1. Chapter 3 HEREDITY AND VARIATION

2. 3.1 Cell division All living organisms grow and reproduce & are made up of cells Cells reproduce by dividing and passing on their genes to daughter cells Each cell has its own nucleus which controls the cells activities through the genetic material, DNA which acts as a set of instruction or code for life 3. What is genes? Genes are sections of DNA that code for the production of protein and are arranged along the chromosomes Carry the inherited characteristics of a person Genes occurs in pairs, therefore each inherited characteristics is carried by two genes One gene from each pair comes from the father and the mother Example of inherited characteristics carried by genes are: - Colour of the eyes - Pattern of finger prints - Type of hair - Blood group - Intelligence 4. Deoxyribonucleic acids (DNA) Is a double helix, made up of a series of genes to encode genetic information This determine the individual characteristics of an organism All the information in DNA is transmitted in the form of a genetic code Should any information not be followed, the baby will be born with physical or mental defects DNA molecule is very complicated and resembles a twisted ladder or staircase 5. Chromosomes Is a thread-like structure found in the nucleus of a cell Chromosome and the genes on them are composed of DNA It exist in pair and are identicalhomologous A pair of chromosomes has genes arranged in the same way. As a result the genes also exist in pairs Human has two sets of chromosomes in the body(one set from the father and another set from the mother) 6. Mitosis Occur in somatic cells all cells body except reproductive cells Is the type of cell division that produces genetically identical cells During mitosis, DNA replicates in the parent cell, which divides to produce two new cells, each containing an exact copy of the DNA as in the parent cell This allows growth of multicellular organism from unicellular zygotes Replacements of cells and repair of tissues is possible through mitosis followed by cell division Cells are constantly dying and being replaced by identical cells Is the basis of asexual reproductionthe production of new individuals of a species by one parent organism 7. Mitosis Mitosis is the type of cell division that produces genetically identical cells During mitosis, DNA replicates in the parent cell, which divides to produce two new cells, each containing an exact copy of the DNA as in the parent cell Cells produced are diploid (2n) Replacement of cells and repair of tissues is possible through mitosis (Ex: cell replacement is rapid in the skin & in the lining of the gut Is actively carried out at the ends of plant shoots and roots 8. Chromosomes thicken and shorten, and become clearly visible Each chromosome duplicates itself and forms two chromatids attached at the centromere The nuclear membrane dissapears (a) 9. A fibrous spindle is formed between the chromosomes Each chromosome is attached to a spindle by its centromere The chromosome arrange themselves at the (b) 10. The centromere of each chromosome splits into two The chromatids separate and move to opposite poles. (c) 11. A nuclear membrane is formed around the chromosomes at each pole The cell begins to divide and form two daughter cells Each daughter cell has the same number of chromosomes as the parent cell In a plant cell, the cytoplasm does not constrict. Instead a new cell wall is developed across the cytoplasm (d) 12. Importance of mitosis Important for the following reasons: (a) Growth (b) Replacing old or damaged cells Enables organisms to reproduce themselves (Ex: by asexual reproduction; binary fission in amoeba) Ensures that the new cells produced have the same number of chromosomes as the parent cells 13. Meiosis Meiosis is the process by which a nucleus divides by two divisions into four nuclei, each containing half the number of chromosome of the mother cell Resulting nuclei are haploid (n) The cells produced are genetically different to the parent cell and to each other Occurs in 2 stages that is meiosis 1 and meiosis 2 Produces genetic variation (caused by crossing over between homologous chromosome) In animals, meiosis occurs in testes and ovaries, whereas in plants, it occurs in the anthers and ovaries 14. Importance of meiosis 1. Production of gamete the only process that produces gametes 2. Reduction in chromosome number ensures that each gamete has only half the number of chromosomes of the parent cell 3. Maintenance of chromosome number in somatic cells the somatic cells have their number of chromosomes maintained 4. Production of variation causes genetic variation in the descendants(can produce individuals who do not resemble their parents closely * Variation occurs in TWO ways: 1. Different combination of chromosomes 2. Crossing over 15. Comparing and contrasting mitosis with meiosis SIMILARITIES of MITOSIS and MEIOSIS: - Both are processes of cell division - In each process, duplication of chromosomes occurs once 16. MITOSIS DIFFERENCES MEIOSIS To produce new cells for growth and repair of damaged tissues Aim of process To produce gametes (reproductive cells) Somatic cells (non reproductive cells) Type of cell involved Reproductive cells which produce gametes Two daughter cells Product Four gametes Nucleus of parent cells divides once Division of nucleus Nucleus of parent cell divides twice Duplication of chromosomes takes place Duplication of chromosomes Duplication of chromosomes takes place in the first cell division Same as in parent cell,46 Number of chromosomes in daughter cells Half the number of those in the parent cell,23 Does not occur Crossing over Occurs when the chromosomes are at the equatorial plane Same as in parent cell Combination of Different from that of the parent cell DIFFERENCES OF MITOSIS AND MEIOSIS 17. Exercises : CELL DIVISION 1. Which of the following is the simplest unit of inheritance? A. Nucleus B. Chromatid C. Chromosome D. Gene 2. How many chromosomes are present in the nucleus of a human cheek cell? A. 23 B. 24 C. 46 D. 48 18. 3. How many chromosomes can be found in the nucleus of a human ovum? A. 23 B. 24 C. 46 D. 48 4. What is the importance of mitosis in somatic cells? A. It maintains the number of chromosomes in all new cells which are produced B. It occurs in all organs of the body so long as the cells are actively growing C. It prevents the body from being reduced in size due to the death of old cells D. It produces new cells which will support the cellular activities of the old cells. 19. 5. What is the importance of meiosis in living organisms? A. It maintains the number of chromosomes of zygotes produced from fusion of sex cells B. It occurs only in parts of the living organism which are actively growing C. It replaces old cells which are no longer able to carry out their cellular functions D. It repairs damaged tissue in the body due to injury or disease 6. Where is DNA found in a living organism? A. Cell membrane B. Cytoplasm C. Nucleus D. Cell wall 20. 7. What is mitosis and meiosis? 8. Describe the process of mitosis. 9. Describe the process of meiosis 10. What is the importance of mitosis and meiosis? 21. 3.2 The principles and mechanism of inheritance The study of inherited characteristic is called heredity Genes carry the inherited characteristics of a person Each characteristic of a person is controlled by a pair of genes A gene for a certain characteristic may be a dominant gene or a recessive gene 22. Dominant genes and recessive genes A dominant gene is powerful and can mask the effect of a less powerful or recessive gene in a pair A recessive gene is weak and cannot show its effect if it is paired with a dominant gene Each pair of genes is represented by two letters, a capital letter used for a dominant gene and a small letter for a recessive gene 23. Monohybrid inheritance Mendel chose true-breeding varieties of the garden pea as the parents Mendel crossed a tall plant with a dwarf plant. Progeny from this cross, called the F1 generation (first filial generation) were all tall All the F1 plants were then self-pollinated to obtain the F2 generation (second filial generation) The F2 generation included both tall and dwarf plants in the ratio of 3 tall to 1 dwarf(phenotype ratio) 24. It is shown that only the dominant(strong) trait appeared in the F1 generation. The recessive (weak) trait is blocked by the strong trait from expressing itself and is therefore hidden in the F1 generation 25. The mechanism of trait inheritance The character of height is determined by two contrasting traits, tall and dwarf. The gene for tall is dominant while the gene for dwarf is recessive If the gene for tall is represented by T and the gene for dwarf is represented by t, there will be three genetic combinations(genotype) for plant height Genotype is the term used to describe the genetic make-up or constitution of a person. It includes the genes inherited from both parents,whether they are dominant or recessive Phenotype is the physical traits such as colour of the eyes, height and ect.. 26. 3 possible combination are: Phenotype Genotype Tall (pure strain) TT (homozygous dominant) Tall (monohybrid) Tt (heterozygous) Dwarf (pure strain) tt (homozygous recessive) 27. Pure bred crossed with pure bred 28. Dominant traits and recessive traits in human Dominant traits Recessive traits Right handed Left handed Tall Short Black/Curly hair Brown/Straight hair Black/Brown eyes Blue eyes With ear lobe Without ear lobe Can roll the tongue Cannot roll the tongue Has normal vision Colour blind Has dimples Has no dimple Skin is pigmented Skin has no pigments(albino) Long eye lashes Short eye lashes 29. The principles and mechanism of inheritance For questions 1 to 4 , use the information given below: 1. Which of the following crosses is likely to produce the ratio 3 brown eyed: 1 blue eyed? A. BB x bb B. Bb x bb C. BB x Bb D. Bb x Bb 2. Which of the following crosses is likely to produce the ratio 1 brown eyed: 1 blue eyed? A. BB x bb B. Bb x bb C. BB x Bb D. Bb x Bb The brown eyed trait, B, is dominant over the blue eyed trait, b 30. 3. Which of the following crosses is likely to produce ALL brown eyed individuals? A. BB x bb B. Bb x bb C. bb x bb D Bb x Bb 4. Which of the following crosses is likely to produce all blue eyed individuals? A. BB x bb B. Bb x bb C. bb x bb D. Bb x Bb 31. For questions no 5 to 8, refer to the information below: 5. Which of the traits are recessive? A. Straight hair, no dimples B. Straight hair, dimples C. Curly hair, dimples D. Curly hair, no dimples A girl has curly hair and dimples on her face. Her mother has straight hair and no dimples on her face. Her father has curly hair and has dimples on his face. Her mother is homozygous for both traits. Assume that the two traits are inherited independently of each other 32. 6. If the curly hair traits is represented by the letter C and dimples by the letter D, what is the mothers genotype for the two traits? A. CC and DD B. cc and Dd C. cc and dd D. Cc and Dd 7. If the curly hair is represented by the letter C and dimples by the letter D, which of the following is the possible genotype of the girlss father? A. cc and dd B. CC and Dd C. cc and DD D. Cc and dd 33. 8. What is the genotype of the girl? A. cc and dd B. CC and Dd C. Cc and DD D. Cc and Dd 34. 3.3 Sex Determination and the occurrence of twins in human beings Sex Chromosome Autosomes are chromosomes which are not sex chromosomes The characteristic features of a person are determined by the autosomes The sex of a person is determined by the sex chromosome In a woman, the sex chromosomes are a pair of identical chromosomes called X chromosomes In a man, the sex chromosomes consists of one X chromosome and a smaller Y chromosome 35. Therefore, chromosomes in female can be written as 44+XX while male as 44+XY 36. Mechanism of Sex Determination Fathers sperm determine the sex of the baby If the baby inherits Y chromosome from its father boy If the baby inherits X chromosome from its fathergirl THUS, the sex of the baby depends on which sperm fertilizes the egg cell(ovum). 37. Parent Phenotype: Male Female Parent Genotype: 44+XY 44+XX Parents gametes: Offspring genotype : 44+XX 44+XX 44+XY 44+XY Offspring phenotype: Female Female Male Male Ratio: (1 girl : 1 boy) 22+X 22+Y 22+X 22+X 38. Identical and non-identical twins (1) Identical twins When an ovum is fertilized by a sperm, a zygote is formed The zygote then divides into 2 separate cells in the womb producing 2 embryos identical twins Identical twins are two normal, genetically identical individuals of the same sex and share a common placenta during developmental stage 39. (2) Non-Identical twins Some women may produce two ova at the same time. When the two different ova are fertilized at the same time by two different sperms, two zygotes are formed non-identical twins(fraternal twins) Non-identical twins are genetically different individuals, can may be of the same sex or different sex and each have their own placenta during developmental stage 40. Comparing and contrasting identical twins with non-identical twins 1. SIMILARITIES: Two foetuses develop in the uterus at the same time Two babies are born at the same time 41. 2. Differences Identical twins Differences Non-Identical twins One ovum and one sperm No of gametes involved Two ova and two sperms One ovum is fertilized and one zygote is formed Fertilization Two ova are fertilized and two zygotes are formed The single zygote divides into two zygotes Division of zygote formed The two zygotes formed do not divide The two foetuses share one placenta Number of placentas The two foetuses have two separate placentas Have identical chromosomes and genes Chromosomes and genes Have different chromosomes and genes Have the same sex-both boys or both girls Sex of twins The twins may have the same sex or opposite sexes The twins have similar characteristic feature Characteristic features The twins have different characteristic features 42. Siamese twins Sometimes a zygote can not divides into two completely, so the two zygotes are still linked to each other The linked zygote develop into foetuses and are born as twins joined together in some parts. Siamese twins are identical twins that are formed when the two embryos do not separate completely during the developmental stage and are attached to one another 43. Exercises :Sex determination and the occurrence of twins in human beings 1. What are autosomes? 2. Name the chromosome which determines that zygote will develop into a male 3. Why is the Y-chromosome smaller than the X- chromosome? 4. Why do identical twins have identical features? 5. State the possible sex of non-identical twins. 44. 3.4 Mutation Is a change in the amount or the chemical structure of DNA Can occur spontaneously or induced by substances called mutagens The new genetic material produced are called mutants Mutation occur in gamete cell can be inherited Mutation occur in somatic cell can only be inherited by daughter cells produced by mitosis 45. Types of mutation 1. Chromosomal mutation 2. Gene mutation 46. Chromosomal mutation Is the result of changes in the number or structure of chromosomes Can cause changes in the characteristics of the organism Changes in the no of chromosomes are usually the result of errors occurring during meiosis or mitosis These changes may involve the loss of a pair or a single chromosome, or the increase in an entire haploid set of chromosomespolyploidy 47. Consequences of chromosomal mutation 1. Downs syndrome Individuals suffering from Downs syndrome have 47 chromosomes Additional chromosome is autosome no 21 Symptoms: mental retardation, reduced resistance to disease, congenital heart abnormalities, a short stocky body, a thick neck and the characteristics folds of skin over the inner corner of the eye Termed as mongolism 48. 2. Klinefelters syndrome Have an extra X chromosome, so that the usual karyotype of XY is replaced by one of XXY Male with underdeveloped testis, infertility and some female characteristic such as breast enlargement 49. 3. Turners syndrome Lack one X chromosome Such women are XO, rather than the normal XX. Have ovaries and breast that do not develop, do not experience the menstrual cycle and do not ovulate 50. ** Downs syndrome and other related chromosomal abnormalities occur more frequently in children born to older women related to the age of the mothers egg cells 51. 2. Gene Mutation/point mutation Is a change in the structure of the DNA at a single locus Difficult to detect because physical changes on specific genes are not visible and cannot be observed under a microscope Often have undesirable effects but are sometimes beneficial to the mutant organism If gene mutation produces a beneficial phenotype, the frequency of the mutation in the population will increase from generation to generation as a result of natural selection 52. Consequences of gene mutation 1. Colour Blindness Is a sex-linked recessive trait Recessive gene for colour blindness is found on the human X chromosome Most common form is red-green colour blindness(cannot distinguishes red & green) Females have pair of genes that control color blindness but male have only one If a male inherits one gene for CB,he has this disorder Female can only has this disorder if she inherits two recessive genes,one from each parent 53. 2. Albinism Occurs when the gene which is responsible for normal pigmentation undergoes mutation The gene responsible for albinism is recessive Albino human beings are unable to produce the black pigment (melanin) So, they have no distinct colour in their skin, hair or eyes 54. Causes of mutation 1. Radioactive substances - Radioactive radiation such as alpha, beta and gamma radiation - X-ray also can cause mutation - Can leads to cancer - Can change the structure of functions of gene and chromosome 55. 2. High Intensity Ultraviolet Light - Ultraviolet from the sun can destroy the cells in the skin and cause mutation - Can leads to skin cancer 3. Chemical substances - Exposure of certain chemical substances like formaldehyde, pesticides, benzene,nicotine - Can cause cancer and foetal deformity 56. 4. Changes in environment - After a long period of time, changes in the environment can cause organisms to adapt themselves and mutate into new species - These new species with their adapted or mutated features are most suitable for living in the changing environment 57. Advantages and disadvantages of mutation Advantages Disadvantages 1. Mutation produces genetic variation. This can give rise to new species. 1. Can cause genetic disorder such as albinism and haemophilia which can be passed on future generations 2. Plants and animals mutate in response to adverse changes in the environment. Mutation gives them a better chance to survive 2. Can cause abnormalities in children 58. Exercise: Mutation 1. What is mutation 2. What pigment gives our skin colour? 3. Why is haemophilia known as a sex-linked disease? 4. State one disorder in a woman with Turners syndrome. 5. Illustrate what will happen if a normal female (carrier) haemophilia married to a normal male. 59. Variation Each person has unique features which enable you to differentiate him or her from other friends No two people are exactly the same These differences among individuals in a population or a species are called variation The differences may be the result of genetic differences, the influences of the environment, or a combination of genetic and environmental influences. 60. In human usually the character referred to are the physical characters such as height, weight, skin colour, etc.. The differences in physiological processes such as our blood glucose level and immunity level are also considered There are two types of variation: (1) Continuous variation (2) Discontinuous variation 61. Continuous Variation The characteristics can be measured or graded on a scale such as human heights, weight,skin colour, etc.. Cannot be grouped into distinct and discrete phenotypes Usually quantitative (they can be measured) and are determined by genes and influenced by environmental factors. In *CV, a normal distribution curve is obtained From the distribution curve, it can be seen that individuals show gradual difference from one extreme to another extreme There are very few individuals who are very short or very tall 62. Discontinuous variation The characters are discrete and cannot be measured or graded on a scale such as ABO blood group, color blindness, presence or absence of earlobe,etc.. Are qualitative(they cannot be measured or graded) usually determined by a single gene with distinct effect. Discrete distribution is obtained 63. The differences of continuous variation and discontinuous variation 64. Factors that cause variation Factors that cause variation Genetic Factor Crossing Over Mutation Gene Mutation Chromosome Mutation Environmental factor Adaptation 65. Importance of variation Causes natural differences in appearance among individuals of the same species enables us to identify different individuals Improves the ability to survive in a changing environment Gives rise to a large variety of plants and animals. 66. Exercises: Variation 1. What is variation 2. What is continuous variation. Give an example. 3. State two important factors which cause variation 4. Why is the ability to roll ones tongue considered as a discontinuous variation? 5. State one important use of variation in daily life.