishes are cold-blooded aquatic vertebrates that breathe by gills and swim with fins. They are divided into cartilaginous fishes (Chondrichthyes) and bony fishes (Osteichthyes). This topic covers general characters, classification, scales, swim bladder, migration and parental care, with Indian examples.
Key points
Fishes (मासे) are poikilothermic (cold-blooded) and live in fresh water and the sea.
Body is usually streamlined and covered by scales.
Paired (pectoral, pelvic) and unpaired (dorsal, anal, caudal) fins help in swimming and balance.
Respiration by gills; a few also breathe air (lungfish, Clarias, Anabas).
Heart is two-chambered (one auricle and one ventricle) and is called a venous heart because it pumps only deoxygenated blood.
Lateral line system senses water movement and pressure.
Kidneys are mesonephric.
Most are oviparous with external fertilisation.
Important terms
Placoid scales – tooth-like scales of cartilaginous fishes (sharks).
Lung fishes (Dipnoi): Protopterus, Lepidosiren, Neoceratodus – show links to amphibians.
Indian major carps: rohu, catla, mrigal – widely cultured.
5 important questions
Q1. Why is the fish heart called a venous heart?
The fish heart receives only deoxygenated (venous) blood from the body and pumps it to the gills for oxygenation. It never carries oxygenated blood, so it is called a venous heart.
Q2. What is the function of the swim bladder?
The swim bladder is a gas-filled sac in bony fishes. It helps keep the fish at a desired depth by changing the amount of gas (hydrostatic organ). In some fishes it also helps in breathing or making sound.
Q3. Describe the different types of scales in fishes.
Placoid: dermal, tooth-like with basal plate and spine; sharks.
Ganoid: rhomboid, covered with ganoin; Lepisosteus.
Cycloid: thin, circular, smooth edge; rohu.
Ctenoid: thin, comb-like teeth on hind margin; perch.
Draw one sketch of each.
Q4. Write a note on migration in fishes.
Regular movement from one place to another for breeding, feeding or climate.
Amphibians are cold-blooded vertebrates that live both in water and on land. They were the first vertebrates to move onto land. Frogs, toads, salamanders and caecilians are amphibians. This topic covers general characters, classification, the frog as a type, metamorphosis and parental care.
Key points
Amphibia means "double life" (amphi = both, bios = life). Frog is बेडूक in Marathi.
Poikilothermic; show hibernation (winter sleep) and aestivation (summer sleep).
Skin is moist, glandular and without scales (except in some caecilians); used in respiration.
Two pairs of pentadactyl limbs (limbless in caecilians).
Respiration: gills in larvae; lungs, skin (cutaneous) and buccal lining (buccopharyngeal) in adults.
Heart is three-chambered – two auricles and one ventricle.
Cloaca present: common opening for digestive, excretory and reproductive tracts.
Kidneys mesonephric; adult frog is ureotelic (excretes urea); tadpole is ammonotelic.
Two occipital condyles on the skull.
Eggs laid in water; fertilisation mostly external in frogs.
Development includes a tadpole larva and metamorphosis controlled by thyroxine.
Important terms
Metamorphosis – change from tadpole to adult: loss of tail and gills, growth of limbs and lungs.
Nictitating membrane – thin, transparent third eyelid protecting the eye in water.
Tympanum – eardrum seen as a round area behind the eye.
Amplexus – mating embrace in frogs; male clasps female so eggs and sperm are released together.
Nuptial pad – rough pad on the first finger of the male frog in the breeding season.
Vocal sacs – sacs in male frogs that make loud croaking.
Neoteny – retention of larval features in a sexually mature animal (e.g. axolotl larva of Ambystoma).
Key facts
Order
Features
Example
Apoda (Gymnophiona)
Limbless, worm-like, burrowing
Ichthyophis, Uraeotyphlus
Urodela (Caudata)
Tail present throughout life
Salamander, newt, Ambystoma
Anura (Salientia)
Tailless adults, long hind legs
Frog (Rana, Euphlyctis), toad (Bufo), tree frog (Hyla)
The Indian bullfrog is Hoplobatrachus tigerinus (old name Rana tigrina).
Frog's tongue is bifid (two-lobed) and attached at the front – flicked out to catch insects.
Frog RBCs are nucleated and oval.
5 important questions
Q1. What is the difference between hibernation and aestivation?
Hibernation is a period of inactivity during winter to avoid cold. Aestivation is a period of inactivity during summer to avoid heat and dryness. Frogs show both, burying themselves in mud.
Q2. What is neoteny?
Neoteny is the condition in which a larva becomes sexually mature and reproduces without completing metamorphosis. Example: axolotl larva of Ambystoma; it is linked with low thyroxine or iodine.
Q3. Describe the metamorphosis of frog.
Eggs → tadpole with external gills, tail, no limbs; herbivorous.
Internal gills covered by operculum; then hind limbs, then forelimbs appear.
Lungs develop; gills disappear.
Tail is absorbed (by lysosomal enzymes).
Gut shortens; becomes carnivorous; ureotelic excretion.
Controlled by thyroxine; iodine needed.
Q4. Give the general characters of Amphibia.
Dual life; cold-blooded.
Moist glandular skin; cutaneous respiration.
Three-chambered heart.
Two occipital condyles; cloaca.
Pentadactyl limbs (absent in Apoda).
Eggs in water; tadpole larva; metamorphosis.
Q5. Explain parental care in amphibians.
Alytes (midwife toad): male carries egg strings on hind legs.
Pipa (Surinam toad): eggs develop in pockets on female's back.
Rhinoderma: male keeps eggs in vocal sac.
Ichthyophis: female coils around eggs.
Tree frogs make foam nests.
Purpose: protect eggs and young from predators and drying.
Exam tip
Remember the three orders with one Indian example each, and that the frog heart is three-chambered. Parental care examples are frequently asked.
Reptiles are cold-blooded, mostly terrestrial vertebrates with dry, scaly skin. They were the first vertebrates fully adapted to life on land, because they lay shelled (cleidoic) eggs with protective membranes. Lizards, snakes, turtles, tortoises and crocodiles are reptiles. This topic covers characters, classification, poisonous and non-poisonous snakes, and the snake bite and venom.
Key points
Reptilia means "creeping" (repere = to creep). Snake is साप in Marathi.
Poikilothermic; skin dry, without glands, covered by epidermal scales or scutes.
Lungs are the only respiratory organs.
Heart is three-chambered (two auricles, partly divided ventricle); in crocodiles it is four-chambered.
The "Big Four" venomous snakes of India: Indian cobra (Naja naja), common krait (Bungarus caeruleus), Russell's viper (Daboia russelii) and saw-scaled viper (Echis carinatus).
Neurotoxic venom (cobra, krait) affects the nervous system – breathing failure. Haemotoxic venom (vipers) affects blood and blood vessels – bleeding, swelling.
5 important questions
Q1. What is a cleidoic egg?
A cleidoic egg is a large-yolked egg covered by a protective shell, with membranes like amnion and allantois. It does not dry out on land, so reptiles and birds can breed away from water.
Q2. Name the Big Four venomous snakes of India.
Indian cobra (Naja naja), common krait (Bungarus caeruleus), Russell's viper (Daboia russelii) and saw-scaled viper (Echis carinatus).
Q3. Distinguish between poisonous and non-poisonous snakes.
Poisonous: fangs and poison glands present; head often triangular (vipers); ventral scales broad, covering full width.
Krait: large hexagonal vertebral scales along the back; cobra: hood.
Non-poisonous: no fangs; ventral scales may be narrow (python); tail gradually tapering.
Note: these rules have exceptions; treat every bite seriously.
Q4. Give the general characters of Reptilia.
Dry scaly skin; no glands.
Lungs only; cold-blooded.
Three-chambered heart (four in crocodiles).
One occipital condyle.
Uricotelic; metanephric kidneys.
Internal fertilisation; cleidoic eggs; amniotes.
Q5. Write a note on snake venom and first aid for snake bite.
Venom is modified saliva from modified salivary glands; mixture of proteins and enzymes.
First aid: keep patient calm and still; immobilise the bitten limb; remove rings and tight items.
Do not cut, suck or apply ice; avoid tight tourniquets.
Take to hospital quickly for antivenom (polyvalent ASV in India).
Exam tip
Learn the Big Four with scientific names, and the type of venom of each. Draw a sketch of the head of a poisonous snake showing fangs and poison gland.
Birds (Aves) are warm-blooded vertebrates with feathers, and forelimbs modified into wings. Most can fly, and their whole body is adapted for flight. This topic covers general characters, flight adaptations, types of feathers, migration and the classification of birds, with Archaeopteryx as a connecting link.
Key points
Birds (पक्षी) are the only animals with feathers.
Homeothermic (warm-blooded): body temperature stays constant.
Forelimbs are wings; hind limbs are for walking, perching or swimming; the hind limb has scales.
Beak without teeth (in modern birds).
Pneumatic bones – light, hollow bones with air spaces.
Heart is four-chambered; only the right aortic arch remains.
Lungs are compact and connected to air sacs; air flows through lungs in one direction, giving very efficient breathing.
Syrinx is the voice box (at the base of the trachea).
Digestive tract has a crop (stores food) and a gizzard (grinds food).
No urinary bladder; uricotelic.
One occipital condyle; oviparous with large cleidoic eggs; parental care well developed.
Usually only the left ovary and oviduct are functional in females.
Important terms
Contour feathers (quill feathers) – large feathers giving shape; flight feathers are remiges (wings) and rectrices (tail).
Down feathers (plumules) – soft feathers that trap air for warmth.
Filoplumes – hair-like feathers.
Keel (carina) – ridge on the breast bone (sternum) for attachment of large flight muscles.
Synsacrum – fused bones of the lower back and pelvis giving rigid support.
Pygostyle – fused last tail vertebrae.
Furcula (wishbone) – fused clavicles.
Key facts
Ratitae (flightless running birds): no keel – ostrich, emu, kiwi, rhea.
Archaeopteryx (Jurassic fossil, found in Germany): has feathers and wings like birds, and teeth, a long bony tail and clawed fingers like reptiles. It is a connecting link between reptiles and birds.
Birds evolved from reptilian (theropod dinosaur) ancestors; they have scales on their legs as a reptilian feature.
Migration – seasonal, two-way, long-distance movement for breeding and food. India receives winter migrants such as the Siberian crane (historically at Bharatpur), flamingos and many ducks. Birds navigate using the sun, stars, Earth's magnetic field and landmarks.
5 important questions
Q1. What are pneumatic bones?
Pneumatic bones are hollow bones filled with air spaces connected to air sacs. They make the skeleton light, which helps birds in flight.
Q2. Why is Archaeopteryx called a connecting link?
Archaeopteryx shows both reptile features (teeth, long bony tail, clawed fingers) and bird features (feathers, wings, beak-like jaws). Hence it links reptiles and birds.
Q3. Describe flight adaptations in birds.
Streamlined, spindle-shaped body; feathers.
Forelimbs modified into wings.
Light, pneumatic bones; keel for flight muscles; synsacrum, pygostyle.
Air sacs and efficient lungs; four-chambered heart; high metabolism.
Toothless beak; no urinary bladder; one ovary – reduce weight.
Good eyesight; large cerebellum for balance.
Q4. Describe the structure of a contour (quill) feather.
Central shaft: lower hollow calamus (quill) and upper solid rachis.
Vane on both sides made of barbs.
Each barb bears barbules with hooklets that interlock.
Superior and inferior umbilicus on the calamus; after-feather near the superior umbilicus.
Chromosomes are thread-like bodies in the nucleus that carry genes. Each species has a fixed number and shape of chromosomes. This topic covers the parts of a chromosome, types based on centromere position, karyotype and idiogram, and two special giant chromosomes – polytene and lampbrush chromosomes.
Key points
The word chromosome ("coloured body") was coined by Waldeyer (1888).
A metaphase chromosome has two identical sister chromatids joined at the centromere (primary constriction).
The kinetochore on the centromere is where spindle fibres attach.
Some chromosomes have a secondary constriction; the part beyond it is the satellite (SAT chromosome). The secondary constriction may carry the nucleolar organiser region (NOR).
Telomeres are the protective ends of the chromosome; they stop ends from sticking together.
Chromosomes are made of DNA, histone and non-histone proteins and a little RNA.
Autosomes control body characters; sex chromosomes (X, Y) decide sex. Humans have 22 pairs of autosomes and 1 pair of sex chromosomes (2n = 46).
Types by centromere position:metacentric (centre, V-shaped at anaphase), submetacentric (slightly off-centre, L-shaped), acrocentric (near one end, J-shaped) and telocentric (at the tip, rod-shaped; not found in humans).
Karyotype is the set of chromosomes of a species arranged by size and shape; its diagram is an idiogram.
Polytene chromosomes are found in the salivary glands of dipteran larvae (Drosophila, Chironomus). They were discovered by E. G. Balbiani (1881).
They form by repeated DNA replication without cell division (endoreduplication), giving many parallel strands (about 1,000 in Drosophila). Homologues stay paired (somatic pairing).
They show dark bands and light interbands. Swollen active regions called puffs or Balbiani rings are sites of active RNA synthesis.
Lampbrush chromosomes are seen in the diplotene stage of meiosis in oocytes of amphibians, fishes, reptiles and birds. They were first seen by Flemming (1882) and named by Rückert (1892).
They are very long; paired loops extend sideways from a central axis, giving a "lamp-brush" look. The loops are sites of active transcription, making RNA for the growing egg.
Important terms
Chromomere – bead-like thickening along a chromosome.
Endoreduplication (endomitosis) – DNA copies itself repeatedly without cell division.
Chromocentre – point where centromeres of all polytene chromosomes join in Drosophila.
Arm ratio – length of long arm ÷ length of short arm.
Heteropycnosis – darker or lighter staining of some chromosome regions.
Key facts
Feature
Polytene
Lampbrush
Discovered by
Balbiani (1881)
Flemming (1882); named by Rückert
Found in
Salivary glands of dipteran larvae
Oocytes of vertebrates
Stage
Interphase
Diplotene of meiosis I
Structure
Many strands, bands and puffs
Central axis with paired loops
Activity sites
Puffs / Balbiani rings
Loops
5 important questions
Q1. What are telomeres?
Telomeres are the specialised ends of chromosomes made of short repeated DNA sequences. They protect the ends from damage and stop chromosomes from joining each other. They shorten with every cell division.
Q2. What are Balbiani rings?
They are very large puffs on polytene chromosomes of Chironomus salivary glands. At these sites the chromosome loosens and actively makes mRNA for salivary proteins.
Q3. Describe the structure of a chromosome and its types.
Sex determination is the mechanism that decides whether an individual develops as male or female. In most animals it is controlled by sex chromosomes, but in some it depends on chromosome sets or the environment. Genes on the sex chromosomes show a special pattern of inheritance called sex-linked inheritance. This topic covers the chromosomal, genic balance, haplodiploid and environmental methods, and sex-linked, sex-limited and sex-influenced traits.
Key points
Henking (1891) first saw the "X body" in insects; McClung (1902) linked it with sex.
XX–XY type: female homogametic (XX), male heterogametic (XY). Found in humans, other mammals and Drosophila.
XX–XO type: female XX, male has only one X (XO). Found in grasshoppers, cockroaches and bugs.
ZW–ZZ type: female heterogametic (ZW), male homogametic (ZZ). Found in birds, some reptiles and butterflies.
ZO–ZZ type: female ZO, male ZZ; found in some moths.
In humans, the SRY gene on the Y chromosome triggers testis formation, so the Y decides maleness.
Genic balance theory (C. B. Bridges) for Drosophila: sex depends on the ratio of X chromosomes to autosome sets (X/A). X/A = 1 or more → female; 0.5 or less → male; between 0.5 and 1 → intersex. The Y is needed for male fertility, not for maleness.
Haplodiploidy: in honeybees, ants and wasps, fertilised eggs (diploid) become females (queen or workers) and unfertilised eggs (haploid) become males (drones) by parthenogenesis.
Environmental sex determination: in many turtles and crocodiles the incubation temperature of eggs decides sex; in the marine worm Bonellia, larvae that settle on a female's proboscis become tiny males.
Sex-linked genes lie on the X chromosome. T. H. Morgan (1910) discovered sex-linkage with white-eye in Drosophila.
X-linked recessive traits (haemophilia, red-green colour blindness) are far more common in males, because one recessive allele is enough in a male (hemizygous).
They show criss-cross inheritance: a father passes the X-linked trait to his grandson through his daughter (a carrier).
Y-linked (holandric) genes pass only from father to son, e.g. hypertrichosis of the ear (hairy ear rims).
Important terms
Homogametic sex – produces one kind of gamete with respect to sex chromosomes.
Heterogametic sex – produces two kinds of gametes.
Hemizygous – having only one copy of a gene (X-linked genes in males).
Carrier – heterozygous female carrying a recessive X-linked allele without showing it.
Sex-limited trait – expressed in only one sex (beard in men, milk production in cows).
Sex-influenced trait – autosomal, but dominant in one sex and recessive in the other (pattern baldness).
Key facts
Cross: carrier woman (XᴴXʰ) × normal man (XᴴY), where h = haemophilia.
Gametes
Xᴴ
Y
Xᴴ
XᴴXᴴ normal daughter
XᴴY normal son
Xʰ
XᴴXʰ carrier daughter
XʰY haemophilic son
Result: 50% of daughters are carriers; 50% of sons are haemophilic.
Haemophilia is called the "royal disease" because Queen Victoria was a carrier.
5 important questions
Q1. What is criss-cross inheritance?
It is the passing of an X-linked trait from a father to his grandson through his daughter, or from a mother to her son. It happens because a son gets his only X from his mother.
Q2. Distinguish between sex-limited and sex-influenced traits.
A sex-limited trait appears only in one sex, though genes are present in both (e.g. beard). A sex-influenced trait appears in both sexes but is dominant in one sex and recessive in the other (e.g. baldness, more common in men).
Q3. Describe the chromosomal mechanisms of sex determination.
XX–XY with human example and cross.
XX–XO in grasshopper.
ZW–ZZ in birds; ZO–ZZ in moths.
Role of SRY gene.
Diagrams of each cross.
Q4. Explain the genic balance theory of sex determination in Drosophila.
Bridges' experiments with triploid females.
X/A ratio table: normal female, normal male, metafemale, metamale, intersex, triploid female.
Role of autosomes (male-determining) and X (female-determining).
Y only for fertility.
Q5. Explain sex-linked inheritance with suitable examples.
Morgan's white-eye experiment in Drosophila (reciprocal crosses).
Haemophilia and colour blindness in humans with crosses.
Characteristics: more males affected, criss-cross pattern, no father-to-son transmission of X-linked traits.
Y-linked inheritance.
Exam tip
Always show crosses in a checkerboard with clear X and Y symbols; a cross without a result statement loses marks.
A mutation is a sudden, heritable change in the genetic material. It may affect a single gene (gene or point mutation) or the structure or number of chromosomes (chromosomal aberration). Mutations are the raw material of evolution but can also cause disease. This topic covers the types and causes of mutations, mutagens, structural changes in chromosomes and changes in chromosome number.
Key points
The term mutation was given by Hugo de Vries (1901) from his work on the evening primrose (Oenothera lamarckiana). Mutation is उत्परिवर्तन in Marathi.
H. J. Muller (1927) first produced mutations artificially using X-rays in Drosophila (Nobel Prize 1946).
Spontaneous mutations occur naturally (errors in DNA replication); induced mutations are caused by mutagens.
Physical mutagens: X-rays, gamma rays, UV rays (UV forms thymine dimers), high temperature.
Chemical mutagens: nitrous acid, base analogues (5-bromouracil), alkylating agents (mustard gas, EMS), acridine dyes.
Gene (point) mutations:substitution – transition (purine replaced by purine, or pyrimidine by pyrimidine) and transversion (purine ↔ pyrimidine); frameshift – insertion or deletion of bases, which shifts the reading frame.
Effects of substitution: silent (same amino acid), missense (different amino acid, e.g. sickle-cell anaemia) or nonsense (stop codon).
Structural chromosomal aberrations:deletion (loss of a segment), duplication (a segment repeated), inversion (a segment turned 180°) and translocation (a segment moves to a non-homologous chromosome).
Inversions are paracentric (centromere outside the inverted part) or pericentric (centromere inside).
Numerical changes:euploidy – change in whole sets (monoploid n, triploid 3n, tetraploid 4n); aneuploidy – gain or loss of one or a few chromosomes.
Aneuploidy usually arises by non-disjunction (chromosomes fail to separate at meiosis).
Polyploidy is common and useful in plants but rare in animals, since it upsets sex determination.
Trisomy (2n + 1) – one extra chromosome (Down syndrome, trisomy 21).
Nullisomy (2n − 2) – one pair missing.
Reciprocal translocation – exchange of segments between two non-homologous chromosomes.
Bar eye – duplication of a band region of the X chromosome in Drosophila.
Key facts
Type
Example
Missense point mutation
Sickle-cell anaemia (glutamic acid → valine, 6th position of β-globin)
Deletion
Cri-du-chat syndrome (part of short arm of chromosome 5 lost)
Duplication
Bar eye in Drosophila
Translocation
Philadelphia chromosome (chromosomes 9 and 22) in chronic myeloid leukaemia
Trisomy
Down syndrome (47, +21)
Monosomy
Turner syndrome (45, X)
5 important questions
Q1. Distinguish between transition and transversion.
In transition, a purine is replaced by another purine (A ↔ G) or a pyrimidine by another pyrimidine (C ↔ T). In transversion, a purine is replaced by a pyrimidine or vice versa.
Q2. What is non-disjunction?
Non-disjunction is the failure of homologous chromosomes or sister chromatids to separate during cell division. It produces gametes with an extra or a missing chromosome, leading to aneuploidy such as Down syndrome.
Q3. Classify mutations and describe gene mutations.
Human genetic disorders are diseases caused by defects in genes or chromosomes. Some follow simple Mendelian inheritance (autosomal or X-linked), while others come from abnormal chromosome number or structure. This topic covers pedigree analysis, important Mendelian disorders, chromosomal syndromes, and prenatal diagnosis and genetic counselling.
Key points
Pedigree analysis is the study of a trait over several generations of a family using a chart. Squares = males, circles = females, shaded = affected, half-shaded or dotted = carriers.
Autosomal recessive disorders appear only in homozygous recessive persons; parents are usually normal carriers; more common in marriages between relatives (consanguinity).
Examples: albinism (no melanin; defect in the enzyme tyrosinase), phenylketonuria (PKU) (lack of phenylalanine hydroxylase; phenylalanine builds up and harms the brain), sickle-cell anaemia, thalassaemia, cystic fibrosis.
Sickle-cell anaemia: a point mutation in the β-globin gene (chromosome 11) replaces glutamic acid with valine at the 6th position. Red cells become sickle-shaped at low oxygen. HbˢHbˢ persons are ill; HbᴬHbˢ carriers have sickle-cell trait and some resistance to malaria. It is common in parts of Vidarbha and among some tribal communities in India.
Thalassaemia: reduced synthesis of α or β globin chains, causing severe anaemia; major cases need regular blood transfusion.
Autosomal dominant disorders need only one defective allele and appear in every generation: Huntington's disease (chromosome 4; nerve degeneration in middle age), achondroplasia (dwarfism), polydactyly.
X-linked recessive disorders:haemophilia (blood fails to clot), red-green colour blindness, Duchenne muscular dystrophy. Mostly males are affected.
Chromosomal disorders usually result from non-disjunction during gamete formation.
Down syndrome (trisomy 21, 47 chromosomes): described by Langdon Down (1866); cause found by Lejeune (1959). Features: short stature, round face, slanting eyes, open mouth with furrowed tongue, broad palm with a single simian crease, mental retardation. Risk rises with mother's age.
Klinefelter syndrome (47, XXY): tall male, small testes, sterile, may show breast development (gynaecomastia).
It is an autosomal recessive disorder in which the enzyme phenylalanine hydroxylase is missing. Phenylalanine is not converted to tyrosine; it accumulates and is changed to phenylpyruvic acid, which damages the brain. A low-phenylalanine diet from birth prevents damage.
Q2. Why are carriers of sickle-cell trait resistant to malaria?
In heterozygous carriers some red cells sickle when infected, and they are destroyed quickly along with the malaria parasite. So Plasmodium falciparum cannot multiply well in them.
Q3. Describe Down syndrome.
Cause: trisomy 21 by non-disjunction; link with maternal age.