Heredity & Evolution
Overview: Mendel's Experiments, Sex Determination, Acquired vs Inherited Traits, Speciation, Evolution.
1. Heredity
Transmission of characters from parents to offspring.
Mendel's Work: Used Garden Pea (Pisum sativum).
- Dominant Trait: Expressed in F1 generation (e.g., Tallness 'T').
- Recessive Trait: Suppressed in F1 (e.g., Dwarfness 't').
- Genotype: Genetic makeup (TT, Tt, tt).
- Phenotype: Physical appearance (Tall, Dwarf).
Monohybrid Ratio: 3:1 (Phenotypic)
Dihybrid Ratio: 9:3:3:1 (Round-Yellow : Round-Green : Wrinkled-Yellow : Wrinkled-Green)
2. Sex Determination
Humans have 23 pairs of chromosomes.
- 22 pairs: Autosomes.
- 1 pair: Sex Chromosomes (XX in Female, XY in Male).
Determination: Sperm determines sex. (X-egg + X-sperm = Girl; X-egg + Y-sperm = Boy).
3. Evolution
Gradual change in heritable traits over generations.
- Homologous Organs: Same structure, different function (Frog forelimb, Human hand). Indicates common ancestor.
- Analogous Organs: Different structure, same function (Bird wing, Bat wing).
- Fossils: Preserved traces of past organisms. (Archaeopteryx - link between reptiles and birds).
NCERT In-Text Questions (Solved)
Page 143
Q1. If a trait A exists in 10% of population of an asexually reproducing species and
trait B exists in 60%, which arose earlier?
Trait B arose earlier. In asexual reproduction, traits are passed with minimal
variation. Since B is present in larger percentage (60%), it must have originated earlier and passed
down to more generations than A (10%).
Q2. How variation creates survival advantage?
Variation allows organisms to adapt to changing environments. E.g., bacteria that can
withstand heat (heat-resistant variant) will survive a heat wave, while others die.
Page 147
Q1. How Mendel showed traits may be dominant or recessive?
He crossed pure Tall (TT) with pure Dwarf (tt). F1 generation were all Tall (Tt). This
showed Tallness is dominant. In F2, dwarf plants reappeared (tt). This showed Dwarfness is recessive
(hidden in F1).
Q2. How Mendel showed traits are inherited independently?
In Dihybrid cross (Round-Yellow x Wrinkled-Green), he got new combinations (Round-Green,
Wrinkled-Yellow) in F2 generation. This showed that inheritance of shape (Round/Wrinkled) is independent
of color (Yellow/Green).
Q3. Anatomy of 'A' blood group man marrying 'O' blood group woman. Daughter is 'O'.
Blood group dominance: A and B are dominant over O.
Man is 'A' but has 'O' daughter, so his genotype must be IAIO (Heterozygous).
Woman is 'O', genotype IOIO.
Daughter 'O' inherits IO from both.
Man is 'A' but has 'O' daughter, so his genotype must be IAIO (Heterozygous).
Woman is 'O', genotype IOIO.
Daughter 'O' inherits IO from both.
Q4. Sex determination in humans?
Females are XX (homogametic). Males are XY (heterogametic). Offspring sex depends on
sperm type. 50% chance for Boy/Girl.
Page 150
Q1. Ways in which individuals with a particular trait may increase in population?
1. Natural Selection: Trait gives survival advantage (e.g., Green beetles in green
bushes).
2. Genetic Drift: Accidental change in frequency (e.g., Elephant steps on blue beetles, only red survive).
2. Genetic Drift: Accidental change in frequency (e.g., Elephant steps on blue beetles, only red survive).
Q2. Why traits acquired during lifetime not inherited?
Acquired traits (like learning skills, muscles from gym) cause changes in somatic cells,
not in germ cells (DNA). Only DNA changes are passed to offspring.
Q3. Why small number of surviving tigers a cause of worry?
Small population means very less genetic variation. If a disease spreads or environment
changes drastically, they might all be wiped out (Extinction).
Page 156
Q1. Factors leading to speciation?
Genetic drift, Natural selection, Geographical isolation (Mountains/Rivers separating
populations), DNA changes.
Q2. Will geographical isolation be a factor for self-pollinating plant species?
No. Self-pollinating plants do not depend on other plants for reproduction. They don't
need gene flow between populations. So isolation doesn't affect speciation much.
Q3. Will geographical isolation be a factor for asexually reproducing organisms?
No. They reproduce mostly by fission/budding alone. No mixing of genes with others.
Isolation doesn't stop gene flow (as there is none).
Page 158
Q1. Examples of Homologous organs?
Forelimbs of Humans, Birds, Whales, and Bats. Similar basic structure (bones), different
functions.
Q2. Wings of butterfly and bat?
Analogous Organs. They have different structure (Bat has skin fold on
fingers, Butterfly has membrane) but same function (Flying). Convergent evolution.
Q3. Are fossils formed layer by layer?
Yes. Older fossils are found in deeper layers of earth. Newer fossils are in upper
layers. Helps in dating fossils.
Page 161
Q1. Why human beings who look different belong to same species?
They can reproduce among themselves to produce fertile offspring. Genetic difference is
negligible. All evolved from common ancestor in Africa.
Q2. Evolutionary terms: Bacteria, Spider, Fish, Chimpanzee?
They are not "better" or "worse". They are all evolved to survive in their own
environments. Complex body doesn't mean better survival. Bacteria can survive in hot springs where
humans can't.
NCERT Exercise Questions
Complete solutions for Chapter 8 exercises.
Q1. A Mendelian experiment consisted of breeding tall pea plants bearing violet
flowers with short pea plants bearing white flowers. The progeny all bore violet flowers, but almost
half of them were short. Genetic make up of tall parent?
(c) TtWW.
Reason: All progeny represented Violet flowers, so Violet color is Dominant (WW). Half were short, meaning the tall parent must be Heterozygous (Tt) to produce short offspring (tt) when crossed with short parent (tt).
Reason: All progeny represented Violet flowers, so Violet color is Dominant (WW). Half were short, meaning the tall parent must be Heterozygous (Tt) to produce short offspring (tt) when crossed with short parent (tt).
Q2. Example of Homologous organs?
(d) All of the above.
(Our arm and dog's fore-leg, Our teeth and elephant's tusks, Potato and runners of grass).
(Our arm and dog's fore-leg, Our teeth and elephant's tusks, Potato and runners of grass).
Q3. In evolutionary terms, we have more in common with?
(a) A Chinese school-boy.
Reason: We belong to the same species (Homo sapiens). Chimpanzees are our cousins, not same species.
Reason: We belong to the same species (Homo sapiens). Chimpanzees are our cousins, not same species.
Q4. Study of evolution tells us origin?
It tells us about the gradual development of life from simple to complex forms. It suggests all life
evolved from abiotic material (inorganic molecules) on primitive earth.
Q5. How is sex of child determined?
Determined by the chromosome carried by the sperm.
- If sperm carrying X chromosome fertilizes egg (X) -> Girl (XX).
- If sperm carrying Y chromosome fertilizes egg (X) -> Boy (XY).
Q6. Variations in population?
Variations provide raw material for evolution. Some variations give survival advantage (e.g., bacteria
resistant to heat). Natural selection selects these beneficial variations, helping species survive
changing conditions.
Q7. Explain how sexual reproduction gives rise to more viable variations?
Sexual reproduction involves fusion of DNA from two parents. This mixing creates new combinations of
variants. Errors in DNA copying also add to variation. Natural selection acts on these variations to
select the best accepted ones (viable).
Q8. How is equal genetic contribution of male and female parents ensured?
Gametes (Sperm/Egg) are formed by Meiosis (Reduction division), so they have half the number of
chromosomes (Haploid, n). When fusion occurs (Fertilization), the Zygote gets 'n' from father and 'n'
from mother, restoring the full set (Diploid, 2n).
Q9. Only advantageous variations accumulated?
Mostly yes, via Natural Selection. If a variation is harmful, organism dies. If neutral, it might
persist (Genetic Drift). But generally, advantageous ones increase in frequency as they help in
survival/reproduction.
Key Ratios & Facts
Mendelian ratios and evolutionary terms.
Mendelian Ratios
F2 Mono Phenotype
3 : 1 (Tall:Short)
F2 Mono Genotype
1 : 2 : 1 (TT:Tt:tt)
F2 Dihybrid Phenotype
9 : 3 : 3 : 1
Sex Ratio
1 : 1 (Male:Female)
50 Important Facts
1. Genetics: Study of heredity and variation.
2. Variation leads to evolution.
3. Sexual reproduction produces more variation.
4. Mendel is the Father of Genetics.
5. Mendel used Garden Pea (Pisum sativum).
6. Gene is the functional unit of heredity.
7. Alleles are alternative forms of a gene.
8. Dominant trait is expressed in F1.
9. Recessive trait is suppressed in F1.
10. Homozygous: Same alleles (TT or tt).
11. Heterozygous: Different alleles (Tt).
12. Phenotype: Physical appearance.
13. Genotype: Genetic constitution.
14. TT and Tt are tall phenotypes.
15. tt is short phenotype.
16. Monohybrid Cross involves one pair of
contrasting traits.
17. Dihybrid Cross involves two pairs.
18. Law of Segregation: Alleles separate during
gamete formation.
19. Law of Independent Assortment seen in Dihybrid
cross.
20. DNA carries genetic information.
21. Humans have 23 pairs of chromosomes.
22. 22 pairs are Autosomes.
23. 1 pair is Sex Chromosomes.
24. Male: XY. Female: XX.
25. Gametes are haploid (n).
26. Zygote is diploid (2n).
27. Sex is determined by the sperm.
28. Reptiles (turtles) sex determined by incubation
temperature.
29. Snails can change sex.
30. Evolution is gradual change.
31. Acquired traits are not inherited.
32. Inherited traits are passed via germ cells.
33. Natural Selection (Darwin): Survival of fittest.
34. Genetic Drift: Change in gene frequency by
chance.
35. Speciation: Formation of new species.
36. Geographical isolation leads to speciation.
37. Homologous organs have same structure, different
origin.
38. Analogous organs have different structure, same
function.
39. Homologous organs show Divergent evolution.
40. Analogous organs show Convergent evolution.
41. Wings of bat and bird are analogous.
42. Forelimbs of frog and human are homologous.
43. Fossils provide evidence of evolution.
44. Archaeopteryx is connecting link between
reptiles/birds.
45. Vestigial organs are non-functional (Appendix).
46. Artificial Selection: Breeding cabbage, broccoli
from wild cabbage.
47. Human evolution started in Africa.
48. All humans belong to same species Homo sapiens.
49. DNA dating studies evolutionary relationships.
50. Evolution is not progress, but adaptation.
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