Chapter 13

1. Chapter 13 Meiosis and Sexual Life Cycles

2. Overview: Hereditary Similarity and Variation • Living organisms are distinguished by their ability to reproduce their own kind • Heredity is the transmission of traits from one generation to the next • Variation shows that offspring differ in appearance from parents and siblings • Genetics is the scientific study of heredity and variation

4. Concept 13.1: Offspring acquire genes from parents by inheriting chromosomes • In a literal sense, children do not inherit particular physical traits from their parents • It is genes that are actually inherited

5. Inheritance of Genes • Genes are the units of heredity • Genes are segments of DNA • Each gene has a specific locus on a certain chromosome • One set of chromosomes is inherited from each parent • Reproductive cells called gametes (sperm and eggs) unite, passing genes to the next generation

6. Comparison of Asexual and Sexual Reproduction • In asexual reproduction, one parent produces genetically identical offspring by mitosis • In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents Video: Hydra Budding

7. LE 13-2 Parent Bud 0.5 mm

8. Concept 13.2: Fertilization and meiosis alternate in sexual life cycles • A life cycle is the generation-to-generation sequence of stages in the reproductive history of an organism

9. Sets of Chromosomes in Human Cells • Each human somatic cell (any cell other than a gamete) has 46 chromosomes arranged in pairs • A karyotype is an ordered display of the pairs of chromosomes from a cell • The two chromosomes in each pair are called homologous chromosomes, or homologues • Both chromosomes in a pair carry genes controlling the same inherited characteristics

10. LE 13-3 Pair of homologous chromosomes 5 µm Centromere Sister chromatids

11. The sex chromosomes are called X and Y • Human females have a homologous pair of X chromosomes (XX) • Human males have one X and one Y chromosome • The 22 pairs of chromosomes that do not determine sex are called autosomes

12. Each pair of homologous chromosomes includes one chromosome from each parent • The 46 chromosomes in a human somatic cell are two sets of 23: one from the mother and one from the father • The number of chromosomes in a single set is represented by n • A cell with two sets is called diploid (2n) • For humans, the diploid number is 46 (2n = 46)

13. LE 13-4 Key Maternal set of chromosomes (n = 3) 2n = 6 Paternal set of chromosomes (n = 3) Two sister chromatids of one replicated chromosomes Centromere Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set)

14. Gametes are haploid cells, containing only one set of chromosomes • For humans, the haploid number is 23 (n = 23) • Each set of 23 consists of 22 autosomes and a single sex chromosome • In an unfertilized egg (ovum), the sex chromosome is X • In a sperm cell, the sex chromosome may be either X or Y

15. Behavior of Chromosome Sets in the Human Life Cycle • At sexual maturity, the ovaries and testes produce haploid gametes • Gametes are the only types of human cells produced by meiosis, rather than mitosis • Meiosis results in one set of chromosomes in each gamete • Fertilization, the fusing of gametes, restores the diploid condition, forming a zygote • The diploid zygote develops into an adult

16. LE 13-5 Key Haploid gametes (n = 23) Haploid (n) Ovum (n) Diploid (2n) Sperm cell (n) MEIOSIS FERTILIZATION Ovary Testis Diploid zygote (2n = 46) Mitosis and development Multicellular diploid adults (2n = 46)

17. The Variety of Sexual Life Cycles • The alternation of meiosis and fertilization is common to all organisms that reproduce sexually • The three main types of sexual life cycles differ in the timing of meiosis and fertilization

18. In animals, meiosis produces gametes, which undergo no further cell division before fertilization • Gametes are the only haploid cells in animals • Gametes fuse to form a diploid zygote that divides by mitosis to develop into a multicellular organism

19. LE 13-6 Key Haploid Diploid Haploid multicellular organism (gametophyte) Haploid multicellular organism Gametes n n Mitosis Mitosis Mitosis Mitosis n n n n n n n n n Spores n MEIOSIS FERTILIZATION Gametes Gametes n MEIOSIS FERTILIZATION MEIOSIS FERTILIZATION Zygote 2n 2n 2n 2n Zygote Diploid multicellular organism (sporophyte) 2n Diploid multicellular organism Mitosis Mitosis Zygote Most fungi and some protists Animals Plants and some algae

20. Plants and some algae exhibit an alternation of generations • This life cycle includes two multicellular generations or stages: one diploid and one haploid • The diploid organism, the sporophyte, makes haploid spores by meiosis • Each spore grows by mitosis into a haploid organism called a gametophyte • A gametophyte makes haploid gametes by mitosis

21. LE 13-6b Key Haploid Diploid Haploid multicellular organism (gametophyte) Mitosis Mitosis n n n n n Spores Gametes MEIOSIS FERTILIZATION 2n 2n Zygote Diploid multicellular organism (sporophyte) Mitosis Plants and some algae

22. In most fungi and some protists, the only diploid stage is the single-celled zygote; there is no multicellular diploid stage • The zygote produces haploid cells by meiosis • Each haploid cell grows by mitosis into a haploid multicellular organism • The haploid adult produces gametes by mitosis

23. LE 13-8b MEIOSIS II: Separates sister chromatids TELOPHASE I AND CYTOKINESIS TELOPHASE II AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II Haploid daughter cells forming Cleavage furrow Sister chromatids separate Two haploid cells form; chromosomes are still double During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes

24. LE 13-9 MITOSIS MEIOSIS Chiasma (site of crossing over) Parent cell (before chromosome replication) MEIOSIS I Propase Prophase I Chromosome replication Chromosome replication Tetrad formed by synapsis of homologous chromosomes Duplicated chromosome (two sister chromatids) 2n = 6 Chromosomes positioned at the metaphase plate Tetrads positioned at the metaphase plate Metaphase I Metaphase Anaphase Sister chromatids separate during anaphase Anaphase I Homologues separate during anaphase I; sister chromatids remain together Telophase Telophase I Haploid n = 3 Daughter cells of meiosis I 2n 2n MEIOSIS II Daughter cells of mitosis n n n n Daughter cells of meiosis II Sister chromatids separate during anaphase II

26. Origins of Genetic Variation Among Offspring • The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation • Three mechanisms contribute to genetic variation: • Independent assortment of chromosomes • Crossing over • Random fertilization

27. Crossing Over • Crossing over produces recombinant chromosomes, which combine genes inherited from each parent • Crossing over begins very early in prophase I, as homologous chromosomes pair up gene by gene • In crossing over, homologous portions of two nonsister chromatids trade places • Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome Animation: Genetic Variation

28. LE 13-11 Nonsister chromatids Prophase I of meiosis Tetrad Chiasma, site of crossing over Metaphase I Metaphase II Daughter cells Recombinant chromosomes

29. Random Fertilization • Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg) • The fusion of gametes produces a zygote with any of about 64 trillion diploid combinations • Crossing over adds even more variation • Each zygote has a unique genetic identity

30. Evolutionary Significance of Genetic Variation Within Populations • Natural selection results in accumulation of genetic variations favored by the environment • Sexual reproduction contributes to the genetic variation in a population, which ultimately results from mutations

32. Concept 14.1: Mendel used the scientific approach to identify two laws of inheritance • Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments

33. Mendel’s Experimental, Quantitative Approach • Advantages of pea plants for genetic study: • There are many varieties with distinct heritable features, or characters (such as color); character variations are called traits • Mating of plants can be controlled • Each pea plant has sperm-producing organs (stamens) and egg-producing organs (carpels) • Cross-pollination (fertilization between different plants) can be achieved by dusting one plant with pollen from another

34. LE 14-2 Removed stamens from purple flower Transferred sperm- bearing pollen from stamens of white flower to egg- bearing carpel of purple flower Parental generation (P) Stamens Carpel Pollinated carpel matured into pod Planted seeds from pod Examined offspring: all purple flowers First generation offspring (F1)

35. Mendel chose to track only those characters that varied in an “either-or” manner • He also used varieties that were “true-breeding” (plants that produce offspring of the same variety when they self-pollinate)

36. In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called hybridization • The true-breeding parents are the P generation • The hybrid offspring of the P generation are called the F1 generation • When F1 individuals self-pollinate, the F2 generation is produced

37. The Law of Segregation • When Mendel crossed contrasting, true-breeding white and purple flowered pea plants, all of the F1 hybrids were purple • When Mendel crossed the F1 hybrids, many of the F2 plants had purple flowers, but some had white • Mendel discovered a ratio of about three to one, purple to white flowers, in the F2 generation

38. LE 14-3 P Generation (true-breeding parents) Purple flowers White flowers F1 Generation (hybrids) All plants had purple flowers F2 Generation

40. The first concept is that alternative versions of genes account for variations in inherited characters • For example, the gene for flower color in pea plants exists in two versions, one for purple flowers and the other for white flowers • These alternative versions of a gene are now called alleles • Each gene resides at a specific locus on a specific chromosome

41. LE 14-4 Allele for purple flowers Homologous pair of chromosomes Locus for flower-color gene Allele for white flowers

42. The second concept is that for each character an organism inherits two alleles, one from each parent • Mendel made this deduction without knowing about the role of chromosomes • The two alleles at a locus on a chromosome may be identical, as in the true-breeding plants of Mendel’s P generation • Alternatively, the two alleles at a locus may differ, as in the F1 hybrids

43. The third concept is that if the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance • In the flower-color example, the F1 plants had purple flowers because the allele for that trait is dominant

44. The fourth concept, now known as the law of segregation, states that the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes • Thus, an egg or a sperm gets only one of the two alleles that are present in the somatic cells of an organism • This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis

45. Mendel’s segregation model accounts for the 3:1 ratio he observed in the F2 generation of his numerous crosses • The possible combinations of sperm and egg can be shown using a Punnett square, a diagram for predicting the results of a genetic cross between individuals of known genetic makeup • A capital letter represents a dominant allele, and a lowercase letter represents a recessive allele

46. LE 14-5_2 P Generation Purple flowers PP White flowers pp Appearance: Genetic makeup: p P Gametes F1 Generation Appearance: Genetic makeup: Purple flowers Pp Gametes: 1 1 p P 2 2 F1 sperm P p F2 Generation P PP Pp F1 eggs p Pp pp 3 : 1

47. Useful Genetic Vocabulary • An organism with two identical alleles for a character is said to be homozygous for the gene controlling that character • An organism that has two different alleles for a gene is said to be heterozygous for the gene controlling that character • Unlike homozygotes, heterozygotes are not true-breeding

48. Because of the different effects of dominant and recessive alleles, an organism’s traits do not always reveal its genetic composition • Therefore, we distinguish between an organism’s phenotype, or physical appearance, and its genotype, or genetic makeup

49. The Testcross • How can we tell the genotype of an individual with the dominant phenotype? • Such an individual must have one dominant allele, but the individual could be either homozygous dominant or heterozygous • The answer is to carry out a testcross: breeding the mystery individual with a homozygous recessive individual • If any offspring display the recessive phenotype, the mystery parent must be heterozygous

50. LE 14-7 Dominant phenotype, unknown genotype: PP or Pp? Recessive phenotype, known genotype: pp If Pp, then 1 2 offspring purple and 1 2 offspring white: If PP, then all offspring purple: p p p p P P Pp Pp Pp Pp P P pp pp Pp Pp