Genetics
Introduction
Genetics is the study of inheritance and variation. In ICSE Class 10 Biology, you study Mendel's laws of inheritance, the mechanism of sex determination in humans, and the inheritance of blood groups.
Key Terms
| Term | Definition |
|---|---|
| Gene | Unit of inheritance (segment of DNA) |
| Allele | Alternative form of a gene (dominant/recessive) |
| Genotype | Genetic makeup (e.g., TT, Tt, tt) |
| Phenotype | Observable characteristic |
| Homozygous | Both alleles are the same (TT or tt) |
| Heterozygous | Alleles are different (Tt) |
| Dominant | Allele that expresses itself in heterozygous condition |
| Recessive | Allele that is masked in heterozygous condition |
Mendel's Laws
Law of Dominance
When two homozygous individuals with contrasting traits are crossed, the F₁ generation shows only one of the traits (the dominant trait).
Law of Segregation
During gamete formation, the two alleles for a trait separate (segregate) so that each gamete carries only one allele.
Law of Independent Assortment
When two or more pairs of contrasting traits are considered, the alleles of different genes assort independently during gamete formation.
Monohybrid Cross
Example: Height in Pea Plants
P generation: TT (tall) × tt (dwarf) Gametes: T and t F₁ generation: All Tt (tall)
F₁ self-cross (Tt × Tt):
| T | t | |
|---|---|---|
| T | TT (tall) | Tt (tall) |
| t | Tt (tall) | tt (dwarf) |
Phenotypic ratio: 3 tall : 1 dwarf Genotypic ratio: 1 TT : 2 Tt : 1 tt
Dihybrid Cross
Example: Seed Shape and Colour
P generation: RRYY (round yellow) × rryy (wrinkled green) F₁ generation: All RrYy (round yellow)
F₁ self-cross (RrYy × RrYy):
| Gametes | RY | Ry | rY | ry |
|---|---|---|---|---|
| RY | RRYY | RRYy | RrYY | RrYy |
| Ry | RRYy | RRyy | RrYy | Rryy |
| rY | RrYY | RrYy | rrYY | rrYy |
| ry | RrYy | Rryy | rrYy | rryy |
Phenotypic ratio: 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green
Sex Determination in Humans
Humans have 23 pairs of chromosomes: 22 pairs of autosomes + 1 pair of sex chromosomes.
| Sex | Chromosomes | Genotype |
|---|---|---|
| Female | XX | Homogametic |
| Male | XY | Heterogametic |
Mechanism:
- All eggs carry X chromosome.
- Sperms carry either X or Y chromosome (50% each).
- If X sperm fertilises egg → XX → female.
- If Y sperm fertilises egg → XY → male.
Probability of having a boy or girl = 50% each (the father determines the sex of the child).
Blood Group Inheritance (ABO System)
| Blood group | Genotype | Antigen on RBC | Antibody in plasma |
|---|---|---|---|
| A | IᴬIᴬ or Iᴬi | A | Anti-B |
| B | IᴮIᴮ or Iᴮi | B | Anti-A |
| AB | IᴬIᴮ | A and B | None (universal recipient) |
| O | ii | None | Anti-A and Anti-B (universal donor) |
Inheritance Pattern
- Iᴬ and Iᴮ are co-dominant (both express when together).
- i is recessive to both Iᴬ and Iᴮ.
Common Mistakes and Fixes
| Mistake | Fix |
|---|---|
| Confusing genotype and phenotype | Genotype = genes; Phenotype = physical trait |
| Thinking dominant allele is more common | Dominance is about expression, not frequency |
| Forgetting the Punnett square setup | Male gametes on top, female gametes on side |
| Mixing co-dominance with incomplete dominance | Co-dominance = BOTH express; Incomplete = BLEND |
| Writing incorrect gamete combinations in dihybrid cross | Each gamete gets ONE allele from each gene pair |
ICSE Exam Focus
This chapter carries 6–8 marks. Key topics: monohybrid and dihybrid crosses (Punnett squares, ratios), Mendel's laws, sex determination, blood group genetics.
Marks Blueprint: Monohybrid cross — 3 marks, Dihybrid cross — 3 marks, Sex determination/Blood groups — 2 marks.
Self-Test Questions
-
State Mendel's law of segregation. Explain with a monohybrid cross.
-
In pea plants, tall (T) is dominant over dwarf (t). What is the phenotypic ratio of F₂ when Tt is self-pollinated?
-
Explain sex determination in humans with the help of a diagram.
-
A child has blood group O. The mother has blood group A. What are the possible blood groups of the father?
-
In a dihybrid cross between RrYy × RrYy, what is the probability of getting round green seeds?
-
Differentiate between dominant and recessive traits.
