Reproduction lets a species go on after its members die. Some organisms just split or grow buds (asexual reproduction). Most plants and animals join a male and a female cell (sexual reproduction). Traits pass from parents to children through genes on chromosomes. Over many generations, these small changes add up to evolution.
On this page you will learn:
- Asexual reproduction, the parts of a flower, and cloning
- Human reproduction, the 46 chromosomes and how a baby's sex is decided
- Mendel's pea experiments and sex-linked traits such as colour blindness
- DNA, RNA, DNA fingerprinting and Dolly the sheep
- ABO blood groups: donors and receivers
- Darwin, Lamarck, fossils and homologous organs
46
Chromosomes in a human body cell
23 pairs; asked in SI 2016
XX / XY
Sex chromosomes
Female / male
3 : 1
Mendel's F2 ratio
Tall : dwarf pea plants
Asexual reproduction and the flower
Rule in one line: asexual means one parent and copies; in plants, the flower is the sex organ.
| Method | How it works | Examples |
|---|---|---|
| Fission | The cell splits into two (binary) or many (multiple) | Amoeba (binary), Plasmodium (multiple) |
| Budding | A bud grows on the parent and breaks off | Hydra, yeast |
| Fragmentation | The body breaks into pieces; each grows into a new one | Spirogyra |
| Regeneration | A cut part grows into a whole organism | Planaria, Hydra |
| Spore formation | Tiny spores spread and grow in good conditions | Bread mould (Rhizopus), ferns |
| Vegetative propagation | New plants from roots, stems or leaves | Potato eyes, ginger, Bryophyllum leaf, rose cutting |
FROM FLOWER TO FRUIT
- 1PollinationPollen reaches the stigma — by wind, insects or water.
- 2FertilisationThe male cell joins the egg in the ovule and forms a zygote.
- 3SeedThe ovule becomes the seed; the zygote becomes the embryo.
- 4FruitThe ovary grows into the fruit. Its wall becomes the fruit wall, the pericarp.
QUICK CHECK
PYQ · PSSSB Excise Inspector 2023 — Which of the following is not a part of the flower?
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Answer: D — Pericarp
The pericarp is the wall of the fruit. It forms from the ovary wall after fertilisation, so it is not a part of the flower.
QUICK CHECK
PYQ · PPSC SEVE 2016 — In making the saffron spice, which one of the following parts of the plant is used?
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Answer: D — Stigma
Saffron threads are the dried stigmas of Crocus sativus. The stigma is the top of the pistil that catches pollen.
Human reproduction and chromosomes
Rule in one line: body cells have 46 chromosomes; sperm and egg have 23 each; the father's sperm decides the sex.
| Organ | Job |
|---|---|
| Testes | Make sperms and the hormone testosterone |
| Ovaries | Release eggs (ova) and make oestrogen |
| Oviduct (fallopian tube) | Where fertilisation happens |
| Uterus | The embryo grows here, fed through the placenta |
| Mother's egg | Father's sperm | Child |
|---|---|---|
| X | X | XX — girl |
| X | Y | XY — boy |
QUICK FACT
Identical twins come from one zygote that splits into two. Non-identical twins come from two eggs. In IVF, egg and sperm are joined outside the body ('test-tube baby'). In some reptiles, the temperature of the eggs decides the sex.
QUICK CHECK
PYQ · Punjab Police SI 2016 — The number of chromosomes in a normal human body cell is
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Answer: D — 46
46 chromosomes, in 23 pairs. Sperm and egg carry 23 each, so the zygote gets 46 again. PPSC TM 2015 asked the same fact.
QUICK CHECK
PYQ · PPSC EO 2018 — The sex of a newborn baby is determined by the chromosome inherited from:
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Answer: B — The father
The mother always gives an X. The father gives either X (girl) or Y (boy). PPSC SSO 2018 repeated this question.
QUICK CHECK
PYQ · RSMSSB LDC 2024 — Sex determination in humans is based on _____ type chromosome.
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Answer: A — X Y
Humans follow the XY type: XX is female and XY is male. The XO type is found in some insects, such as grasshoppers.
Heredity: Mendel's peas
Rule in one line: each trait has two forms; the dominant form shows, the recessive form hides.
KEY TERM
Dominant and recessive
Mendel crossed pure tall pea plants with pure dwarf ones. All first-generation (F1) plants were tall. So tallness is dominant and dwarfness is recessive. When F1 plants bred among themselves, the F2 plants were tall and dwarf in the ratio 3 : 1. With two traits at once, the F2 ratio is 9 : 3 : 3 : 1.
Tall × dwarf→F1: all tall→F2: 3 tall : 1 dwarf
| Trait | Dominant | Recessive |
|---|---|---|
| Stem height | Tall | Dwarf |
| Seed shape | Round | Wrinkled |
| Seed colour | Yellow | Green |
| Flower colour | Violet | White |
| Flower position | Axial (along the stem) | Terminal (at the tip) |
| Pod shape | Inflated (full) | Constricted |
| Pod colour | Green | Yellow |
QUICK CHECK
PYQ · RSMSSB LDC 2024 — Which of the following is NOT a dominant trait in a Garden Pea as studied by Mendel?
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Answer: B — Terminal flower position
Axial flowers are dominant; terminal flowers are recessive. Inflated pod, yellow seed and green pod are all dominant.
KEY TERM
Sex-linked traits
Some genes sit on the X chromosome. Colour blindness and haemophilia (blood that does not clot) are caused by recessive genes on the X. A man has only one X, so one faulty gene shows its effect. A woman has two X's, so a normal X usually hides the faulty one. That is why these problems are more common in men.
NOT SEX-LINKED
Down's syndrome comes from an extra copy of chromosome 21, not from the X. Skin colour is controlled by many genes on other chromosomes.
QUICK CHECK
PYQ · PPSC TM 2015 — An example of human X – linked trait:
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Answer: C — Colour blindness
Colour blindness is carried on the X chromosome. Down's syndrome is an extra chromosome 21.
QUICK CHECK
PYQ · PPSC PCSEB 2018 — Consider the two statements given below, one labelled as Assertion (A) and the other labelled as Reason (R). Assertion (A): There are more colourblind men than women throughout the world. Reason (R): Colourblindness is a sex-linked disease resulting from a recessive gene on the Y chromosome. In the context of the above statements, which one of the following is correct?
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Answer: C — A is true but R is false
A is true: more men are colour-blind. R is false: the gene is on the X, not the Y.
DNA, genes and cloning
Rule in one line: DNA stores the code; RNA carries it; proteins do the work.
FROM GENE TO PROTEIN
- 1DNAThe gene, in the nucleus
- 2mRNACopied from DNA: transcription
- 3ProteinBuilt at ribosomes: translation
KEY TERM
DNA fingerprinting
Parts of every person's DNA are unique, except in identical twins. DNA fingerprinting compares these parts. Police use it to identify criminals from blood, hair or skin left at a crime scene. It also settles questions about who a child's parents are. Alec Jeffreys of Britain developed it in 1984.
QUICK CHECK
PYQ · RSMSSB LDC 2024 — The nitrogenous base not present in DNA is:
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Answer: A — Uracil
DNA has A, T, G and C. Uracil takes the place of thymine in RNA.
QUICK CHECK
PYQ · RSMSSB LDC 2024 — Identify the correct process for DNA to Protein formation: DNA →(a)→ mRNA →(b)→ Protein.
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Answer: B — a: transcription, b: translation
DNA is first copied into mRNA (transcription). The mRNA message is then read to build protein (translation).
QUICK CHECK
PYQ · RSMSSB LDC 2024 — Which of the following techniques is useful identification tool in forensic science?
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Answer: B — DNA fingerprinting
DNA fingerprinting matches DNA from a crime scene to a person. Gene therapy treats disease; it does not identify people.
QUICK CHECK
PYQ · PPSC TM 2015 — In 1997 the world was astonished by the announcement of generation of a new born lamb which was a product of cloning. The name of the lamb is :
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Answer: C — Dolly
Dolly, cloned at the Roslin Institute, Scotland, by Ian Wilmut's team. She was born in 1996; the news came out in 1997.
ABO blood groups
Rule in one line: you carry antibodies against the antigen you do not have.
| Group | Antigen on red cells | Antibody in plasma | Can receive from |
|---|---|---|---|
| A | A | Anti-B | A, O |
| B | B | Anti-A | B, O |
| AB | A and B | None | All groups — universal recipient |
| O | None | Anti-A and anti-B | O only — but gives to all: universal donor |
REMEMBER
AB has nothing to fight (no antibodies), so it accepts all. O has nothing on its cells, so it can be given to all.
QUICK CHECK
PYQ · PSSSB Election Kanungo 2018 — The universal recipient blood group is:
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Answer: C — AB
AB plasma has no anti-A or anti-B, so it does not attack donated red cells of any ABO group. O is the universal donor.
QUICK CHECK
PYQ · RSMSSB LDC 2024 — Match List-I (Blood Group) with List-II (Antibodies in plasma): a-A, b-B, c-AB, d-O with I-Anti-A, II-Anti-A,B, III-Anti-B, IV-Nil.
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Answer: D — a-III, b-I, c-IV, d-II
A has anti-B (III), B has anti-A (I), AB has none (IV), and O has both anti-A and anti-B (II).
Evolution
Rule in one line: nature selects the variations that suit the environment; acquired traits are not inherited.
DARWIN'S NATURAL SELECTION IN FOUR STEPS
- 1VariationMembers of a species differ a little.
- 2StruggleMore young are born than food and space allow.
- 3SelectionThose with useful variations survive and breed more.
- 4New speciesOver many generations, the changes add up (speciation).
QUICK CHECK
PYQ · PSSSB Election Kanungo 2018 — The Lamarckian theory of organic evolution is known as the theory of:
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Answer: C — Inheritance of acquired characters
Lamarck said traits gained in a lifetime pass to the young. Natural selection is Darwin's idea; continuity of germplasm is Weismann's.
QUICK CHECK
PYQ · PPSC SEVE 2016 — Charles Darwin's theory of evolution is based on which one of the following?
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Answer: D — Natural Selection
Darwin: natural selection (1859). Use and disuse is Lamarck; mutation theory is Hugo de Vries.
More facts that are asked once in a whileTap to open
- A gene is a part of DNA that controls one trait.
- Mendel published his work in 1866. It was rediscovered in 1900.
- Vestigial organs, such as the human appendix, no longer do much work.
- Fossils are the preserved remains of old organisms. Their age is found by methods such as radiocarbon dating.
- Darwin studied finches and other animals of the Galapagos Islands on his voyage in HMS Beagle.
- Besides ABO, blood has the Rh factor: Rh-positive or Rh-negative.
- Pre-natal sex testing of the foetus is banned in India to stop female foeticide.
Traps and practice
Last revision — where the options try to fool you.
COMMON TRAPS
- The pericarp is part of the fruit, not the flower. Saffron is the stigma, not the petal.
- Body cells have 46 chromosomes; sperm and egg have 23.
- The father, not the mother, decides the sex of the child.
- Colour blindness is on the X chromosome, not the Y.
- Green seed is recessive, but green pod is dominant. Terminal flower is recessive.
- AB is the universal recipient; O is the universal donor.
- Darwin = natural selection; Lamarck = acquired characters (use and disuse).
CHECK YOURSELF
Why do children of the same parents look different from each other?
Show answer
Each child gets half its genes from the mother and half from the father. Every egg and sperm carries a different mix. Only identical twins share the same genes.
FLASHCARDS
Reproduction and heredity flashcards
- Father of genetics
- Gregor Mendel
- Chromosomes in a human body cell
- 46 (23 pairs)
- Base found in RNA but not DNA
- Uracil
- DNA → mRNA
- Transcription
- First mammal cloned from an adult cell
- Dolly the sheep
- Universal recipient
- AB
- Saffron comes from
- Stigma of Crocus
- Link between reptiles and birds
- Archaeopteryx
QUICK BYTES
- Asexual: fission, budding, fragmentation, regeneration, spores, vegetative
- Ovary → fruit; ovule → seed; ovary wall → pericarp
- 46 chromosomes; XX girl, XY boy; father decides
- Mendel: 3 : 1 and 9 : 3 : 3 : 1
- Colour blindness, haemophilia: X-linked, more in men
- DNA: A T G C; RNA: A U G C
- AB universal recipient; O universal donor
- Darwin 1859: natural selection; Lamarck: acquired characters