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Page 1 of 5 SKILL: DETERMINING RELEVANCE Are Traits Linked? 1.5 3.0 (white eyes) (notch wings) Distance between genes on this map 27.5 (tan body) Fruit flies are easy to breed in a laboratory and have an assortment of easily recognized genetic traits—different eye colors, body patterns, limb characteristics, and wing shapes. For these reasons, early geneticists studied fruit flies to learn how certain traits were inherited. Sometimes the experiments produced puzzling results. Here is an example from the laboratory of Thomas Hunt Morgan. Observations • In a batch of fruit flies, most red-eyed individuals were born with short wings. • In the same batch, at least one fruit fly was born with red eyes and normal-sized wings. 56.7 (forked bristles) Hypotheses Morgan and his coworkers made these hypotheses about the inherited traits: Genes have a particular location on a chromosome. A gene map shows the location. • The gene for red eyes and the gene for short wings are linked together on a fruit fly’s chromosomes. These linked genes are usually inherited together. • Sometimes during meiosis, one of the linked genes will “cross over” from one chromosome to a homologous one. When this happens, a fruit fly will be born with one but not both of the linked genes—red eyes without short wings. • Genes that are farthest from each other on a chromosome are most likely to become separated and cross over during meiosis. Genes that are closest (linked) to each other are least likely to. Further Discoveries By studying the results of many breeding experiments, Morgan and his student, Alfred Sturtevant, could determine which genes were closest and farthest from each other on the same chromosome. From this information, they drew a simple map showing the location of each of the fruit fly’s linked genes. Determine Relevance On Your Own Look at the map of a chromosome on this page. Which of the traits are most likely to be inherited together? Which might be most easily separated and cross over during meiosis? As a Group Is it reasonable to think that information about a fruit fly’s genes could apply to the genes of a human being? Discuss this topic in a small group and see if the group can agree. Chapter 4: Patterns of Heredity 123 Page 2 of 5 Chapter Review In sexual reproduction, genes are passed from parents to offspring in predictable patterns. CONTENT REVIEW CLASSZONE.COM KEY CONCEPTS SUMMARY 1 alleles Living things inherit traits in patterns. Offspring inherit alleles, which are forms of genes, from their parents. Alleles can be dominant or recessive. The alleles you have are your genotype; the observable characteristics that come from your genotype are your phenotype. a gene 2 A a B B c C d d E e F F g G h H sexual reproduction p. 102 gene p. 102 heredity p. 102 allele p. 103 phenotype p. 106 genotype p. 106 dominant p. 107 recessive p. 107 Patterns of heredity can be predicted. Punnett squares show possible outcomes of heredity. Ratios and percentages can be used with Punnett squares to express the probability of particular outcomes. B b BB Bb Bb bb VOCABULARY VOCABULARY B = black fur b = brown fur Punnett square p. 110 ratio p. 112 probability p. 112 percentage p. 112 B b 3 Meiosis is a special form of cell division. VOCABULARY • At the beginning of meiosis I, the parent cell has two copies of each chromosome pair. • During meiosis I, the homologs of the chromosome pair separate; there are two cells, each with two copies of one homolog from each pair. • During meiosis II, the two copies of each homolog separate; each daughter cell has one homolog. gamete p. 118 egg p. 118 sperm p. 118 fertilization p. 118 meiosis p. 119 124 Unit 1: Cells and Heredity Meiosis I Meiosis II 1n 1n 1n 1n Page 3 of 5 Reviewing Vocabulary Make a frame for each of the vocabulary terms listed below. Write the term in the center. Think about how each term is related to the Big Idea of the chapter. Decide what information to frame it with. Use definitions, examples, descriptions, parts, or pictures. 1. allele 3. ratio 2. heredity 4. probability Describe how the vocabulary terms in the following pairs of words are related to each other. Explain the relationship in a one- or two-sentence answer. Underline each vocabulary word or term in your answers. 11. In guinea pigs, the allele for black fur (B) is dominant, and the allele for brown fur (b) is recessive. If a BB male mates with a Bb female, what percentage of offspring are likely to have black fur? a. 100 percent c. 50 percent b. 75 percent d. 25 percent 12. If one parent has two dominant alleles and another parent has two recessive alleles, the offspring will have a. the recessive phenotype b. the dominant phenotype c. two dominant alleles d. two recessive alleles 13. Cells that contain half the usual number of chromosomes are a. fertilized egg cells c. alleles b. gametes d. diploid cells 5. phenotype, genotype 6. dominant, recessive Reviewing Key Concepts Multiple Choice Choose the letter of the best answer. 7. Which is an example of an acquired trait? a. eye color c. blood type b. hair color d. ability to read 8. The unit of heredity that determines a particular trait is known as a. a chromosome c. a gene b. a gamete d. a phenotype 9. A human female would have which set of sex chromosomes? a. XX c. XY b. YY d. XxYy 10. If one copy of a dominant allele is present in a genotype, then the trait the allele codes for is a. expressed in the phenotype b. not expressed in the phenotype c. partially expressed in the phenotype d. not expressed in an offspring’s phenotype 14. The process that produces haploid (1n) cells is known as a. mitosis c. meiosis b. reproduction d. fertilization 15. What happens when fertilization occurs? a. Two 2n cells combine in a new cell. b. Two 1n cells combine into a new cell. c. Two 2n daughter cells are produced. d. Two 1n daughter cells are produced. 16. Which does not occur during meiosis? a. Four haploid daughter cells are produced. b. Two diploid daughter cells are produced. c. Only cells that are gametes are produced. d. Daughter cells are produced that contain half the chromosomes of the parent cell. Short Answer Write a short answer to each question. 17. In what case would a recessive allele be expressed in the phenotype of an offspring? 18. Describe the purpose of a Punnett square. 19. How does the number of chromosomes in a person’s sex cells compare with the number of chromosomes in the body cells? Chapter 4: Patterns of Heredity 125 Page 4 of 5 Thinking Critically 20. INFER How was Mendel able to infer that each offspring of two parent pea plants had a pair of “factors” for a particular trait? 27. ANALYZE This diagram shows the process of fertilization. Which of the cells shown are haploid? Explain. egg cell (female gamete) 21. COMMUNICATE Briefly describe how heredity works. Use the terms gene and chromosome in your explanation. 22. APPLY Can a dwarf pea plant ever have a dominant allele? Explain. 23. ANALYZE How is a Punnett Square used to show both the genotype and phenotype of both parents and offspring? 24. APPLY In rabbits, the allele for black fur is dominant over the allele for white fur. Two black rabbits have a litter of eight offspring. Six of the offspring have black hair and two have white hair. What are the genotypes of the parents? Explain. fertilized egg cell 1n fertilization sperm cell (male gamete) 2n 1n 28. SUMMARIZE Briefly describe what happens during meiosis I and meiosis II. What is the function of meiosis? Use the Punnett square below to answer the next two questions. B b b Bb bb b Bb bb 25. CALCULATE A parent has one dominant allele for black fur (B) and one recessive allele for white fur (b). The other parent has two recessive alleles for white fur. In this cross what is the chance that an offspring will be born with black fur? With white fur? 29. INFER Look again at the picture on pages 98–99. Now that you have finished the chapter, how would you change or add details to your answer to the question on the photograph? 30. SYNTHESIZE Write one or more paragraphs explaining how Mendel’s observations of pea plants contributed to the study of modern genetics. Use these terms in your explanation. gene allele trait genotype phenotype dominant recessive 26. CALCULATE What is the percentage chance that an offspring will have the recessive phenotype? If you need to create graphs or other visuals for your project, be sure you have grid paper, poster board, markers, or other supplies. 126 Unit 1: Cells and Heredity Page 5 of 5 Standardized Test Practice For practice on your state test, go to . . . TEST PRACTICE CLASSZONE.COM Analyzing data The chart below shows the phenotypes of pea-plant offspring. Phenotypes of Pea Plants Phenotype Number of Offspring Regular (D) 12 Dwarf (d) 4 Use the chart to answer the questions below. 1. What percentage of pea plants showed the dominant phenotype? a. 100 percent b. 75 percent c. 50 percent d. 25 percent 2. What percentage of pea plants showed the recessive phenotype? a. 100 percent b. 75 percent c. 50 percent d. 25 percent 3. What is the genotype of the dwarf pea plants? a. DD b. Dd c. dd d. cannot tell 4. What are the possible genotypes of the regular pea plants? a. DD and dd b. DD and Dd c. Dd and dd d. cannot tell 5. What are the genotypes of the parents? a. Dd and dd b. DD and Dd c. Dd and Dd d. dd and dd 6. Which statement is true, based on the data in the chart? a. If both parents were Dd, then none of the offspring would be dwarf. b. If both parents were DD, then none of the offspring would be dwarf. c. If one parent were Dd and the other were dd, then none of the offspring would be regular. d. If one parent were DD and the other parent were dd, then none of the offspring would be regular. Extended Response 7. Traits for a widow’s peak hairline (W) and curly hair (C) are controlled by dominant alleles. A family of eight has three children with widow’s peaks. All six children have curly hair. Use your knowledge of heredity to write one or two paragraphs explaining the possible genotypes of the parents. 8. A student proposes a hypothesis that traits that are dominant are more common in the general population than traits with recessive alleles. Describe a procedure you might use to test this hypothesis. Chapter 4: Patterns of Heredity 127