Principles of Inheritance and Variation

Genetics deals with inheritance, the process by which characters pass from parent to progeny, and variation, the degree by which progeny differ from their parents. In this chapter you will study Gregor Mendel's hybridisation experiments on garden pea between 1856 and 1863 using seven pairs of contrasting traits, the monohybrid cross with its 3:1 phenotypic and 1:2:1 genotypic ratios, the test cross, and the dihybrid cross giving a 9:3:3:1 ratio, from which the laws of dominance, segregation and independent assortment follow. You will also learn deviations from Mendel's findings: incomplete dominance in snapdragon flower colour, codominance and multiple alleles in the ABO blood groups, pleiotropy and polygenic inheritance of human skin colour. The chapter then covers the chromosomal theory of inheritance of Sutton and Boveri, Morgan's work on linkage and recombination in Drosophila, and sex determination by the XO, XY, ZW and haplodiploid mechanisms. Finally, you will study mutation, pedigree analysis, Mendelian disorders such as haemophilia, colour blindness, sickle-cell anaemia, phenylketonuria and thalassemia, and chromosomal disorders like Down, Klinefelter and Turner syndromes.

Detailed Worksheet: Principles of Inheritance and Variation Section A - Definitions (10 marks) 1. Define allele, homozygous and heterozygous with one example from Mendel's pea plants. (2 marks) 2. State the law of segregation and the law of independent assortment. (2 marks) 3. What is a test cross? Why is it performed? (2 marks) 4. Distinguish between incomplete dominance and codominance with one example of each. (2 marks) 5. What is aneuploidy? Name one human disorder caused by trisomy and one caused by monosomy of a chromosome. (2 marks) Section B - Calculations and Applications (15 marks) 6. A pure tall pea plant (TT) is crossed with a pure dwarf plant (tt). Work out the F1 and F2 generations with a Punnett square, giving the phenotypic and genotypic ratios. If 800 F2 plants are raised, how many are expected to be dwarf? (3 marks) 7. In a dihybrid cross between pure round yellow (RRYY) and wrinkled green (rryy) pea plants, calculate the expected numbers of each phenotype among 1,600 F2 seeds. State the dihybrid ratio. (3 marks) 8. A red-flowered snapdragon (RR) is crossed with a white-flowered one (rr). Work out the F1 and F2 phenotypes and ratios. Explain why the phenotypic and genotypic ratios in F2 are the same. (3 marks) 9. A man with blood group A (heterozygous) marries a woman with blood group B (heterozygous). Work out the possible blood groups of their children and the probability of each. (3 marks) 10. A colour-blind man marries a woman with normal vision whose father was colour blind. Work out the probability that (i) a son will be colour blind and (ii) a daughter will be colour blind. Show the cross. (3 marks) Section C - Diagrams (10 marks) 11. Draw a Punnett square for the dihybrid cross RrYy x RrYy, showing all sixteen combinations, and mark the phenotypic classes. (4 marks) 12. Draw the standard symbols used in pedigree analysis for male, female, affected individuals, mating, consanguineous mating and offspring. (3 marks) 13. Draw a labelled diagram showing sex determination in humans (XX-XY type), with gametes and offspring. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Morgan crossed yellow-bodied, white-eyed females of Drosophila with brown-bodied, red-eyed males and intercrossed the F1. Explain why the F2 ratios deviated from 9:3:3:1, and analyse why the cross involving white eye and miniature wing showed 37.2% recombination while yellow body and white eye showed only 1.3%. How did Sturtevant use such data? (5 marks) 15. Sickle-cell anaemia is caused by a single base substitution. Explain the molecular basis, including the codon change and the amino acid change at the sixth position of the beta-globin chain. Analyse why carriers (HbA HbS) are generally healthy but may show symptoms under low oxygen, and work out the probability of an affected child from two carrier parents. (5 marks) 16. Compare sex determination in humans, grasshoppers, birds and honeybees. Explain why in honeybees a male has a father only through its grandfather, and why the sex of a human child is determined by the father. (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Show all crosses with gametes and Punnett squares. Draw all diagrams neatly in pencil and label every part.