1. Unit Overview – pages 250-251 Genetics Mendel and Meiosis Meiosis

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

3. Section 10.2 Summary – pages 263-273 In the body cells of animals and most plants, chromosomes occur in pairs. Diploid and haploid cells Diploid : A cell with two of each kind of chromosome (2n)

4. Section 10.2 Summary – pages 263-273 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. Haploid: a cell containing one of each kind of chromosome (n) Diploid and haploid cells

5. Section 10.2 Summary – pages 263-273 This fact supports Mendel’s conclusion that parent organisms give one allele for each trait to each of their offspring. Diploid and haploid cells Chromosome Numbers of Common Organisms OrganismBody Cell (2n) Fruit fly8 Garden pea14 Corn20 Tomato24 Leopard Frog2613 Apple34 Human46 Chimpanzee 48 Dog78 Adder’s tongue fern 1260 Gamete (n) 4 7 10 12 17 23 24 39 630

6. Chromosome Numbers of Common Organisms OrganismBody Cell (2n) Fruit fly8 Garden pea14 Corn20 Tomato24 Leopard Frog2613 Apple34 Human46 Chimpanzee 48 Dog78 Adder’s tongue fern 1260 Section 10.2 Summary – pages 263-273 Gamete (n) 4 7 10 12 17 23 24 39 630 This table shows the diploid and haploid number of chromosomes of some`species. Diploid and haploid cells

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

8. Section 10.2 Summary – pages 263-273 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 chromosomes Homologous Chromosome 4 aA Terminal Axial Inflated D Constricted d Tall T Short t

9. Section 10.2 Summary – pages 263-273 When cells divide by mitosis, the new cells have exactly the same number and kind of chromosomes as the original cells. Why meiosis? 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 The F1 pea plants would have cell nuclei with 28 chromosomes, and the F2 plants would have cell nuclei with 56 chromosomes. Why meiosis?

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

12. Section 10.2 Summary – pages 263-273 Meiosis consists of two separate divisions, known as meiosis I and meiosis II. The eight phases of meiosis are prophase I, metaphase I, anaphase I, telophase I, prophase II, metaphase II, anaphase II, and telophase II. Why meiosis?

13. Section 10.2 Summary – pages 263-273 Meiosis I begins with one diploid (2n) cell. By the end of meiosis II, there are four haploid (n) cells. Why meiosis?

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

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

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

17. Section 10.2 Summary – pages 263-273 The long stringy chromatin coils up into visible chromosomes The spindles form Synapsis: the homologous chromosomes line up forming a four part structure called a tetrad Crossing over may occur Prophase I

18. Section 10.2 Summary – pages 263-273 Meiosis Provides for Genetic Variation Crossing over: sister chromatids pair so tightly that non-sister chromatids from homologous pairs can break and exchange genetic material

19. Section 10.2 Summary – pages 263-273 Chromosomes become attached to the spindle fibers by their centromeres and tetrads line up on the midline of the cell Metaphase I Anaphase I Homologous pairs separate, sister chromatids remain attached

20. Section 10.2 Summary – pages 263-273 Chromosomes unwind, spindles break down, cytoplasm divides Two new diploid cells are formed Telophase I

21. Section 10.2 Summary – pages 263-273 The spindles form Prophase II Metaphase II Chromosomes become attached to the spindle fibers by their centromeres and chromosomes line up on the midline of the cell

22. Section 10.2 Summary – pages 263-273 The centromere of each chromosome splits and sister chromatids separate and move to opposite poles Anaphase II Telophase II Chromosomes unwind, spindles break down, cytoplasm divides, nuclei re-form

23. Section 10.2 Summary – pages 263-273 Four new haploid cells are formed MEIOSIS I MEIOSIS II Possible gametes Chromosome AChromosome BChromosome a Chromosome b

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

25. 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 Genetic recombination

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

27. Section 10.2 Summary – pages 263-273 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. Meiosis explains Mendel’s results

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

29. Section 10.2 Summary – pages 263-273 Nondisjunction: the failure of homologous chromosomes to separate properly during meiosis Nondisjunction

30. Section 10.2 Summary – pages 263-273 Recall that during meiosis I, one chromosome from each homologous pair moves to each pole of the cell. In nondisjunction, both chromosomes of a homologous pair move to the same pole of the cell. Nondisjunction

31. Section 10.2 Summary – pages 263-273 The effects of nondisjunction are often seen after gametes fuse. Nondisjunction When a gamete with an extra chromosome is fertilized by a normal gamete, the zygote will have an extra chromosome. This condition is called trisomy.

32. Section 10.2 Summary – pages 263-273 Although organisms with extra chromosomes often survive, organisms lacking one or more chromosomes usually do not. Nondisjunction When a gamete with a missing chromosome fuses with a normal gamete during fertilization, the resulting zygote lacks a chromosome. This condition is called monosomy.

33. Section 10.2 Summary – pages 263-273 An example of monosomy that is not lethal is Turner syndrome, in which human females have only a single X chromosome instead of two. Nondisjunction

34. Section 10.2 Summary – pages 263-273 Nondisjunction Male parent (2n) Female parent (2n) Meiosis Nondisjunction Meiosis Nondisjunction Abnormal gamete (2n) Abnormal gamete (2n) Zygote (4n)

35. Section 10.2 Summary – pages 263-273 When a gamete with an extra set of chromosomes is fertilized by a normal haploid gamete, the offspring has three sets of chromosomes and is triploid. Nondisjunction The fusion of two gametes, each with an extra set of chromosomes, produces offspring with four sets of chromosomes—a tetraploid.

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

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