Topic 16: Inheritance

Question 1

Definitions I:
(a) Haploid
(b) Diploid
(c) Allele

Answer 1

(a) Haploid: Single chromosome set (n). (1 mark)
(b) Diploid: Two chromosome sets (2n). (1 mark)
(c) Allele: Variant form of a gene. (1 mark)

Question 2

Explain haploid and diploid in a human sperm and skin cell. [2 Marks]

Answer 2

Sperm: Haploid (n), 23 chromosomes (1 mark); Skin: Diploid (2n), 46 chromosomes (1 mark). (2 marks)

Question 3

In a pea plant, explain homologous chromosomes. [2 Marks]

Answer 3

Homologous pairs: Matching chromosomes (1 mark); Carry same genes, different alleles (1 mark). (2 marks)

Question 4

Explain the need for meiosis in a cat’s ovary. [2 Marks]

Answer 4

Meiosis halves chromosome number (1 mark); Ensures gametes fuse to diploid (1 mark). (2 marks)

Question 5

In a lily cell, describe chromosome behaviour in meiosis prophase I. [3 Marks]

Answer 5

Chromosomes condense, pair up (1 mark); Crossing over swaps segments (1 mark); Nuclear envelope breaks (1 mark). (3 marks)

Question 6

Identify metaphase II in a frog cell photomicrograph. [2 Marks]

Answer 6

Chromosomes single, at equator (1 mark); Spindle pulls, metaphase II (1 mark). (2 marks)

Question 7

In a mouse, explain how crossing over increases gamete variety. [3 Marks]

Answer 7

Homologous chromosomes exchange DNA (1 mark); Creates new allele combinations (1 mark); Diverse gametes form (1 mark). (3 marks)

Question 8

Explain random fusion’s role in a human zygote’s diversity. [2 Marks]

Answer 8

Random gamete fusion (1 mark); Produces unique offspring genotypes (1 mark). (2 marks)

Question 9

Define gene, locus, and dominant in a fruit fly. [3 Marks]

Answer 9

Gene: DNA coding protein (1 mark); Locus: Gene’s chromosome position (1 mark); Dominant: Expressed over recessive (1 mark). (3 marks)

Question 10

In peas, construct a Punnett square for a monohybrid cross (Tt x Tt). [3 Marks]

Answer 10

Tt x Tt: TT, Tt, Tt, tt (1 mark); 1:2:1 genotype ratio (1 mark); Tall/short phenotype (1 mark). (3 marks)

Question 11

In rabbits, predict a dihybrid cross outcome with codominance (RrYy x RrYy). [4 Marks]

Answer 11

RrYy x RrYy: 9:3:3:1 phenotypes (1 mark); Codominance mixes traits (1 mark); Punnett shows combos (1 mark); E.g., RY, Ry (1 mark). (4 marks)

Question 12

In mice, use a test cross to identify a genotype (Bb x bb). [3 Marks]

Answer 12

Bb x bb: 1:1 Bb:bb (1 mark); Offspring reveal B dominance (1 mark); Test confirms genotype (1 mark). (3 marks)

Question 13

Calculate chi-squared for a pea cross (observed: 75:25, expected: 3:1). [3 Marks]

Answer 13

χ² = Σ(O-E)²/E, expected 75:25 (1 mark); Calc: 0 + 0 = 0 (1 mark); No significant difference (1 mark). (3 marks)

Question 14

Explain how the HBB gene affects sickle cell anaemia in humans. [4 Marks]

Answer 14

HBB codes haemoglobin (1 mark); Mutation alters beta chain (1 mark); Sickled RBCs form (1 mark); Anaemia results (1 mark). (4 marks)

Question 15

In barley, explain gibberellin’s role in stem elongation via Le/le alleles. [4 Marks]

Answer 15

Le: Functional enzyme, gibberellin made (1 mark); le: Non-functional, less gibberellin (1 mark); Gibberellin elongates stem (1 mark); Le dominant (1 mark). (4 marks)

Question 16

In E. coli, explain lac operon control of lactose metabolism. [3 Marks]

Answer 16

Lactose binds repressor (1 mark); RNA polymerase transcribes lac genes (1 mark); Enzymes digest lactose (1 mark). (3 marks)

Question 17

In a wheat cell, explain gibberellin’s activation of growth genes. [2 Marks]

Answer 17

Gibberellin degrades DELLA (1 mark); Frees transcription factors, boosts growth (1 mark). (2 marks)