Reproduction, Heredity and Evolution

What to remember

• Reproduction makes new individuals. Asexual reproduction has one parent and gives identical offspring. Sexual reproduction needs two gametes and gives variation.
• Heredity passes characters through genes on chromosomes (DNA). Mendel's monohybrid ratio is 3:1 in the F₂ and the dihybrid ratio is 9:3:3:1. Humans have 23 pairs of chromosomes; sex is decided by the father's X or Y.
• Evolution is gradual change of species over time. Darwin explained it by natural selection. Fossils, homologous organs and embryology give evidence.

Asexual reproduction

Vegetative propagation in forestry and nurseries: cutting, layering, grafting and budding give plants identical to the parent, and fruit and flowers come earlier. Teak and eucalyptus can be multiplied by clonal methods; bamboo spreads by rhizomes. Seedless plants such as banana are propagated vegetatively. Tissue culture grows new plants from small tissue pieces in a nutrient medium.

Asexual reproduction gives little variation. That is a risk if disease strikes.

Sexual reproduction in flowering plants

Parts of a flower: sepals (calyx), petals (corolla), stamen (male: anther and filament), pistil or carpel (female: stigma, style, ovary). The anther makes pollen grains; the ovary has ovules with the egg cell.

• Pollination: transfer of pollen from anther to stigma. Self-pollination is within the same flower or plant. Cross-pollination is between flowers of different plants, by wind, insects, water or birds.
• Fertilisation: the pollen grain forms a pollen tube that reaches the ovule. A male gamete fuses with the egg to form the zygote. In flowering plants double fertilisation also forms the endosperm (food store).
• After fertilisation: ovule → seed; ovary → fruit.
• Seed dispersal by wind (maple, drumstick), water (coconut), animals (burr fruits) and explosion (pea pods). Dispersal helps forests spread and regenerate.

Many bamboo species flower together after long gaps and then die. This is called gregarious flowering.

Human reproduction

Male system: testes (make sperm and testosterone, held in the scrotum), vas deferens, seminal vesicles, prostate gland, urethra.

Female system: ovaries (make eggs and oestrogen), fallopian tubes (oviducts), uterus (womb), vagina.

• Puberty: the age of sexual maturity, controlled by hormones.
• Fertilisation: sperm meets egg in the fallopian tube. The zygote divides, travels to the uterus and implants in its wall.
• Placenta: the organ that links mother and embryo; it brings nutrients and oxygen and removes waste.
• Gestation period: about 9 months (about 40 weeks) in humans.
• Menstrual cycle: about 28 days; ovulation occurs about the middle of the cycle.

Contraception: barrier methods (condoms), hormonal pills, intrauterine devices (IUDs) and surgical methods (vasectomy in males, tubectomy in females). Condoms also reduce risk of sexually transmitted diseases such as HIV/AIDS.

Animal reproduction: fish and frogs have external fertilisation; birds, reptiles and mammals have internal fertilisation. Mammals are viviparous (give birth to young); birds are oviparous (lay eggs).

Heredity

Gregor Mendel (Father of Genetics) worked on the garden pea (*Pisum sativum*) and gave the laws of inheritance.

Monohybrid cross: tall (TT) × dwarf (tt) → F₁ all tall (Tt). F₁ self → F₂ phenotype ratio 3 tall : 1 dwarf, genotype ratio 1 TT : 2 Tt : 1 tt.

Dihybrid cross: round yellow × wrinkled green → F₂ ratio 9 : 3 : 3 : 1.

Mendel's laws: law of dominance, law of segregation (alleles separate in gametes), and law of independent assortment (traits are inherited independently).

Chromosomes and DNA: genes sit on chromosomes in the nucleus. DNA (deoxyribonucleic acid) is a double helix described by Watson and Crick. Its four bases are adenine, thymine, guanine and cytosine. Humans have 46 chromosomes (23 pairs); gametes carry 23.

Sex determination in humans: females are XX; males are XY. Eggs always carry X; sperm carry X or Y. So the father's sperm decides the child's sex. Probability of a boy or a girl is 50 per cent each.

Variation arises from sexual reproduction, crossing over and mutations. A mutation is a sudden change in DNA.

Evolution

• Lamarck: proposed "use and disuse" and inheritance of acquired characters. Acquired characters are not inherited.
• Darwin: proposed natural selection (survival of the fittest) in his book *On the Origin of Species* (1859). Variations that help survival are passed on.
• Speciation: formation of new species when populations are isolated.

Human evolution (broadly): *Australopithecus*, *Homo habilis*, *Homo erectus*, *Homo sapiens*. Humans did not evolve from today's apes; both share a common ancestor.

Conservation link: loss of habitat and genetic diversity reduces the chance of species adapting. Protected forests keep gene pools alive.

More on reproduction and inheritance

Parts of a seed: the seed coat, the embryo (plumule, radicle, cotyledons) and food store. Germination needs water, air and suitable temperature; light is not needed in most seeds. The radicle grows into the root and the plumule into the shoot. Dicot seeds (gram) have two cotyledons; monocot seeds (maize) have one.

Forest regeneration: natural regeneration is by seeds, coppice (shoots from stumps) and root suckers. Species such as sal and teak coppice well. Foresters use nurseries, direct seeding and planting of clonal cuttings. Seed viability and storage matter for plantations.

Types of reproductive strategies

Cell division: mitosis gives two identical cells with the same chromosome number and is used in growth and repair. Meiosis makes gametes with half the chromosome number (reduction division) and occurs in the testes and ovaries. Fertilisation restores the full number.

Hormones in reproduction: testosterone (male), oestrogen and progesterone (female). The pituitary hormones FSH and LH control the cycle.

Genetic disorders: colour blindness and haemophilia are sex-linked recessive disorders on the X chromosome, more common in males. Down syndrome is caused by an extra chromosome 21 (47 chromosomes). Sickle-cell anaemia is a recessive blood disorder.

Biotechnology links: hybrid seeds, genetically modified crops and cloning rely on genes. Dolly the sheep was the first mammal cloned from an adult cell.

Evolution and classification: organisms with common ancestry are grouped together. Natural selection acts on variation: in a population, individuals with helpful traits survive and breed more. Example often given is the peppered moth, in which dark forms increased in polluted areas. Antibiotic resistance in bacteria is a present-day example of natural selection.

Extinction: when a species disappears forever. Habitat loss, hunting and climate change are main reasons. Forest officers protect habitats to keep species from becoming extinct.

Exam traps

• 1. Pollination vs fertilisation: pollination is transfer of pollen; fertilisation is fusion of gametes.
• 2. Ovary becomes fruit; ovule becomes seed.
• 3. Homologous vs analogous: same origin vs same function.
• 4. Father decides the child's sex, not the mother.
• 5. Fertilisation in humans occurs in the fallopian tube, not the uterus.
• 6. Phenotype ratio 3:1 vs genotype ratio 1:2:1.
• 7. Lamarck vs Darwin: acquired characters vs natural selection.
• 8. Budding vs fission: budding gives an outgrowth; fission divides the parent.

One-liners

• 1. Mendel is the Father of Genetics.
• 2. Mendel used the garden pea.
• 3. F₂ monohybrid phenotype ratio is 3:1.
• 4. Dihybrid F₂ ratio is 9:3:3:1.
• 5. Humans have 23 pairs of chromosomes.
• 6. Females are XX and males are XY.
• 7. DNA structure was given by Watson and Crick.
• 8. Natural selection is Darwin's theory.
• 9. Human gestation is about 9 months.
• 10. Placenta nourishes the embryo.
• 11. Amoeba reproduces by binary fission.
• 12. Yeast reproduces by budding.

Practice questions

1. Amoeba reproduces by

   1. budding
   2. fragmentation
   3. spore formation
   4. binary fission
   Answer

   D. binary fission

   The cell divides into two.

2. Yeast commonly reproduces by

   1. binary fission
   2. regeneration
   3. pollination
   4. budding
   Answer

   D. budding

   A small bud grows from the parent.

3. Spirogyra reproduces by

   1. seeds
   2. budding
   3. spores
   4. fragmentation
   Answer

   D. fragmentation

   The filament breaks into pieces.

4. Bryophyllum reproduces vegetatively through its

   1. leaves
   2. stem tubers
   3. seeds
   4. roots
   Answer

   A. leaves

   Plantlets develop at leaf notches.

5. Potato is propagated vegetatively by

   1. leaves
   2. tubers
   3. rhizomes
   4. bulbs
   Answer

   B. tubers

   Buds on tubers give new plants.

6. Ginger spreads by

   1. bulbs
   2. spores
   3. rhizomes
   4. tubers
   Answer

   C. rhizomes

   Rhizomes are underground stems.

7. Bread mould (Rhizopus) reproduces by

   1. spores
   2. bulbs
   3. budding
   4. seeds
   Answer

   A. spores

   Spore formation is asexual.

8. Growing new plants from small tissue pieces in a nutrient medium is called

   1. grafting
   2. layering
   3. hybridisation
   4. tissue culture
   Answer

   D. tissue culture

   It gives many plants from a small piece.

9. Transfer of pollen from anther to stigma is

   1. germination
   2. pollination
   3. fertilisation
   4. dispersal
   Answer

   B. pollination

   Fertilisation is fusion of gametes.

10. After fertilisation, the ovary becomes the

    1. flower
    2. stigma
    3. fruit
    4. seed
    Answer

    C. fruit

    The ovule becomes the seed.

11. The male part of a flower is the

    1. stamen
    2. carpel
    3. pistil
    4. sepal
    Answer

    A. stamen

    Anther and filament form the stamen.

12. Double fertilisation in flowering plants also forms the

    1. embryo sac
    2. endosperm
    3. pollen
    4. cotyledon only
    Answer

    B. endosperm

    Endosperm stores food.

13. Seeds of drumstick and maple are mostly dispersed by

    1. water
    2. explosion
    3. animals
    4. wind
    Answer

    D. wind

    They have wings or hairs.

14. Coconut is dispersed mainly by

    1. animals
    2. wind
    3. water
    4. explosion
    Answer

    C. water

    The fibrous husk floats.

15. Fertilisation in humans normally occurs in the

    1. uterus
    2. fallopian tube
    3. ovary
    4. vagina
    Answer

    B. fallopian tube

    Sperm meets the egg in the oviduct.

16. The organ that nourishes the embryo in the womb is the

    1. testis
    2. ovary
    3. bladder
    4. placenta
    Answer

    D. placenta

    It links mother and embryo.

17. The testes produce sperm and the hormone

    1. testosterone
    2. oestrogen
    3. thyroxine
    4. insulin
    Answer

    A. testosterone

    Testosterone controls male characters.

18. Surgical sterilisation of males is called

    1. vasectomy
    2. tubectomy
    3. caesarean
    4. IUD
    Answer

    A. vasectomy

    The vas deferens is cut.

19. Gregor Mendel is called the

    1. Father of Botany
    2. Father of Evolution
    3. Father of Genetics
    4. Father of Medicine
    Answer

    C. Father of Genetics

    He worked on garden pea.

20. Mendel's experimental plant was the

    1. sunflower
    2. maize
    3. mango
    4. garden pea
    Answer

    D. garden pea

    Pisum sativum.

21. In a monohybrid cross, the F₂ phenotypic ratio is

    1. 1 : 2 : 1
    2. 3 : 1
    3. 9 : 3 : 3 : 1
    4. 1 : 1
    Answer

    B. 3 : 1

    Three dominant to one recessive.

22. The F₂ genotypic ratio in a monohybrid cross is

    1. 1 : 1
    2. 1 : 2 : 1
    3. 9 : 3 : 3 : 1
    4. 3 : 1
    Answer

    B. 1 : 2 : 1

    1 TT : 2 Tt : 1 tt.

23. The dihybrid F₂ phenotypic ratio is

    1. 1 : 1 : 1 : 1
    2. 1 : 2 : 1
    3. 9 : 3 : 3 : 1
    4. 3 : 1
    Answer

    C. 9 : 3 : 3 : 1

    Independent assortment of two traits.

24. A plant with genotype Tt is

    1. homozygous dominant
    2. homozygous recessive
    3. a mutant
    4. heterozygous
    Answer

    D. heterozygous

    Two different alleles.

25. The double helix structure of DNA was given by

    1. Watson and Crick
    2. Hooke and Schleiden
    3. Mendel and Darwin
    4. Lamarck and Wallace
    Answer

    A. Watson and Crick

    They proposed it in 1953.

26. How many chromosomes does a normal human body cell have?

    1. 48
    2. 23
    3. 44
    4. 46
    Answer

    D. 46

    23 pairs.

27. A human egg contains how many chromosomes?

    1. 44
    2. 23
    3. 22
    4. 46
    Answer

    B. 23

    Gametes have half the number.

28. The sex of a human child is decided by

    1. the placenta
    2. the age of the mother
    3. the father's sperm
    4. the mother's egg
    Answer

    C. the father's sperm

    Sperm carry X or Y; eggs carry X.

29. A male human has the sex chromosomes

    1. XY
    2. YY
    3. XO
    4. XX
    Answer

    A. XY

    Females are XX.

30. Darwin's theory of evolution is based on

    1. natural selection
    2. use and disuse
    3. spontaneous generation
    4. mutation only
    Answer

    A. natural selection

    Survival of the fittest.

31. Inheritance of acquired characters was proposed by

    1. Mendel
    2. Darwin
    3. Watson
    4. Lamarck
    Answer

    D. Lamarck

    It is not supported by evidence.

32. A human forelimb and a bat's wing are

    1. fossils
    2. homologous organs
    3. analogous organs
    4. vestigial organs
    Answer

    B. homologous organs

    Same basic structure, different function.

33. Wings of a bird and an insect are

    1. atavistic organs
    2. vestigial organs
    3. analogous organs
    4. homologous organs
    Answer

    C. analogous organs

    Same function, different origin.

34. The appendix in humans is an example of a

    1. analogous organ
    2. vestigial organ
    3. homologous organ
    4. sex organ
    Answer

    B. vestigial organ

    It is reduced and has little use.

35. Archaeopteryx is a fossil linking

    1. plants and fungi
    2. fish and frogs
    3. apes and humans
    4. reptiles and birds
    Answer

    D. reptiles and birds

    It shows features of both.

36. Gregarious flowering followed by death is seen in many species of

    1. bamboo
    2. neem
    3. mango
    4. teak
    Answer

    A. bamboo

    Bamboo flowers once after long intervals.

37. A cross between a tall (TT) and a dwarf (tt) pea gives F₁ plants that are

    1. all dwarf
    2. half tall, half dwarf
    3. all tall
    4. three-fourths tall
    Answer

    C. all tall

    Tall is dominant; F₁ is Tt.

38. In a cross Tt × Tt, what fraction of offspring is dwarf (tt)?

    1. 3/4
    2. 1/4
    3. 1/8
    4. 1/2
    Answer

    B. 1/4

    The genotype ratio is 1 : 2 : 1.

39. In a cross Tt × Tt with 200 offspring, how many are expected to be tall?

    1. 150
    2. 50
    3. 100
    4. 200
    Answer

    A. 150

    3/4 of 200 = 150.

40. A couple's chance of having a girl in each birth is

    1. 75 per cent
    2. 100 per cent
    3. 50 per cent
    4. 25 per cent
    Answer

    C. 50 per cent

    Sperm carry X or Y with equal chance.

41. Statements: 1. Asexual reproduction involves one parent. 2. Sexual reproduction produces more variation. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are true.

42. Statements: 1. The ovule develops into the fruit. 2. The ovary develops into the fruit. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    The ovule becomes the seed.

43. Statements: 1. Mother decides the sex of the child. 2. Eggs always carry the X chromosome. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    The sperm decides the sex.

44. Statements: 1. Humans evolved from present-day apes. 2. Humans and apes share a common ancestor. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Both groups evolved from a common ancestor.

45. Statements: 1. DNA is a double helix. 2. Genes are sections of DNA on chromosomes. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are true.