Heredity — Practice

Question 1

How many chromosomes do human body cells have?

Accepted Answer

✦ Answer: 46 chromosomes (23 pairs). 22 pairs of autosomes + 1 pair of sex chromosomes (XX in females, XY in males).

Question 2

Who determines the sex of the child?

Accepted Answer

✦ Answer: The FATHER. Mother gives X (always). Father gives X (→ girl, XX) or Y (→ boy, XY).

Question 3

Cross a tall plant (Tt) with a dwarf plant (tt). Find the offspring ratio.

Accepted Answer

Step 1 — Set up Punnett square.
  Tall parent gametes: T, t
  Dwarf parent gametes: t, t

Step 2 — Combine.
  |   | T  | t  |
  | t | Tt | tt |
  | t | Tt | tt |

Step 3 — Count results.
  Tt: 2 (tall, carrier of dwarf gene)
  tt: 2 (dwarf)

Step 4 — Ratio.
  2:2 = 1:1
  Tall:Dwarf = 1:1

Step 5 — Phenotype vs Genotype.
  Phenotype: 1:1 (tall to dwarf)
  Genotype: 1:1 (Tt to tt)

✦ Answer: Offspring are 50% Tt (tall) and 50% tt (dwarf). Ratio = 1:1.

Question 4

Two brown-eyed parents have a blue-eyed child. Explain genetically with a Punnett square.

Accepted Answer

Step 1 — Information.
  Brown (B) is DOMINANT.
  Blue (b) is RECESSIVE.
  Both parents are brown-eyed but child is blue-eyed.

Step 2 — Possible parent genotypes.
  Brown-eyed could be BB or Bb.
  For a blue-eyed (bb) child, BOTH parents must carry the b allele.
  So both parents must be Bb (heterozygous carrier).

Step 3 — Punnett square.
  |   | B  | b  |
  | B | BB | Bb |
  | b | Bb | bb |

Step 4 — Results.
  BB: 1 (homozygous brown — pure brown)
  Bb: 2 (heterozygous brown — appears brown, but carrier)
  bb: 1 (homozygous recessive — BLUE eyes!)

Step 5 — Probability.
  25% chance of BB (brown)
  50% chance of Bb (brown, carrier)
  25% chance of bb (BLUE)

Step 6 — Explanation.
  Both parents look BROWN (because they have at least one B allele), but each CARRIES a b allele. When both pass the b allele, child is bb (blue-eyed).

Step 7 — Importance of recessive traits.
  Many genetic diseases are recessive (sickle cell, thalassaemia). Two healthy parents can have an affected child if both are carriers.

Step 8 — Indian context.
  Genetic counselling helps couples plan pregnancies, especially for traits like thalassaemia (common in Indian populations).

✦ Answer: Both parents must be heterozygous (Bb) — outwardly brown but carrying blue allele. Punnett square shows 1:2:1 ratio (BB:Bb:bb). 25% chance of blue-eyed child (bb). This is how recessive traits 'reappear' — both parents need to be carriers.

Practice — Heredity

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