10.2 Section Objectives – page 263

1. 10.2 Section Objectives – page 263 Section Objectives Analyze how meiosis maintains a constant number of chromosomes within a species. Infer how meiosis leads to variation in a species. Relate Mendel’s laws of heredity to the events of meiosis.

2. Section 10.2 Summary – pages 263-273 Genes, Chromosomes, and Numbers Genes do not exist free in the nucleus of a cell; they are lined up on chromosomes. Typically, a chromosome can contain a thousand or more genes along its length.

3. Section 10.2 Summary – pages 263-273 Diploid and haploid cells In the body cells of animals and most plants, chromosomes occur in pairs. A cell with two of each kind of chromosome is called a diploid cell and is said to contain a diploid, or 2n, number of chromosomes.

4. Section 10.2 Summary – pages 263-273 Diploid and haploid cells This pairing supports Mendel’s conclusion that organisms have two factors—alleles—for each trait. Organisms produce gametes that contain one of each kind of chromosome. A cell containing one of each kind of chromosome is called a haploidcell and is said to contain a haploid, or n, number of chromosomes.

5. Section 10.2 Summary – pages 263-273 Diploid and haploid cells This fact supports Mendel’s conclusion that parent organisms give one allele for each trait to each of their offspring. Chromosome Numbers of Common Organisms Organism Body Cell (2n) Gamete (n) 4 Fruit fly 8 Garden pea 14 7 10 Corn 20 12 Tomato 24 Leopard Frog 26 13 Apple 34 17 Human 46 23 Chimpanzee 24 48 Dog 78 39 1260 630 Adder’s tongue fern

6. Section 10.2 Summary – pages 263-273 Diploid and haploid cells Chromosome Numbers of Common Organisms This table shows the diploid and haploid number of chromosomes of some species. Organism Body Cell (2n) Gamete (n) 4 Fruit fly 8 Garden pea 14 7 10 Corn 20 12 Tomato 24 Leopard Frog 26 13 Apple 34 17 Human 46 23 Chimpanzee 24 48 Dog 78 39 1260 630 Adder’s tongue fern

7. Section 10.2 Summary – pages 263-273 Homologous chromosomes The two chromosomes of each pair in a diploid cell are calledhomologous chromosomes. Each pair of homologous chromosomes has genes for the same traits.

8. Section 10.2 Summary – pages 263-273 Homologous chromosomes On homologous chromosomes, these genes are arranged in the same order, but because there are different possible alleles for the same gene, the two chromosomes in a homologous pair are not always identical to each other. Homologous Chromosome 4 a A Terminal Axial Inflated D d Constricted T t Short Tall

9. Section 10.2 Summary – pages 263-273 Why meiosis? When cells divide by mitosis, the new cells have exactly the same number and kind of chromosomes as the original cells. Imagine if mitosis were the only means of cell division. Each pea plant parent, which has 14 chromosomes, would produce gametes that contained a complete set of 14 chromosomes.

10. Section 10.2 Summary – pages 263-273 Why meiosis? The F1 pea plants would have cell nuclei with 28 chromosomes, and the F2 plants would have cell nuclei with 56 chromosomes.

11. Section 10.2 Summary – pages 263-273 Why meiosis? There must be another form of cell division that allows offspring to have the same number of chromosomes as their parents. This kind of cell division, which produces gametes containing half the number of chromosomes as a parent’s body cell, is called meiosis.

12. Section 10.2 Summary – pages 263-273 Why meiosis? Meiosis consists of two separate divisions, known as meiosis I and meiosis II. Meiosis I begins with one diploid (2n) cell. By the end of meiosis II, there are four haploid (n) cells.

13. Section 10.2 Summary – pages 263-273 Why meiosis? These haploid cells are called sex cells— gametes. Male gametes are called sperm. Female gametes are called eggs. When a sperm fertilizes an egg, the resulting zygote once again has the diploid number of chromosomes.

14. Section 10.2 Summary – pages 263-273 Why meiosis? This pattern of reproduction, involving the production and subsequent fusion of haploid sex cells, is called sexual reproduction. Meiosis Haploid gametes (n=23) Sperm Cell Meiosis Egg Cell Fertilization Multicellular diploid adults (2n=46) Diploid zygote (2n=46) Mitosis and Development

15. Section 10.2 Summary – pages 263-273 The Phases of Meiosis During meiosis, a spindle forms and the cytoplasm divides in the same ways they do during mitosis. However, what happens to the chromosomes in meiosis is very different.

16. Sect ion 10.2 Summary – pages 263-273 The Phases of Meiosis

17. Section 10.2 Summary – pages 263-273 Meiosis Provides for Genetic Variation Cells that are formed by mitosis are identical to each other and to the parent cell. Crossing over during meiosis, however, provides a way to rearrange allele combinations. Thus, variability is increased.

18. Section 10.2 Summary – pages 263-273 Genetic recombination Reassortment of chromosomes and the genetic information they carry, either by crossing over or by independent segregation of homologous chromosomes, is called genetic recombination.

19. Section 10.2 Summary – pages 263-273 Genetic recombination It is a major source of variation among organisms. MEIOSIS I MEIOSIS II Possible gametes Possible gametes Chromosome A Chromosome B Chromosome a Chromosome b

20. Section 10.2 Summary – pages 263-273 Meiosis explains Mendel’s results The segregation of chromosomes in anaphase I of meiosis explains Mendel’s observation that each parent gives one allele for each trait at random to each offspring, regardless of whether the allele is expressed.

21. Section 10.2 Summary – pages 263-273 Meiosis explains Mendel’s results The segregation of chromosomes at random during anaphase I also explains how factors, or genes, for different traits are inherited independently of each other.

22. Section 10.2 Summary – pages 263-273 Polyploidy Organisms with more than the usual number of chromosome sets are called polyploids. Polyploidy is rare in animals and almost always causes death of the zygote.

23. Section 10.2 Summary – pages 263-273 Polyploidy However, polyploidy frequently occurs in plants. Many polyploid plants are of great commercial value.

24. Section 10.2 Summary – pages 263-273 Gene Linkage and Maps If genes are close together on the same chromosome, they usually are inherited together. These genes are said to be linked.

25. Section 10.2 Summary – pages 263-273 Gene Linkage and Maps Linked genes may become separated on different homologous chromosomes as a result of crossing over. When crossing over produces new gene combinations, geneticists can use the frequencies of these new gene combinations to make a chromosome map showing the relative locations of the genes.

26. Section 2 Check Question 1 A cell with two of each kind of chromosome is __________. A. diploid B. haploid C. biploid D. polyploid

27. Section 2 Check Homologous Chromosome 4 The answer is A. The two chromosomes of each pair in a diploid cell are called homologous chromosomes. Each has genes for the same traits. a A Terminal Axial Inflated D d Constricted T t Short Tall

28. Section 2 Check Question 2 Meiosis Haploid gametes (n=23) What is the importance of meiosis in sexual reproduction? Sperm Cell Meiosis Egg Cell Fertilization Diploid zygote (2n=46) Multicellular diploid adults (2n=46) Mitosis and Development

29. Section 2 Check Meiosis is cell division that produces haploid gametes. If meiosis did not occur, each generation would have twice as many chromosomes as the preceding generation. Meiosis Haploid gametes (n=23) Sperm Cell Meiosis Egg Cell Fertilization Diploid zygote (2n=46) Multicellular diploid adults (2n=46) Mitosis and Development

30. Chapter Summary – 10.2 Meiosis • In meiosis, one diploid (2n) cell produces four haploid (n) cells, providing a way for offspring to have the same number of chromosomes as their parents. • In prophase I of meiosis, homologous chromosomes come together and pair tightly. Exchange of genetic material, called crossing over, takes place.

31. Chapter Summary – 10.2 Meiosis • Mendel’s results can be explained by the distribution of chromosomes during meiosis. • Random assortment and crossing over during meiosis provide for genetic variation among the members of a species.

32. Chapter Assessment Question 9 Organisms with more than the usual number of chromosome sets are called __________. A. diploids B. haploid C. triploids D. polyploids

33. Chapter Assessment The answer is D. Polyploidy is rare in animals but occurs frequently in plants. Because the flowers and fruits of polyploid plants are often larger than normal, these plants have great commercial value.