Heredity

Children resemble their parents in many ways, yet no two individuals, not even siblings, are exactly alike. In this chapter you will learn how variations arise during reproduction, why sexual reproduction produces more variation than asexual reproduction, and why variations help a species survive changing conditions. Heredity is the transmission of characters, or traits, from parents to offspring, and the rules of inheritance explain how these traits are passed on reliably. You will study the experiments of Gregor Johann Mendel, who crossed pea plants with contrasting traits such as tall and short or round and wrinkled seeds. From his monohybrid crosses (F2 ratio 3:1) and dihybrid crosses (F2 ratio 9:3:3:1), you will learn the ideas of dominant and recessive traits, genotype and phenotype, and the independent inheritance of traits. You will see how genes, located on chromosomes and made of DNA, control traits by making proteins such as enzymes and hormones. Finally, you will learn how sex is determined in humans by the X and Y chromosomes.

Detailed Worksheet: Heredity Section A - Definitions (10 marks) 1. Define heredity and variation. Why is variation important for a species? (2 marks) 2. Distinguish between dominant and recessive traits with one example from Mendel's experiments. (2 marks) 3. Define genotype and phenotype. Write the genotype and phenotype of a pure tall pea plant. (2 marks) 4. What is a gene? Where are genes located in a cell? (2 marks) 5. What are sex chromosomes? Write the sex chromosome combination of a human male and a human female. (2 marks) Section B - Calculations and Applications (15 marks) 6. In one of Mendel's experiments, the F2 generation from a cross between tall and dwarf pea plants contained 787 tall and 277 dwarf plants. Calculate the ratio of tall to dwarf plants to two decimal places and explain how it is close to the expected ratio. (3 marks) 7. A pure tall plant (TT) is crossed with a pure dwarf plant (tt). Show the F1 and F2 generations using a checkerboard. In an F2 generation of 400 plants, how many would be expected to be tall and how many dwarf? How many of the tall plants would be pure breeding? (3 marks) 8. In a dihybrid cross between pea plants with round yellow seeds (RRYY) and wrinkled green seeds (rryy), the F2 generation had 1600 seeds. Using the expected ratio, calculate the expected number of seeds of each of the four types. (3 marks) 9. Mendel crossed tall plants with each other and got 1052 offspring, of which 790 were tall and 262 were short. Find the approximate ratio and deduce the genotypes of the parent plants. (3 marks) 10. A couple already has two daughters. What is the probability that their third child will be a son? Explain with a cross showing the sex chromosomes of the parents and the gametes. (3 marks) Section C - Diagrams (10 marks) 11. Draw a checkerboard (Punnett square) for a monohybrid cross between a pure tall (TT) and a pure dwarf (tt) pea plant through the F1 and F2 generations, and write the phenotypic and genotypic ratios. (4 marks) 12. Draw a flow diagram to show how sex is determined in human beings, showing the parents, their gametes and the offspring. (3 marks) 13. Draw a flow chart showing how a gene controls a trait such as plant height: gene -> protein (enzyme) -> hormone -> trait. Explain what happens if the gene is altered. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Mendel chose the garden pea for his experiments. Give four reasons why it was a suitable choice. Explain why Mendel's results would have been confusing if he had studied two traits whose genes were present close together on the same chromosome. (5 marks) 15. A man with blood group A marries a woman with blood group O, and their daughter has blood group O. Is this information enough to tell you which of the traits, blood group A or O, is dominant? Why or why not? What further information would help? (5 marks) 16. Explain how the equal genetic contribution of male and female parents is ensured in the progeny. Why do we say that sexual reproduction creates more variation than asexual reproduction, and how does this help in the survival of a species? (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Show crosses with clear symbols for genes and gametes. Write ratios in their simplest form and draw all checkerboards neatly.