Heredity and its history
Children often have their father's nose or their mother's curly hair. Nobody teaches them; the instructions came with the cell they started from. This passing of features from parents to children is heredity.
Heredity (आनुवंशिकता) is the transfer of characters from one generation to the next through genes (जनुके).
In the figure : In the figure heredity passes characters through genes.
Rule 1: What heredity means — Characters are carried by genes on chromosomes and handed from parents to offspring. Gregor Mendel is called the pioneer of modern genetics.
- Easy : A child gets one set of genes from the mother and one from the father.
- Medium : A tall plant (TT) crossed with a short plant (tt) gives only Tt offspring, and all are tall because T is dominant.
- Exam-type : Two Tt plants are crossed: the Punnett square gives TT, Tt, Tt, tt, so 3 tall to 1 short.
Heredity and its history
In the app you can move this figure with your finger.
Open in the appRule 2: Milestones in genetics — Hugo de Vries (1901) gave the mutation theory. Walter Sutton (1902) found paired chromosomes in grasshopper cells. Avery, McCarty and MacLeod (1944) showed DNA is the genetic material (except in some viruses). Jacob and Monod (1961) proposed a model of protein synthesis.
- Easy : Sutton looked at grasshopper cells and saw chromosomes occurring in pairs.
- Medium : The 1944 work moved attention from proteins to DNA as the carrier of heredity.
- Exam-type : Uses of genetics: finding and preventing hereditary disorders, making hybrid crop varieties, using microbes in industry, and recombinant DNA technology.
Heredity: genes pass characters. DNA is genetic material (1944).
Thinking that a child gets all characters from only one parent. Genes come from both.
Memory clue: "Genes ride on chromosomes, from mother and father both".
Spot the difference
| Scientist | Contribution |
|---|---|
| Mendel | Pioneer of genetics |
| De Vries (1901) | Mutation theory |
| Sutton (1902) | Paired chromosomes |
| Avery and others (1944) | DNA is genetic material |
Practice questions
- 1Define heredity.
- 2Who is called the pioneer of modern genetics?
- 3Which scientists showed that DNA is the genetic material?
- 4Who found paired chromosomes in grasshopper cells?
- 5In a cross of TT × tt, what is the genotype of all offspring?
- 6A Tt × Tt cross gives 80 plants. About how many are short?
- 7Name two uses of studying heredity.
- 8Why is a Tt plant tall even though it carries t?
Show answers (try them yourself first)
- 1Transfer of characters from one generation to the next through genes.
- 2Gregor Mendel.
- 3Avery, McCarty and MacLeod (1944).
- 4Walter Sutton (1902).
- 5Tt.
- 6One quarter, about 20.
- 7Preventing hereditary disorders and developing hybrid crop varieties.
- 8T is dominant over t, so its effect shows.
Transcription, translation and mutation
A recipe book stays in the kitchen, but the cook copies one recipe on a card and carries it to the stove. Cells do the same: DNA stays in the nucleus, and a copy called mRNA carries the instructions to the ribosome.
The flow is DNA → RNA → protein: transcription (प्रतिलेखन) then translation (भाषांतर).
In the figure : In the figure the flow of information from DNA to protein.
Rule 1: Transcription and translation — In transcription, mRNA is made on one strand of DNA; uracil replaces thymine. In translation, tRNA anticodons bring amino acids according to the triplet codons (त्रिक संकेत) of mRNA, and the ribosome joins them with peptide bonds. In translocation the ribosome moves one codon along the mRNA.
- Easy : The DNA strand TAC GGA is copied into mRNA as AUG CCU (A pairs with U, T with A, G with C, C with G).
- Medium : The mRNA AUG CCU has two codons, so two amino acids are added to the chain.
- Exam-type : A gene has 300 DNA bases. Triplets: 300 ÷ 3 = 100 codons, so the protein can have about 100 amino acids.
Transcription, translation and mutation
In the app you can move this figure with your finger.
Open in the appRule 2: Mutation — A mutation (उत्परिवर्तन) is a sudden change in a gene. It may cause disorders, for example sickle cell anaemia. Khorana worked on triplet codons and won the Nobel Prize in 1968.
- Easy : Changing one base in a gene can change one amino acid in the protein.
- Medium : If one codon changes, only one amino acid changes, but the shape of the protein may change and so may its work.
- Exam-type : In sickle cell anaemia a changed gene makes red blood cells sickle-shaped, so they carry oxygen poorly.
DNA → mRNA (transcription) → protein (translation). Mutation = sudden gene change.
Writing thymine in mRNA. In RNA it is uracil.
Memory clue: "DNA is the book, mRNA the card, ribosome the cook".
Spot the difference
| Step | Where | Result |
|---|---|---|
| Transcription | Nucleus | mRNA made |
| Translation | Ribosome | Protein made |
| Translocation | Ribosome | Moves one codon |
Practice questions
- 1What is the central dogma?
- 2Which base replaces thymine in RNA?
- 3What does tRNA do?
- 4What is a mutation?
- 5Write the mRNA copied from the DNA strand TTA GCC.
- 6A gene has 450 bases. How many codons does it have?
- 7Name a disorder caused by a gene mutation.
- 8Why does a change of one base sometimes change the whole protein's work?
Show answers (try them yourself first)
- 1DNA to RNA to protein.
- 2Uracil.
- 3It brings amino acids according to the mRNA codons.
- 4A sudden change in a gene.
- 5AAU CGG.
- 6450 ÷ 3 = 150.
- 7Sickle cell anaemia.
- 8A different amino acid can change the protein's shape, and shape decides its function.
Evolution and its evidences
Look at your hand, a bat's wing and a whale's flipper. They do very different jobs, yet inside they have the same pattern of bones. It is as if they were all made from one old design.
Evolution (उत्क्रांती) is the slow change in living things over a long time that forms new species.
In the figure : In the figure homologous limbs show a common ancestor.
Rule 1: Evolution — Life began in the ocean from simple compounds. The change took a very long time, at most about 300 crore years. Evidence comes from many fields.
- Easy : Morphological evidence: whales and fishes both have a streamlined shape.
- Medium : Anatomical evidence: the human hand, ox foreleg, whale flipper and bat wing have the same bone pattern, so they are homologous organs (समजात अवयव) with a common ancestor.
- Exam-type : Different jobs, same design: the hand grips, the flipper swims, the wing flies, but the bones inside match.
Rule 2: Vestigial organs — Vestigial organs (अवशेषी अवयव) are organs that are small and of little use now but were useful in ancestors: the coccyx, wisdom teeth, ear muscles and body hair. The appendix is vestigial in humans but functional in ruminants.
- Easy : Wisdom teeth are the last molars, often too small to help us chew.
- Medium : The human coccyx is the remains of the tail that our ancestors had.
- Exam-type : The appendix is large and useful in cattle that digest grass, but small and unneeded in humans.
Homologous organs: same design, common ancestor. Vestigial organs: coccyx, wisdom teeth.
Thinking vestigial organs never had any use. They were useful to ancestors.
Memory clue: "Same bones, one ancestor; small leftovers, old story".
Spot the difference
| Evidence | Example |
|---|---|
| Morphological | Similar body shape |
| Anatomical | Same bones in hand and flipper |
| Vestigial organs | Coccyx, wisdom teeth |
Practice questions
- 1Define evolution.
- 2Where did life begin?
- 3What are homologous organs?
- 4Name two vestigial organs in humans.
- 5Give two examples of animals whose forelimbs are homologous to the human hand.
- 6Why is the appendix called vestigial in humans but not in cattle?
- 7A bird's wing and a butterfly's wing both fly. Are they homologous? Think about their structure.
- 8Why do homologous organs suggest a common ancestor?
Show answers (try them yourself first)
- 1Slow change in organisms over a long time, forming new species.
- 2In the ocean, from simple compounds.
- 3Organs with the same basic structure but possibly different functions.
- 4Coccyx and wisdom teeth.
- 5Bat and whale (or ox).
- 6It is functional in grass-eating animals but of little use in humans.
- 7No; their structure is different, only the function is the same.
- 8Having the same bone plan in different animals is best explained by descent from one ancestor.
Fossils, connecting links and embryology
Digging a well, workers sometimes find a shell shaped like a creature that no longer lives. It is a fossil, a stone memory of life from long ago. Fossils help us read the history of life like pages of a book.
A fossil (जीवाश्म) is the preserved remains or impression of an ancient organism, found in sedimentary rocks.
In the figure : In the figure connecting links join two animal groups.
Rule 1: Fossils and carbon dating — Palaeontological evidence comes from fossils in sedimentary rocks. Carbon dating (कार्बन कालमापन) compares C-14 with C-12 to find the age; the method was developed by Willard Libby (Nobel Prize 1960).
- Easy : A deeper rock layer usually holds older fossils, so layers act like a calendar.
- Medium : C-14 has a half-life of about 5730 years. A fossil with half of its original C-14 is about 5730 years old.
- Exam-type : A fossil keeps only one quarter of its C-14. Two half-lives have passed: 2 × 5730 = 11,460 years.
Rule 2: Connecting links and embryology — Connecting links (दुवे प्राणी) share features of two groups: Peripatus (annelids and arthropods), platypus (reptiles and mammals), lungfish (fishes and amphibians). Embryological evidence: early embryos of vertebrates look very similar, which shows a common origin.
- Easy : The platypus lays eggs like a reptile but feeds its young with milk like a mammal.
- Medium : A lungfish has gills like a fish and also lungs like an amphibian, linking the two groups.
- Exam-type : Early embryos of fish, bird and human all show gill slits and a tail, which hints at a common ancestor.
Fossils in sedimentary rocks; C-14 dating. Links: Peripatus, platypus, lungfish.
Thinking carbon dating works on any rock. It is used on remains of once-living things.
Memory clue: "Peripatus joins worms, platypus joins reptile, lungfish joins water and land".
Spot the difference
| Connecting link | Joins |
|---|---|
| Peripatus | Annelida and Arthropoda |
| Platypus | Reptiles and mammals |
| Lungfish | Fishes and amphibians |
Practice questions
- 1What is a fossil?
- 2In which kind of rock are fossils found?
- 3Who developed carbon dating?
- 4What is a connecting link? Give one example.
- 5A fossil has one eighth of its original C-14 (half-life 5730 years). Find its age.
- 6Which two groups does Peripatus link?
- 7What does the similarity of early vertebrate embryos show?
- 8Why can lower rock layers help to date fossils?
Show answers (try them yourself first)
- 1Preserved remains or impression of an ancient organism.
- 2Sedimentary rocks.
- 3Willard Libby.
- 4An animal showing features of two groups, e.g. platypus.
- 51/8 means 3 half-lives: 3 × 5730 = 17,190 years.
- 6Annelida and Arthropoda.
- 7A common origin.
- 8Lower layers were laid down earlier, so fossils in them are usually older.
Darwin, Lamarck, speciation and human evolution
A giraffe stretches up to eat leaves from a tall tree. Why is its neck so long? Two scientists gave two different answers, and only one of them has stood the test of time.
Natural selection (नैसर्गिक निवड) was proposed by Darwin; use and disuse (वापर-अवापर) by Lamarck.
In the figure : In the figure Lamarck and Darwin explained the giraffe differently.
Rule 1: Darwin and Lamarck — Darwin (Origin of Species): individuals vary, the fittest survive and pass on their useful variations. Lamarck: organs used more grow stronger, unused organs weaken, and acquired characters are inherited. This was disproved.
- Easy : Darwin: some giraffes were born with slightly longer necks, reached more leaves and survived to have young.
- Medium : Lamarck: a giraffe stretched its neck and passed the longer neck to its young. Heredity does not work like this.
- Exam-type : A weightlifter's big muscles are not passed to his child, because acquired characters are not inherited.
In the figure : In the figure the steps of human evolution.
Rule 2: Speciation and human evolution — Speciation (जातिनिर्मिती): new species form through genetic variation and geographical or reproductive isolation. Human evolution: lemur-like ancestors, apes, Ramapithecus, Australopithecus (40 lakh years ago), skilled man (20 lakh), erect man (15 lakh), Neanderthal man, Cro-Magnon man (50,000 years). Agriculture began about 10,000 years ago and writing about 5,000 years ago.
- Easy : A river or mountain splits one group of animals into two; over time each group changes and may become a new species.
- Medium : Order of human evolution by time: Australopithecus (40 lakh) before skilled man (20 lakh) before erect man (15 lakh).
- Exam-type : From erect man (15 lakh years ago) to Cro-Magnon (50,000 years ago) is about 15,00,000 − 50,000 = 14,50,000 years.
Darwin = natural selection; Lamarck = use and disuse (rejected). Cro-Magnon 50,000 years ago.
Saying "survival of the strongest". Darwin meant the best fitted to the surroundings.
Memory clue: "Darwin selects, Lamarck uses".
Spot the difference
| Idea | Darwin | Lamarck |
|---|---|---|
| Main idea | Natural selection | Use and disuse |
| Acquired characters | Not inherited | Inherited (wrong) |
| Status | Accepted | Disproved |
Practice questions
- 1Who wrote the Origin of Species?
- 2What is natural selection?
- 3What did Lamarck propose?
- 4What is speciation?
- 5When did agriculture begin?
- 6Arrange by age, oldest first: Cro-Magnon, Australopithecus, erect man.
- 7Why was Lamarck's idea rejected?
- 8How can isolation lead to a new species?
Show answers (try them yourself first)
- 1Charles Darwin.
- 2Those best fitted to their surroundings survive and pass on their variations.
- 3Use and disuse of organs and inheritance of acquired characters.
- 4Formation of new species.
- 5About 10,000 years ago.
- 6Australopithecus, erect man, Cro-Magnon.
- 7Characters acquired in life are not passed on through genes.
- 8Separated groups cannot mix genes and gather different variations until they cannot interbreed.
Quick revision
- DNA to mRNA (transcription) to protein (translation).
- Evidences: morphological, anatomical, vestigial organs, fossils, connecting links, embryology.
- Darwin = natural selection; Lamarck = use and disuse (rejected).