1. IGCSE BIOLOGY SECTION 3 LESSON 4

2. Content Section 3 Reproduction and Inheritance a)Reproduction - Flowering plants - Humans b) Inheritance

3. Content Lesson 4 b) Inheritance 3.21 understand that the sex of a person is controlled by one pair of chromosomes, XX in a female and XY in a male 3.22 describe the determination of the sex of offspring at fertilisation, using a genetic diagram 3.23 understand that division of a diploid cell by mitosis produces two cells which contain identical sets of chromosomes 3.24 understand that mitosis occurs during growth, repair, cloning and asexual reproduction 3.25 understand that division of a cell by meiosis produces four cells, each with half the number of chromosomes, and that this results in the formation of genetically different haploid gametes 3.26 understand that random fertilisation produces genetic variation of offspring 3.27 know that in human cells the diploid number of chromosomes is 46 and the haploid number is 23 3.28 understand that variation within a species can be genetic, environmental, or a combination of both 3.29 understand that mutation is a rare, random change in genetic material that can be inherited 3.30 describe the process of evolution by means of natural selection 3.31 understand that many mutations are harmful but some are neutral and a few are beneficial 3.32 understand that resistance to antibiotics can increase in bacterial populations, and appreciate how such an increase can lead to infections being difficult to control 3.33 understand that the incidence of mutations can be increased by exposure to ionising radiation (for example gamma rays, X-rays and ultraviolet rays) and some chemical mutagens (for example chemicals in tobacco).

4. Chromosomes In human body cells there are 46 chromosomes, arranged into 23 pairs.

5. Chromosomes In human body cells there are 46 chromosomes, arranged into 23 pairs. This picture is known as the human karyotype.

6. Chromosomes In human body cells there are 46 chromosomes, arranged into 23 pairs. This picture is known as the human karyotype. The human diploid number is 46.

7. Chromosomes In human body cells there are 46 chromosomes, arranged into 23 pairs. This picture is known as the human karyotype. The human diploid number is 46. The human haploid number is 23.

8. Chromosomes In human gametes (sex cells) there are 23 chromosomes, one from each pair.

9. Chromosomes Other species have different numbers of chromosomes – there is no relationship between the size of an organism and the chromosome number.

10. Chromosomes Other species have different numbers of chromosomes – there is no relationship between the size of an organism and the chromosome number. Diploid number = 56 Diploid number = 92

11. Chromosomes Other species have different numbers of chromosomes – there is no relationship between the size of an organism and the chromosome number. OrganismDiploid number of chromosomes Pigeon Dog Cat Bean Pea Meerkat Pig Dolphin 80 78 38 22 14 36 38 44

12. Sex chromosomes One pair of chromosomes are the sex chromosomes – they determine the gender of an individual (male or female) The male has an X sex chromosome and a Y sex chromosome (shown here)

13. Sex chromosomes One pair of chromosomes are the sex chromosomes – they determine the gender of an individual (male or female) The female has two X sex chromosomes

14. Sex chromosomes One pair of chromosomes are the sex chromosomes – they determine the gender of an individual (male or female) Male = XY Female = XX

15. Inheritance of sex Sex chromosomes =XX Sex chromosomes =XY

16. Inheritance of sex Sex chromosomes =XX Sex chromosomes =XY In gamete (sex cell) formation, the sex chromosomes separate XYXX

17. Inheritance of sex XY X X sperm egg

18. Inheritance of sex XY XXXXY XXXXY sperm egg

19. Inheritance of sex XY XXXXY XXXXY sperm egg BABY GIRL BABY BOY

20. Inheritance of sex XY XXXXY XXXXY sperm egg BABY GIRL BABY BOY FERTILISATION is RANDOM -There is an equal chance that an X-sperm or a Y-sperm will fertilise the egg. Therefore, there is an equal chance of having either a baby girl or a baby boy.

21. Inheritance of sex XY XXXXY XXXXY sperm egg BABY GIRL BABY BOY SEXUAL REPRODUCTION means loads of variation, because the genetic material from both parents is mixed together when the sperm fertilises the egg

22. “ understand that division of a diploid cell by mitosis produces two cells which contain identical sets of chromosomes ” “understand that division of a cell by meiosis produces four cells, each with half the number of chromosomes”

23. “ understand that division of a diploid cell by mitosis produces two cells which contain identical sets of chromosomes ” “understand that division of a cell by meiosis produces four cells, each with half the number of chromosomes” MITOSISMEIOSIS The method by which all body cells divide, to maintain the diploid number (46) The method by which sex cells are produced, resulting in the haploid number (23)

24. “ understand that division of a diploid cell by mitosis produces two cells which contain identical sets of chromosomes ” “understand that division of a cell by meiosis produces four cells, each with half the number of chromosomes” MITOSISMEIOSIS The method by which all body cells divide, to maintain the diploid number (46) The method by which sex cells are produced, resulting in the haploid number (23) WARNING! Mitosis and Meiosis are both difficult concepts to grasp, so make sure you take this section slowly and go over it again and again until you’ve grasped all the necessary facts.

25. Mitosis This occurs for growth and repair (and also in asexual reproduction) and before each cell division a copy of each chromosome is made so that each body cell has exactly the same genetic information. The chromosome number is maintained. DIPLOID  DIPLOID

26. Mitosis Parental cell with two pairs of chromosomes

27. Mitosis Parental cell with two pairs of chromosomes Each chromosome replicates itself

28. Mitosis Parental cell with two pairs of chromosomes Each chromosome replicates itself The ‘copies’ are pulled apart. Cell now divides for the only time.

29. Mitosis Parental cell with two pairs of chromosomes Each chromosome replicates itself The ‘copies’ are pulled apart. Cell now divides for the only time. Each ‘daughter’ cell has the same number of chromosomes as the parental cell and contains the same genes as the parental cell

30. Meiosis

31. Diploid number Haploid number

32. Meiosis Diploid number Haploid number Meiosis occurs in the testes and ovaries to produce the gametes (eggs and sperm) for sexual reproduction.

33. Meiosis Parental cell with two pairs of chromosomes

34. Meiosis Parental cell with two pairs of chromosomes Each chromosome replicates itself.

35. Meiosis Parental cell with two pairs of chromosomes Each chromosome replicates itself. Chromosomes part company and move to opposite sides with their copies. Cell divides for the first time.

36. Meiosis Parental cell with two pairs of chromosomes Each chromosome replicates itself. Chromosomes part company and move to opposite sides with their copies. Cell divides for the first time. Copies now separate and the second cell division takes place

37. Meiosis Parental cell with two pairs of chromosomes Each chromosome replicates itself. Chromosomes part company and move to opposite sides with their copies. Cell divides for the first time. Copies now separate and the second cell division takes place Four gametes now formed, each with half the chromosome number of the original parental cell.

38. Why sexual reproduction promotes variation

39. 1. The gametes are produced by meiosis, which ‘shuffles’ the genes.

40. Why sexual reproduction promotes variation 1.The gametes are produced by meiosis, which ‘shuffles’ the genes. 2.Gametes fuse randomly, with one of each pair of alleles coming from each parent.

41. Why sexual reproduction promotes variation 1.The gametes are produced by meiosis, which ‘shuffles’ the genes. 2.Gametes fuse randomly, with one of each pair of alleles coming from each parent. 3.The alleles in a pair may be different and so produce different characteristics.

42. understand that variation within a species can be genetic, environmental, or a combination of both

43. Variation Two possible causes

44. Variation Two possible causes Genetic

45. Variation Two possible causes Genetic As a result of the different genes inherited

46. Variation Two possible causes Genetic As a result of the different genes inherited

47. Variation Two possible causes Genetic As a result of the different genes inherited Environment

48. Variation Two possible causes Genetic As a result of the different genes inherited Environment As a result of the conditions in which they develop

49. Variation Two possible causes Genetic As a result of the different genes inherited Environment As a result of the conditions in which they develop

50. Variation Two possible causes Genetic As a result of the different genes inherited Environment As a result of the conditions in which they develop Usually, variation is due to a combination of genetic and environmental causes

51. “ understand that mutation is a rare, random change in genetic material that can be inherited” “ understand that the incidence of mutations can be increased by exposure to ionising radiation (for example gamma rays, X-rays and ultraviolet rays) and some chemical mutagens (for example chemicals in tobacco) ”

52. Genetic mutations Every time a cell divides, all the DNA in the nucleus must be copied exactly.

53. Genetic mutations Every time a cell divides, all the DNA in the nucleus must be copied exactly. Occasionally a mistake may occur, and bases may be put in the wrong order.

54. Genetic mutations Every time a cell divides, all the DNA in the nucleus must be copied exactly. Occasionally a mistake may occur, and bases may be put in the wrong order. As a result, there will be a different sequence of amino acids, and therefore a different protein will be made.

55. Genetic mutations Every time a cell divides, all the DNA in the nucleus must be copied exactly. Occasionally a mistake may occur, and bases may be put in the wrong order. As a result, there will be a different sequence of amino acids, and therefore a different protein will be made. This change in the order of the bases is called a MUTATION

56. Genetic mutations CausesEffects Mutations occur naturally but …… there is an increased risk if individuals are exposed to mutagenic agents such as ionising radiation (UV, X-rays) radioactive substances and certain chemicals (eg. in cigarette smoke) the greater the dose, the greater the risk. Most mutations are harmful and in … Reproductive cells can cause death or abnormality in body cells they may cause cancer some mutations are neutral and some may increase the survival chances of an organism and its offspring who inherit the gene

57. Types of gene mutations

58. Duplication – the nucleotide (an individual base unit) is inserted twice instead of once Eg: Normal AAT CCG GCA TTC TAT Duplicated AAA TCC GGC ATT CTA duplication here

59. Types of gene mutations Deletion – the nucleotide is completely missed out Eg: Normal AAT CCG GCA TTC TAT Deletion AAC CGC ATT CTA TTC deletion here

60. Types of gene mutations Substitution – a different nucleotide is inserted Eg: Normal AAT CCG GCA TTC TAT Substitution AAG CCG GCA TTC TAT substitution here

61. Types of gene mutations Inversion – the sequence of bases is reversed Eg: Normal AAT CCG GCA TTC TAT Inversion AAT CCG ACG TTC TAT inversion here

62. Chromosome mutations Rather than changes in individual bases, whole chromosomes may be inserted or lost, or bits may be broken off.

63. Chromosome mutations Rather than changes in individual bases, whole chromosomes may be inserted or lost, or bits may be broken off. A relatively common chromosome mutation can be found when the female ova may contain two copies of chromosome 21.

64. Chromosome mutations Rather than changes in individual bases, whole chromosomes may be inserted or lost, or bits may be broken off. A relatively common chromosome mutation can be found when the female ova may contain two copies of chromosome 21. When fertilised by a normal sperm, the offspring will have three copies of chromosome 21. This is called trisomy 21 and the resulting condition is known as Down’s syndrome.

65. understand that resistance to antibiotics can increase in bacterial populations, and appreciate how such an increase can lead to infections being difficult to control

66. Resistance in bacteria Scientists try to control the spread of bacteria using antibiotics.

67. Resistance in bacteria Scientists try to control the spread of bacteria using antibiotics. Gene mutations can result in bacteria becoming resistant to the use of antibiotics.

68. Resistance in bacteria Scientists try to control the spread of bacteria using antibiotics. Gene mutations can result in bacteria becoming resistant to the use of antibiotics. Resistant bacteria have an advantage over non- resistant bacteria and so spread through the population.

69. Resistance in bacteria Scientists try to control the spread of bacteria using antibiotics. Gene mutations can result in bacteria becoming resistant to the use of antibiotics. Resistant bacteria have an advantage over non- resistant bacteria and so spread through the population. This is an example of natural selection in action.

70. “describe the process of evolution by means of natural selection”

72. To be continued ….

73. End of Section 3 Lesson 4 In this lesson we have covered: Chromosomes Inheritance of Sex Mitosis Meiosis Variation Mutations