Heredity — Passing Traits from Parent to Offspring
Heredity is the transmission of characteristics from parents to offspring. The chemical basis of heredity is DNA (deoxyribonucleic acid), which carries genes.
Variation = the differences between individuals of the same species. It arises from: 1. Sexual reproduction (mixing of genetic material from two parents) 2. Mutations (random changes in DNA)
Inherited vs Acquired Traits
| Inherited Traits | Acquired Traits |
|---|---|
| Controlled by genes in DNA | Result of environment or experience |
| Can be passed to offspring | Cannot be passed to offspring |
| Examples: eye colour, height, blood group | Examples: scars, muscular body from exercise, cut hair |
Acquired traits do NOT change the DNA in reproductive cells (eggs/sperm), so they cannot be inherited.
Mendel's Experiments — Laws of Heredity
Gregor Mendel worked with garden pea plants (Pisum sativum) and studied 7 pairs of contrasting traits.
Key Terms
| Term | Meaning |
|---|---|
| Gene | A unit of heredity; a segment of DNA that codes for a trait |
| Allele | One of two or more alternative forms of a gene |
| Dominant | Allele that is expressed when present (shown as capital letter, e.g., T) |
| Recessive | Allele expressed only when two copies are present (shown as lowercase, e.g., t) |
| Homozygous | Two identical alleles (TT or tt) |
| Heterozygous | Two different alleles (Tt) |
| Genotype | Genetic makeup (e.g., TT, Tt, tt) |
| Phenotype | Observable characteristic (e.g., tall, short) |
Law 1 — Law of Dominance
When two individuals with contrasting characters are crossed, only one character (the dominant one) appears in the F₁ generation. The other (recessive) is hidden.
Law 2 — Law of Segregation (Law of Purity of Gametes)
The two alleles for a trait separate during gamete formation. Each gamete carries only one allele. The alleles recombine randomly during fertilisation.
Monohybrid Cross (One trait)
Cross between Tall (TT) and Short (tt) pea plants:
P generation: TT × tt
F₁ generation: All Tt (tall) — only dominant trait visible
F₁ × F₁: Tt × Tt
F₂ generation:
F₂ results:
- Genotypic ratio: 1 TT : 2 Tt : 1 tt
- Phenotypic ratio: 3 Tall : 1 Short = 3:1
Law 3 — Law of Independent Assortment
Genes for different traits assort independently during gamete formation (when the gene pairs are on different chromosomes).
Dihybrid Cross (Two traits)
Cross round yellow (RRYY) × wrinkled green (rryy)
F₁: All RrYy (round yellow — both dominant expressed)
F₂ phenotypic ratio: 9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green = 9:3:3:1
Chromosomes and Sex Determination
Humans have 46 chromosomes (23 pairs):
- 22 pairs = autosomes (same in males and females)
- 1 pair = sex chromosomes
- Females: XX
- Males: XY
How Sex is Determined
| X egg | — | |
|---|---|---|
| X sperm | XX = Female | — |
| Y sperm | XY = Male | — |
- All eggs carry X
- Sperm carry either X or Y
- The father's sperm determines the child's sex
- Probability: 50% male, 50% female each time
Important: The mother always contributes X. The sex of the child is determined by which type of sperm (X or Y) fertilises the egg. It is scientifically wrong (and socially harmful) to blame the mother for the sex of a child.
Evolution
What is Evolution?
Evolution = the gradual change in the inherited characteristics of a species over successive generations.
It explains how all life on Earth is related and how new species arise from pre-existing ones.
Darwin's Theory of Evolution by Natural Selection
1. Variation: Individuals in a population show variation in their traits 2. Overproduction: More offspring are produced than can survive 3. Struggle for existence: Competition for food, space, mates 4. Natural selection: Individuals with favourable traits survive and reproduce 5. Inheritance: Favourable traits are passed to offspring 6. Over time: The population gradually changes → evolution
Evidences for Evolution
1. Homologous organs: Same basic structure, different function — shows common ancestry.
- Human arm, whale flipper, bat wing, horse leg — all have same bones (humerus, radius, ulna, carpals) arranged differently
2. Analogous organs: Similar function, different structure — shows convergent evolution.
- Wings of birds (bone-supported) vs wings of insects (chitinous) — evolved independently
3. Fossils: Remains of ancient organisms preserved in rock layers. Older fossils are found in deeper rock strata.
- Show that complex organisms evolved from simpler ones
- Show intermediate forms (e.g., Archaeopteryx — between reptile and bird)
4. Vestigial organs: Organs present in humans that are non-functional or reduced, but were functional in ancestors.
- Examples: appendix, coccyx (tail bone), body hair, ear muscles
Speciation
The process by which two populations of the same species become so different that they can no longer interbreed → forming new species.
Causes:
- Geographical isolation: Mountains, rivers, oceans separate populations
- Isolated populations face different environments → different selection pressures → accumulate different mutations over generations → become distinct species
Human Evolution
- Humans share ~98.7% of DNA with chimpanzees — common ancestor ~6–7 million years ago
- Homo sapiens (modern humans) originated in Africa ~200,000 years ago
- Migrated to rest of the world, replacing or interbreeding with other hominid species
Board Exam Tips
- 6–8 marks from this chapter
- Monohybrid cross Punnett square — draw it carefully; show all working
- Sex determination diagram — which chromosomes come from mother, which from father
- "The father determines the sex of the child" — explain with XX/XY mechanism
- Homologous vs analogous organs — one real example + one diagram (usually worth 3 marks)
- Evolution is NOT about individual organisms changing — it's about populations changing over generations
- Mendel chose pea plants because: short life cycle, many offspring, easy to control pollination, and had clear contrasting traits
📚 Lakhmir Singh Science Class 10 — Heredity Chapter with Solved Crosses
📝 Oswaal CBSE Question Bank — Heredity and Evolution Board Questions
NCERT Exercise Solutions
Intext Questions
Page 143, Q1: If a trait A exists in 10% of a population of an asexually reproducing species, and a trait B exists in 60% of the same population, which trait is likely to have arisen earlier?
Page 147, Q2: In human beings, the statistical probability of getting either a boy or a girl is 50:50. Explain using sex chromosomes.
During reproduction:
- Mother's eggs always contain X chromosome (only type she can produce)
- Father's sperm can be either X-bearing (50%) or Y-bearing (50%)
If X-sperm fertilises egg → XX → Girl If Y-sperm fertilises egg → XY → Boy
Since X-sperm and Y-sperm are produced in equal numbers and fertilisation is random, the probability of boy or girl is 50:50.
Note: Sex of the child is determined by the father's sperm, not the mother.
Chapter-End Exercise
Q1. A man with blood group A married a woman with blood group O and their daughter has blood group O. Is this information enough to tell you which of the traits — blood group A or O — is dominant? Why or why not?
Reasoning:
- The daughter has blood group O, which means she inherited O alleles from both parents.
- The father has blood group A but passed an O allele to his daughter → his genotype must be AO (heterozygous A)
- The mother has blood group O → genotype OO
- Cross: AO × OO → AO (blood group A) : OO (blood group O) = 1:1
Since blood group A (AO) shows A phenotype even with one O allele, A is dominant over O (which only shows when homozygous).
Q2. How does Mendel's work show that traits may be dominant or recessive?
- F₁ generation: All offspring were tall — the tall trait "dominated"
- F₂ generation (F₁ × F₁): 3 tall : 1 dwarf appeared
Observations: 1. In F₁, dwarf trait completely disappeared — it was hidden (recessive) 2. In F₂, dwarf reappeared unchanged — it was not lost, only masked in F₁ 3. Tall always appeared 3× more often than dwarf → 3:1 ratio
Conclusion: Tall is dominant (expressed whenever present), dwarf is recessive (expressed only when no dominant allele is present). Traits are inherited as discrete units (genes) and can be masked in one generation but reappear in the next.
Q3. Explain the terms: (a) Heredity (b) Variation (c) Natural selection
(b) Variation: Differences in traits among individuals of the same species. Variation arises from mutations, sexual reproduction (recombination), and environmental factors.
(c) Natural selection: The process by which organisms with favorable variations (better adapted to the environment) survive and reproduce more successfully than others. Over generations, favorable traits become more common in the population. Darwin described this as "survival of the fittest."
Q4. What is speciation? List the factors that could lead to speciation.
Factors leading to speciation: 1. Geographical isolation — physical barriers (mountains, rivers, seas) separate populations; they evolve independently 2. Genetic drift — random changes in allele frequency in small isolated populations 3. Natural selection — different environments select for different traits in separated populations 4. Mutations — accumulated mutations over time lead to genetic differences 5. Reproductive isolation — once populations diverge enough that they cannot interbreed, a new species has formed
Example: Galápagos finches — geographically isolated populations evolved into different species with different beak shapes suited to different food sources.
Q5. How are the sex of the offspring determined in human beings? Is it the mother or the father who determines the sex?
- Females: XX (two X chromosomes)
- Males: XY (one X, one Y chromosome)
All eggs from the mother contain one X chromosome. Sperm from the father is of two types: 50% carry X, 50% carry Y.
- X sperm + egg → XX = Female (girl)
- Y sperm + egg → XY = Male (boy)
It is the father who determines the sex of the child, because:
- The mother contributes only X
- The type of sperm (X or Y) that fertilises the egg determines sex
- The father produces both X-bearing and Y-bearing sperm in equal proportions