Inheritance, and Evolution

• Snapdragon flower colour shows incomplete dominance.
• RR = red, Rr = pink, rr = white.
• The offspring appear as pink : white = 1 : 1.

The flower colours of the two parents.

In incomplete dominance every genotype has its own visible colour, so reading the offspring backwards is straightforward: pink offspring must be Rr and white offspring must be rr. A 1 : 1 ratio with only two classes is the signature of a test-cross pattern — one heterozygous parent crossed with one homozygote.

1. No red offspring appear at all, so no RR is ever produced — which means at least one parent cannot supply a second R.
2. White offspring (rr) appear, so both parents must be able to give an r.
3. Pink offspring (Rr) appear, so one parent must be able to give an R.
4. The only pair satisfying all three is Rr × rr — pink × white.
5. Check: Rr × rr gives Rr, Rr, rr, rr = 2 pink : 2 white = 1 : 1. ✓

• Parent 1 is heterozygous: Aa.
• Parent 2 is homozygous recessive: aa.
• A single trait, ordinary complete dominance.

The chance of an offspring with the homozygous recessive phenotype.

This is a test cross, and it behaves quite differently from the carrier × carrier cross. The recessive parent can only ever give an a, so every offspring already has one a guaranteed. The whole outcome then rests on the single gamete from the heterozygous parent: A gives Aa, a gives aa — a straight coin toss.

1. Heterozygote Aa makes two kinds of gamete in equal numbers: A and a.
2. Homozygous recessive aa makes only one kind: a.
3. Combining: A × a gives Aa, and a × a gives aa.
4. So the offspring are 1 Aa : 1 aa.
5. Recessive phenotype = aa = 1 in 2 = 50%.

• Mendel's law of segregation.
• Four candidate statements of what the law rests on.

The fact on which the law of segregation is based.

Segregation says that the two alleles of a gene sit together in a diploid organism but separate cleanly during gamete formation, so each gamete gets one and only one. The observational fact that forces this conclusion is what happens in F₂: the recessive trait, invisible throughout F₁, reappears unchanged. It was never diluted or blended — it was simply hidden and then recovered.

1. In F₁ the hybrid Aa looks entirely like the dominant parent, so a blending theory would predict the recessive character is lost or diluted.
2. In F₂ the recessive character comes back looking exactly as it did in the original parent — not faded, not intermediate.
3. That can only happen if the two alleles stayed separate and unaltered while together, then parted during gamete formation.
4. So the law rests on no blending, and recovery in the next generation.

• Pea seed starch grains, gene B.
• BB produces large starch grains, bb produces small ones.
• The heterozygote is Bb.

What kind of dominance Bb shows when starch grain size is the phenotype.

The same gene can show different dominance relationships depending on which phenotype you look at. Judged by seed shape, Bb peas are round like BB — complete dominance. But judged by starch grain size, Bb grains are intermediate: bigger than bb, smaller than BB. An intermediate heterozygote is the definition of incomplete dominance.

1. BB seeds have large starch grains; bb seeds have small ones.
2. Bb seeds have grains of intermediate size — a blend, not a copy of either parent.
3. A heterozygote showing a blend is incomplete dominance.
4. Note the contrast: for seed shape, Bb is fully round like BB, so the same gene shows complete dominance on that phenotype. Dominance is a property of the phenotype you choose to measure, not of the gene alone.

• A true-breeding violet-flowered pea: VV.
• A white-flowered pea: vv (white is recessive, so white plants are always homozygous).
• Violet is completely dominant over white.

The ratio of violet to white in the offspring of this cross.

Two words decide this. True-breeding means homozygous, so the violet parent is VV, not Vv. And offspring means the immediate next generation, F₁ — not F₂. A VV × vv cross can only produce Vv, and every Vv plant is violet.

1. True-breeding violet gives only V gametes.
2. White is recessive, so a white plant must be vv and gives only v gametes.
3. Every offspring is therefore Vv.
4. Vv is violet, since violet is completely dominant.
5. All offspring are violet: the ratio is 1 : 0.

• Four concepts: complete dominance, incomplete dominance, co-dominance, law of segregation.
• Four features describing what F₁ looks like, plus one about gamete formation.

The correct matching.

Three of the four are about what the heterozygote looks like, and they form a clean progression. Complete dominance: F₁ looks like one parent only. Incomplete dominance: F₁ looks like a blend of the two. Co-dominance: F₁ shows both parental traits, fully and separately. The fourth, segregation, is the odd one out — it is about gametes, not appearance, and it pairs with the only option mentioning gametes.

1. D first, because it is the easiest. Law of segregation is the only concept about gametes, and II is the only feature about gametes. D – II.
2. A. Complete dominance — the heterozygote shows only the dominant trait. A – IV.
3. B. Incomplete dominance — the heterozygote is intermediate, a blend. B – I.
4. C. Co-dominance — the heterozygote shows both parental traits at once. C – III.
5. Answer: A – IV, B – I, C – III, D – II.

• Stanley Miller's 1953 simulation of the early earth.
• The chamber was meant to reproduce the primitive reducing atmosphere.

Which gases were placed in the spark chamber.

Miller was trying to recreate an atmosphere with no free oxygen — a reducing one, in which hydrogen is abundant. The four things he sealed in were methane, ammonia, hydrogen and water vapour, with electric discharges at 800 °C standing in for lightning. After a week the condensed liquid contained amino acids.

1. The early atmosphere had no free oxygen — that rules out any option containing O₂ at once.
2. It was reducing, i.e. rich in hydrogen, so H₂ must be present.
3. The carbon came in as methane, not as carbon dioxide, since CO₂ is an oxidised form of carbon.
4. Water vapour supplied the oxygen and hydrogen for the amino acids.
5. So the set is CH₄, H₂O, NH₃, H₂.

• Modern breeds of domestic dog.
• They descend from wolf ancestors over a few thousand years.

The process by which they arose.

The question turns on who did the choosing. In natural selection the environment decides which individuals survive and reproduce. In artificial selection humans decide — picking the animals with the traits they want and breeding from those. Every dog breed exists because people chose the parents, generation after generation.

1. Ask what would happen to a chihuahua or a dachshund in the wild. Neither would be favoured by any natural pressure.
2. These forms exist because breeders selected them deliberately for size, coat, temperament or working ability.
3. Human-directed selection is artificial selection.
4. Darwin used exactly this — domestic pigeons and dogs — as his everyday illustration of how selection works, before arguing that nature does the same thing unaided.

• Four pairs of organs from different animals.

Which pair is homologous.

Homologous organs share the same basic structural plan inherited from a common ancestor, even if they now do different jobs. Analogous organs are the reverse: different structural plans that arrived at the same job because of similar pressures. The quick test is whether the two animals belong to the same major group. Homology usually sits inside a group; analogy usually crosses between groups.

1. All vertebrates — fish, amphibians, reptiles, birds, mammals — share a common ancestor.
2. Their hearts follow one basic plan, modified from two chambers to four; their brains likewise share fore-, mid- and hindbrain divisions.
3. So vertebrate hearts (across vertebrates) and vertebrate brains are both homologous sets — same plan, common ancestry, varying detail.
4. The other three pairs each cross between unrelated groups, which makes them analogous.

• A seal is a mammal; a penguin is a bird.
• Both have flipper-shaped limbs used for swimming.

What kind of organs these flippers are.

Both animals swim, and water imposes the same demands whatever your ancestry: a flat, paddle-like limb. A seal's flipper is a modified mammalian forelimb; a penguin's is a modified bird wing. Two different starting structures pushed into the same shape by the same environment is convergent evolution, and the resulting organs are analogous.

1. Identify the groups: seal = mammal, penguin = bird. Different classes.
2. Identify the job: both flippers are for swimming. Same function.
3. Different origin plus same function = analogous.
4. The underlying cause is convergent evolution — similar environments producing similar solutions independently.

• Standard dates from the Evolution chapter.

The approximate age of the universe.

The chapter uses four landmark dates and they get shuffled between questions, so they are best learned as one set rather than one at a time: the universe forms, the earth forms, the first cellular life appears, and the dinosaurs die out. Only the first of these is asked here.

1. The Big Bang is dated by the cosmic microwave background and the expansion rate.
2. The current accepted figure is 13.8 billion years.
3. 4.5 billion is the age of the earth, not the universe — the commonest confusion in this set.
4. 65 million years is when the dinosaurs disappeared, a vastly more recent event.

• The embryological line of evidence for evolution.

Who proposed it.

Ernst Haeckel observed that vertebrate embryos of different species pass through similar stages, and proposed that an embryo repeats its evolutionary history as it develops — his “biogenetic law”. The classic example is the gill slits present in the embryos of all vertebrates, including humans. NCERT also notes that this was later disproved by Karl Ernst von Baer, who showed embryos never pass through the adult stages of other animals.

1. The claim is that embryonic development retraces evolutionary ancestry.
2. Its author is Ernst Haeckel.
3. The evidence cited is the set of features shared by early vertebrate embryos, such as gill slits.
4. NCERT then records that von Baer disproved the strong form of the claim — a detail that is itself examined, so keep both names attached to this topic.

• Darwin's finches of the Galapagos islands.
• They now occupy many feeding niches: seeds, insects, cactus, and a tool-using woodpecker-like form.

Which type was the original ancestral form.

A small population of seed-eating ground finches reached the Galapagos from the mainland. With many empty niches and no competition, their descendants diversified into forms with quite different beaks — insect eaters, cactus feeders, a woodpecker-like finch. That process, one ancestral stock radiating into many forms in a new place, is adaptive radiation.

1. The founding birds were ordinary mainland seed eaters.
2. On reaching the islands they found many unoccupied feeding niches.
3. Over generations, beak form diversified to exploit those niches.
4. The specialised forms — insect eater, cactus eater, woodpecker finch — are the results of the radiation, not its starting point.

• A bird wing: a modified vertebrate forelimb, with bones.
• An insect wing: an outgrowth of the body wall, with no bones at all.
• Both are used for flight.

What kind of structures they are, and which pattern of evolution they represent.

Two labels have to be got right and they always travel together. Analogous structures — different construction, same function — are the product of convergent evolution, where unrelated lineages are pushed towards the same solution. Homologous structures — same construction, different functions — are the product of divergent evolution, where one ancestral plan is modified in different directions.

1. Compare construction: a bird wing has humerus, radius, ulna and digits; an insect wing is a thin extension of the cuticle with no skeleton inside.
2. Completely different construction, so not homologous.
3. Same function, flight, arrived at independently: analogous.
4. Analogy is always the signature of convergent evolution.

• Four mammals.
• Marsupials are pouched mammals, and Australia's adaptive radiation of them is the standard NCERT example.

Which one is not a marsupial.

Australian marsupials are the textbook case of adaptive radiation: one ancestral pouched stock filling niches that placental mammals occupy elsewhere. Numbat, koala and wombat are all Australian marsupials. The lemur is a primate from Madagascar — a placental mammal, and part of a different radiation entirely.

1. Numbat — an Australian termite-eating marsupial.
2. Koala — an Australian marsupial that feeds on eucalyptus.
3. Wombat — an Australian burrowing marsupial.
4. Lemur — a primate from Madagascar, placental, not pouched. This is the odd one out.

• Standard dates from the Evolution chapter.

When the first cellular forms of life appeared.

The chapter gives the appearance of the first cells as about 2000 million years ago. Watch the units carefully — this figure is quoted in millions while the age of the earth and the universe are quoted in billions, and the distractors here exploit that switch.

1. The earth itself formed about 4.5 billion years ago.
2. The first cellular forms of life appeared about 2000 million years ago — that is 2 billion, so roughly halfway through the earth's history.
3. Check the other options against the ladder: 200 million is around the age of the dinosaurs, and 3 to 4.5 million years is the range for early hominids.
4. Only 2000 million fits.