Genetics & Heredity
Key Concepts
| # | Concept | Explanation |
|---|---|---|
| 1 | Gene | Functional unit of heredity made of DNA; occupies a specific locus on a chromosome. |
| 2 | Allele | Alternative form of a gene (e.g. T for tall, t for dwarf). |
| 3 | Genotype | Genetic make-up (e.g. TT, Tt, tt); not always visible. |
| 4 | Phenotype | Observable trait (e.g. tall plant, rolling tongue). |
| 5 | Dominant | Allele that expresses in heterozygote (represented by capital letter). |
| 6 | Recessive | Allele that expresses only in homozygote (small letter). |
| 7 | Homozygous | Both alleles identical (TT or tt). |
| 8 | Heterozygous | Alleles different (Tt). |
15 Practice MCQs
1. A pea plant heterozygous for stem height (Tt) is self-pollinated. What fraction of the offspring will be phenotypically dwarf?
**Options:** A) 0 B) ¼ C) ½ D) ¾ **Answer:** B) ¼ **Solution:** Selfing Tt × Tt → 1 TT : 2 Tt : 1 tt; only tt is dwarf = ¼. **Shortcut:** Make a quick Punnett square; recessive phenotype = 25 %. **Tag:** Monohybrid ratio2. Which of the following is NOT a Mendelian trait in humans?
**Options:** A) Widow’s peak B) Attached ear-lobe C) Height D) Ability to roll tongue **Answer:** C) Height **Solution:** Height is polygenic (many genes + environment); others are single-gene. **Shortcut:** “If it varies continuously → polygenic → not Mendelian.” **Tag:** Human genetics3. A human genetic disorder that is autosomal recessive is
**Options:** A) Haemophilia B) Colour-blindness C) Cystic fibrosis D) Huntington’s **Answer:** C) Cystic fibrosis **Solution:** A & B are X-linked recessive; D is autosomal dominant. **Shortcut:** “CF is recessive, autosomal; remember ‘CF = Common & Fibrosis’.” **Tag:** Genetic disorders4. The sex of a child is determined by
**Options:** A) Ovum of mother B) Cytoplasm of zygote C) Sperm of father D) Uterine environment **Answer:** C) Sperm of father **Solution:** Sperm carries either X or Y chromosome; egg always X. **Shortcut:** “Dad decides: give Y → boy, X → girl.” **Tag:** Sex determination5. A cross between red-flowered (RR) and white-flowered (rr) snapdragons produces all pink (Rr) flowers. This is an example of
**Options:** A) Dominance B) Incomplete dominance C) Co-dominance D) Polygenic inheritance **Answer:** B) Incomplete dominance **Solution:** Heterozygote shows intermediate phenotype (pink). **Shortcut:** “No clear winner = incomplete; both show = co-dominance.” **Tag:** Non-Mendel6. A man of blood group A (genotype AO) marries a woman of blood group B (genotype BO). The probability of their child having blood group O is
**Options:** A) 0 B) 25 % C) 50 % D) 75 % **Answer:** B) 25 % **Solution:** AO × BO → 1 AB : 1 AO : 1 BO : 1 OO; OO = O group = ¼. **Shortcut:** Count OO box out of 4; 25 %. **Tag:** Co-dominance (ABO)7. The number of autosomes in a normal human sperm is
**Options:** A) 22 B) 23 C) 44 D) 46 **Answer:** A) 22 **Solution:** 23 chromosomes total; 1 sex (X or Y) + 22 autosomes. **Shortcut:** “Sperm = half; 46/2 = 23; minus 1 sex → 22 autosomes.” **Tag:** Chromosome number8. If a trait skips generations and affects mainly males, its likely mode is
**Options:** A) Autosomal dominant B) Autosomal recessive C) X-linked recessive D) Y-linked **Answer:** C) X-linked recessive **Solution:** Males (XY) express single X; females carriers often unaffected. **Shortcut:** “Skip + males = think X-linked.” **Tag:** Pedigree pattern9. Which nitrogen base is NOT found in RNA?
**Options:** A) Adenine B) Thymine C) Cytosine D) Uracil **Answer:** B) Thymine **Solution:** RNA replaces thymine with uracil. **Shortcut:** “DNA-T, RNA-U.” **Tag:** Nucleic acids10. A pure-breeding tall pea plant is crossed with a dwarf. In F2 the genotypic ratio will be
**Options:** A) 1:1 B) 2:1 C) 1:2:1 D) 3:1 **Answer:** C) 1:2:1 (TT:Tt:tt) **Solution:** F1 all Tt; self → 1 TT : 2 Tt : 1 tt. **Shortcut:** “Genotype = 1:2:1; Phenotype = 3:1.” **Tag:** Mendel’s monohybrid11. The allele that causes sickle-cell anaemia is
**Options:** A) Dominant B) Recessive C) Co-dominant D) X-linked **Answer:** B) Recessive **Solution:** Only homozygous HbS/HbS shows disease; heterozygous carrier. **Shortcut:** “Sickle = recessive; carrier = advantage vs malaria.” **Tag:** Disorders12. A dihybrid cross (AaBb × AaBb) yields a phenotype ratio of
**Options:** A) 1:1:1:1 B) 3:1 C) 9:3:3:1 D) 12:3:1 **Answer:** C) 9:3:3:1 **Solution:** Independent assortment; classic Mendel result. **Shortcut:** “Dihybrid = 9:3:3:1; memorize like mobile PIN.” **Tag:** Dihybrid ratio13. The chromosome theory of inheritance was first proposed by
**Options:** A) Mendel B) Sutton & Boveri C) Watson & Crick D) Morgan **Answer:** B) Sutton & Boveri **Solution:** They linked Mendel’s factors to chromosomes. **Shortcut:** “Sutton = ‘S’ for ‘Site on chromosome’.” **Tag:** History14. A colour-blind woman marries a normal-visioned man. Their son will be
**Options:** A) All normal B) 50 % colour-blind C) All colour-blind D) 75 % colour-blind **Answer:** C) All colour-blind **Solution:** Mom passes Xc to every son; dad gives Y → no counter allele. **Shortcut:** “XcXc × XY → sons = XcY = all affected.” **Tag:** X-linked15. The process by which RNA is made from DNA is called
**Options:** A) Translation B) Replication C) Transcription D) Transduction **Answer:** C) Transcription **Solution:** DNA → RNA = transcription; RNA → protein = translation. **Shortcut:** “Script = write (DNA writes RNA); Translate = language change.” **Tag:** Central dogmaSpeed Tricks
| Situation | Shortcut | Example |
|---|---|---|
| 1. Monohybrid phenotype ratio | Remember “3:1” | 3 tall : 1 dwarf |
| 2. Monohybrid genotype ratio | Remember “1:2:1” | 1 TT : 2 Tt : 1 tt |
| 3. Dihybrid phenotype ratio | Remember “9:3:3:1” | 9 round-yellow : 3 round-green : 3 wrinkle-yellow : 1 wrinkle-green |
| 4. X-linked recessive pedigree | “More boys, cris-cross” | Mom-pass-to-son, dad-to-daughter |
| 5. Blood group cross | Write gametes in boxes | AO × BO → ¼ O, ¼ AB, ¼ A, ¼ B |
Quick Revision
| Point | Detail |
|---|---|
| 1 | DNA ⇒ RNA ⇒ Protein = Central dogma |
| 2 | 46 chromosomes = 44 autosomes + 2 sex chromosomes |
| 3 | Male = 44 + XY; Female = 44 + XX |
| 4 | Dominant allele masks recessive in heterozygote |
| 5 | Test cross: unknown dominant genotype × recessive → checks purity |
| 6 | Mendel chose pea (7 traits, self/in-cross easy, short life) |
| 7 | Law of Segregation: alleles separate during gamete formation |
| 8 | Law of Independent Assortment: genes on different chromosomes assort independently |
| 9 | Mutation: sudden heritable change in DNA sequence |
| 10 | Pedigree square ⬜ = male, circle ○ = female, shaded = affected |
BETA
