Chapter 10: Sexual Reproduction and Genetics

1. Chapter 10: Sexual Reproduction and Genetics 10.1 Meiosis 10.2 Mendelian Genetics 10.3 Gene Linkage and Polyploidy

2. 10.1 Meiosis • Objectives • Explain the reduction in chromosome number that occurs during meiosis • Recognize and summarize the stages of meiosis • Analyze the importance of meiosis in providing genetic variation

3. 10.1 Meiosis • Review Vocabulary • Chromosome – cellular structure that contains DNA • New Vocabulary • Gene • Homologous chromosome • Gamete • Haploid • Fertilization • Diploid • Meiosis • Crossing over

4. Chromosomes and Chromosome Number • Characteristics are passed on to offspring by parents • Each characteristic is called a trait • Instructions for each trait are located on chromosomes found in the nucleus of cells • The DNA on chromosomes is arranged in segments called genes that control the production of proteins • Each chromosome consists of approximately 1500 genes, each gene playing an important role in determining the characteristics and functions of the cell.

5. Chromosomes and Chromosome Number • Human body cells have 46 chromosomes • Each parent contributes 23 chromosomes, resulting in 23 pairs of chromosomes • Homologous chromosomes – one of two paired chromosomes, one from each parent

6. Chromosomes and Chromosome Number • Homologous chromosomes • Same length • Same centromere position • Carry genes that control the same inherited traits

7. Haploid and Diploid Cells • An organism produces gametes to maintain the same number of chromosomes from generation to generation • Gametes are sex cells that have half the number of chromosomes • Human gametes contain 23 chromosomes

8. Haploid and Diploid Cells • The symbol n represents the number of chromosomes in a gamete • A cell with n chromosomes is called a haploid cell (single) • The process by which one haploid gamete combines with another haploid gamete is called fertilization • As a result of fertilization, the cell will contain a total of 2n chromosomes • A cell that contains 2n chromosomes is called a diploid cell (double)

9. Meiosis I • The sexual life cycle in animals involves meiosis, which is a type of cell division that reduces the number of chromosomes (reduction division) • Meiosis produces gametes • When gametes combine in fertilization, the number of chromosomes is restored

10. Meiosis I • Stages of Meiosis I • Reduces the chromosome number by half through the separation of homologous chromosomes • Involves two consecutive cell divisions called meiosis I and meiosis II

11. Meiosis I • Interphase • Chromosomes replicate • Chromatin condenses

12. Meiosis I • Prophase I • Synapsis occurs – the physical binding of homologous chromosomes • Each chromosome consists of two chromatids • The nuclear envelope breaks down • Spindles form

13. Meiosis I • Prophase I • Crossing over produces exchange of genetic material • Crossing over – chromosomal segments are exchanged between a pair of homologous chromosomes

14. Meiosis I • Metaphase I • Chromosome centromeres attach to spindle fibers • Homologous chromosomes line up at the equator

15. Meiosis I • Anaphase I • Homologous chromosomes separate and move to opposite poles of the cell

16. Meiosis I • Telophase I • The spindles break down • Chromosomes uncoil and form two nuclei • The cell divides

17. Meiosis II • Prophase II • A second set of phases begins as the spindle apparatus forms and the chromosomes condense

18. Meiosis II • Metaphase II • A haploid number of chromosomes line up at the equator

19. Meiosis II • Anaphase II • The sister chromatids are pulled apart at the centromere by spindle fibers and move toward the opposite poles of the cell

20. Meiosis II • Telophase II • The chromosomes reach the poles, and the nuclear membrane and nuclei reform

21. Meiosis II • Cytokinesis results in four haploid cells, each with n number of chromosomes

22. The Importance of Meiosis • Meiosis consists of two sets of divisions • Produces four haploid daughter cells that are not identical • Results in genetic variation

23. Mitosis v. Meiosis • Mitosis • One division occurs • DNA replication occurs during interphase • Synapsis of homologous chromosomes does not occur • Two identical cells are formed per cell cycle • Mitosis occurs only in body cells • Mitosis is involved in growth and repair • See Biology Concepts 32: Mitosis v. Meiosis • Meiosis • Two sets of divisions occur during meiosis: meiosis I and meiosis II • DNA replication occurs once before meiosis I • Synapsis of homologous chromosomes occurs during prophase I • Four haploid cells (n) are formed per cell cycle • The daughter cells are not genetically identical because of crossing over • Meiosis occurs in reproductive cells • Meiosis is involved in the production of gametes and providing genetic variation in organisms

24. Meiosis Provides Variation • Depending on how the chromosomes line up at the equator, four gametes with four different combinations of chromosomes can result. • Genetic variation also is produced during crossing over and during fertilization, when gametes randomly combine

25. Sexual Reproduction v. Asexual Reproduction • Asexual Reproduction • The organism inherits all of its chromosomes from a single parent • The new individual is genetically identical to its parent • Example - Bacteria • Sexual Reproduction • Beneficial genes multiply faster over time • Example – Humans • Most protists reproduce both asexually and sexually, depending on environmental conditions • Most plants and many of the more simple animals can reproduce both asexually and sexually

26. 10.2 Mendelian Genetics • Objectives • Explain the significaance of Mendel’s experiments to the study of genetics • Summarize the law of segregation and the law of independent assortment • Predict the possible offspring from a cross using a Punnett Square

27. 10.2 Mendelian Genetics • Review Vocabulary • Segregation – the separation of allelic genes that typically occurs during meiosis • New Vocabulary • Genetics • Allele • Dominant • Recessive • Homozygous • Heterozygous • Genotype • Phenotype • Lae of segregation • Hybrid • Law of independent assortment

28. How Genetics Began • In 1866, Gregor Mendel, an Australian monk and a plant breeder, published his findings on the method and the mathematics of inheritance in garden pea plants • Mendel is known as the father of genetics • Genetics is the science of heredity

29. How Genetics Began • The passing of traits to the next generation is called inheritance, or heredity • Mendel chose pea plants for his study • Easy to grow • Many are true breeding - they consistently produce offspring with only one form of a trait • Usually reproduce by self-fertilization - when a male gamete within a flower combines with a female gamete in the same flower • Mendel performed cross-pollination by transferring a male gamete from the flower of one pea plant to the female reproductive organ in another pea plant • Mendel followed various traits in the pea plants he bred

30. The Inheritance of Traits • The parent generation is also known as the P generation • The offspring of this P cross are called the first filial (F1) generation • The second filial (F2) generation is the offspring from the F1 cross

31. The Inheritance of Traits • Mendel studied seven different traits • Seed or pea color • Flower color • Seed pod color • Seed shape or texture • Seed pod shape • Stem length • Flower position • F1 generation plants from these crosses showed a 3:1 ratio

32. Genes in Pairs • Mendel concluded from his studies that there must be two forms of a trait, controlled by a factor, which is now called an allele • An allele is an alternative form of a single gene passed from generation to generation • Dominant - form of the trait that appeared in the F1 generation • Recessive – form of the trait that was masked in the F1 generation

33. Dominance • Dominant alleles is represented by a capital letter. Yellow-seed forms are a dominant trait (Y) • Recessive alleles are represented by a lowercase letter. Green-seed forms are a recessive trait (y)

34. Dominance • An organism with two of the same alleles for a particular trait is homozygous • YY – yellow-seed plants • yy - green-seed plants • An organism with two different alleles for a particular trait is heterozygous • Yy – dominant allele is observed, therefore, yellow-seed plants

35. Genotype and Phenotype • Genotype – genetic make-up or allelles present • YY, yy, or Yy • Phenotype – physical or observable characteristic • Yellow-seed or green-seed

36. Mendel’s Law of Segregation • Two allelles of each trait separate during meiosis • During fertilization, two alleles for that trait unite • Heterozygous organisms are called hybrids

37. Monohybrid Cross • A cross that involves hybrids for a single trait is called a monohybrid cross • Three possible genotypes YY, Yy and yy • Genotype ratio is 1:2:1 • Phenotype ratio is 3:1 – yellow seeds to green seeds

38. Dihybrid Cross • The simultaneous inheritance of two or more traits in the same plant is a dihybrid cross • Dihybrids are heterozygous for both traits

39. Law of independent Assortment • Random distribution of alleles occurs during gamete formation • Genes on separate chromosomes sort independently during meiosis • Each allele combination is equally likely to occur

40. Punnett Square-Monohybrid Cross • A tool used to predicts the possible offspring of a cross between two known genotypes

41. Punnett Square-Dihybrid Cross • Four types of alleles from the male gametes and four types of alleles from the female gametes can be produced • The resulting phenotypic ratio is 9:3:3:1

42. 10.3 Gene Linkage and Polyploidy • Objectives • Summarize how the process of meiosis produces genetic recombination • Explain how gene linkage can be used to create chromosome maps • Analyze why polyploidy is important to the field of agriculture

43. 10.3 Gene Linkage and Polyploidy • Review Vocabulary • Protein – large, complex polymer essential to all life that provides structure for tissues and organs and helps carry out cell metabolism • New Vocabulary • Genetic recombination • polyploidy

44. Genetic Recombination • The new combination of genes produced by crossing over and independent assortment • Combinations of genes due to independent assortment can be calculated using the formula 2n, where n is the number of chromosome pairs • In humans, the possible number of combinations after fertilization would be 223 x 223, or more than 70 trillion. This number does not include the amount of genetic recombination produced by crossing over

45. Gene Linkage • The linkage of genes on a chromosome results in an exception to Mendel’s law of independent assortment because linked genes usually do not segregate independently • Yet linked genes do not always travel together during meiosis due to crossing over

46. Chromosome Maps • A drawing that shows the sequence of genes on a chromosome and can be created by using cross-over data • Chromosome map percentages are not actual chromosome distances, but they represent relative positions of the genes • Genes that are further apart would have a greater frequency of crossing over.

47. Polyploidy • The occurrence of one or more extra sets of all chromosomes in an organism is polyploidy • A triploid organism, for instance, would be designated 3n, which means that it has three complete sets of chromosomes • In humans, polyploidy is always lethal • In plants, polyploidy is associated with increased vigor and size