Key Concepts
- 1What is Mendel's Law of Segregation?
- 2How is sex determined in humans?
Important Formulas & Facts
During gamete formation, the two alleles of a gene separate so each gamete gets only one allele. In F₂ of monohybrid cross: 3:1 phenotypic ratio, 1:2:1 genotypic ratio (TT:Tt:tt).
Females = XX, Males = XY. Mother always gives X. Father gives X (girl) or Y (boy). So father's sperm determines sex of child. 50% probability each.
Must-Know Questions
Q1Who is known as the 'Father of Genetics'?
Gregor Johann Mendel is known as the Father of Genetics. He conducted experiments on pea plants (Pisum sativum) in his monastery garden and formulated the laws of inheritance.
Q2In a monohybrid cross between tall (TT) and short (tt) pea plants, what is the phenotypic ratio in F₂ generation?
F₁ generation: All Tt (tall) — tall is dominant over short. F₂ generation (Tt × Tt): TT : Tt : Tt : tt = 1:2:1 genotypic ratio. Phenotypic ratio = 3 Tall : 1 Short (3:1). This is Mendel's law of segregation.
Q3How is the sex of a child determined in humans?
Sex determination in humans: 1. Humans have 23 pairs of chromosomes — 22 pairs of autosomes and 1 pair of sex chromosomes. 2. Females have XX sex chromosomes. Males have XY sex chromosomes. 3. All eggs produced by mother contain X chromosome. 4. Sperm produced by father are of two types — 50% carry X and 50% carry Y. 5. If X-bearing sperm fertilises the egg → XX → Female child. 6. If Y-bearing sperm fertilises the egg → XY → Male child. 7. Therefore, the father's sperm determines the sex of the child, NOT the mother. 8. Probability of boy or girl = 50% each.
Q4What are homologous organs? Give examples.
Homologous organs are organs that have the same basic structural plan and origin but perform different functions in different organisms. They indicate common ancestry (divergent evolution). Examples: Forelimbs of humans (grasping), whales (swimming), bats (flying), and horses (running) — all have the same bone pattern (humerus, radius, ulna, carpals, metacarpals, phalanges) but are modified for different functions.
Q5What are analogous organs? How do they differ from homologous organs?
Analogous organs have different basic structure and origin but perform similar functions. They indicate convergent evolution (not common ancestry). Examples: Wings of birds and wings of insects — both used for flying but have completely different structures. Eyes of octopus and eyes of mammals. Difference: Homologous organs share structural origin but differ in function (common ancestor). Analogous organs share function but differ in structure (no common ancestor).
Practice Heredity and Evolution
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