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Secondary 4 Pure Biology Genetics Inheritance Quiz
Free Sec 4 Pure Biology Genetics Inheritance quiz, HY3 Exam version, with questions, answers, and O Level-style practice for Singapore students.
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Secondary 4 Pure Biology Quiz - Genetics Inheritance (Answer Key)
Total Marks: 40
Topic: Genetics Inheritance
Section A Answers
1. ff
Teaching note: Homozygous recessive means two copies of the recessive allele (f). Dominant F would show free earlobes. [1]
2. C. T or t
Teaching note: Gametes are haploid; they carry one allele from each gene pair. Tt parent produces gametes with T or t via segregation. [1]
3. Locus
Teaching note: The locus is the fixed position of a gene on a chromosome. Avoid confusing with "allele" (version of gene). [1]
4. B. Autosomal recessive
Teaching note: Recessive traits skip generations (carriers unaffected) and affect both sexes equally. Dominant usually appears every generation. [1]
5. C. Anaphase I
Teaching note: Homologous chromosomes separate in Anaphase I of meiosis; sister chromatids separate in Anaphase II. [1]
Section B Answers
6. (a) P and p
(b) 50% (or 1/2)
Working: Pp × pp → gametes P, p and p, p. Offspring: Pp, Pp, pp, pp. White (pp) = 2/4 = 1/2. [1+1]
7. Males have one X chromosome (XY); a single recessive allele on X causes the disorder. Females (XX) need two copies to be affected, so they are usually carriers. [2]
Marking: 1 mark for XY explanation, 1 mark for female needing two alleles.
8. Codominance: both alleles expressed equally in heterozygote. Example: AB blood group (I^A I^B both expressed). [2]
Marking: 1 mark definition, 1 mark example.
9. Genetic diagram:
Parents: Hb^A Hb^S × Hb^A Hb^S
Gametes: Hb^A, Hb^S from each
Offspring: Hb^A Hb^A (normal), Hb^A Hb^S (carrier), Hb^S Hb^S (sickle cell) in ratio 1:2:1. [3]
Marking: 1 parent genotypes, 1 gametes, 1 offspring grid/correct ratios.
10.
- Mitosis produces 2 diploid cells; meiosis produces 4 haploid cells.
- Mitosis has no crossing over; meiosis has crossing over in Prophase I. (Or: mitosis for growth, meiosis for gametes) [2]
Section C Answers
11. (a) X^C X^c (carrier female)
(b) 50% (1/2)
Working: II-3 is X^C X^c × X^C Y. Sons get X from mother: 1/2 X^c Y (affected). [1+1]
12. q² = 72/200 = 0.36 → q = √0.36 = 0.6. Frequency of b = 0.6. [3]
Steps: recessive phenotype = bb = q²; solve q. Marking: 1 for q², 1 for sqrt, 1 for answer.
13. Incomplete dominance (or codominance if both shown). RR × WW → RW roan (mixed). Neither allele dominant. [2]
14. (a) Rr (red heterozygous) × rr (white)
(b) Expected 1:1 = 400:400; observed 412:388 close, supports. [1+2]
Marking: 1 genotype, 2 for ratio explanation with numbers.
15. Non-disjunction = failure of homologous chromosomes or sister chromatids to separate in meiosis. Gamete gets 2 copies of chr 21. Fertilisation → trisomy 21 (Down). [3]
16. Sticky ends are single-stranded overhangs that base-pair with complementary cut DNA, allowing gene insertion into plasmid. [2]
17. Possible: selection against affected, genetic counselling, or reduced fitness. [2]
Based on graph decline from 40 to 5.
18. Cross I^A i × I^B i:
Offspring: I^A I^B (AB, 1/4), I^A i (A, 1/4), I^B i (B, 1/4), ii (O, 1/4). [4]
Marking: all groups listed 2, probabilities 2.
19. Gamete mutation passes to offspring (hereditable); somatic mutation only affects individual, not inherited. [2]
20. (a) AUG CCG UUA GGU
(b) Met-Gly-Asn-Pro
Working: DNA T→A, A→U, C→G. Then codons to amino acids. [1+2]