Genetics & Heredity

Key Concepts

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 ratio

2. 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 genetics

3. 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 disorders

4. 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 determination

5. 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-Mendel

6. 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 number

8. 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 pattern

9. 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 acids

10. 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 monohybrid

11. 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: Disorders

12. 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 ratio

13. 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: History

14. 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-linked

15. 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 dogma

Speed Tricks

Quick Revision