Biology Form 5: Chapter 6 (Variation)

Form 5: Chapter 6 – Variation

Chapter 6: Variation 6.1 Variation in Organism

1. Difference in an organism from the same species. 2. Types: Types: Continuous Discontinuous Quantitative Qualitative - Hand span - Blood group - Chest circumference - Eyes colour - Weight - Thumb hyperextension - Height - Ear lobe shape - Rolling of tongue Genetic and environment Genetic 6.2 Causes of variation Genetic 1. Influenced by: a) Crossing over

Occurs between chromatids from a pair of homologous chromosomes during prophase of meiosis I. b) Independent During metaphase of meiosis I. Independent assortment c) Mutations i) Gene ii) Chromosome d) Random fertilization Between any male gamete and any female gamete 2. Crossing over and independent assortment produces genetic recombination (new combinations).

Prepared by: Mr. Gerard Selvaraj


Environmental Phenotype = Genotype + Effects of the environment Mutation

1. A permanent change in the nucleotide sequence of DNA or in the amount of DNA. 2. Reasons: a) Mistakes that occur during replication of DNA during cell division. b) uneven distribution of chromosomes during cell division. c) a chemical or physical agent. Physi Physical agent Chemical agent High energy radiation - benzene - X-rays - formaldehyde - gamma rays - carbon tetrachloride - ultraviolet rays - asbestos - nuclear radiation - mustard gas - tar in tobacco smoke - pesticide 3. Mutations that occur in a body cell will not be inherited but can affect the person during his lifetime. 4. Gene mutation a) occurs at a single locus on a chromosome. b) Involves nucleotide base c) a change in the sequence of bases may result in a defective protein or no protein being produced at all. d) may occur on a recessive or dominant gene. e) causes many inherited diseases like sickle cell anaemia, albinism and polydactylism. f) Sickle cell anaemia; - caused by a mutation on the recessive allele responsible responsible for the production of haemoglobin. - the mutated allele causes the production of abnormal haemoglobin which crystallizes, causing the red blood cells to become sickle shaped or S-shaped. g) Albinism; - Albinos cannot produce an a n enzyme responsible for the production of the skin pigment, melanin. - Albinos have white hair, pinkish eyes and an d skin, and skin that is very sensitive to sunlight. h) Polydactylism; - a polydactyl individual has an extra finger or toe.



5. Chromosome mutation a) an increase or decrease in the number of chromosome or a change in the structure of the chromosome. b) caused by the failure of homologous chromosome to segregate during meiosis I and meiosis II. c) Down Syndrome - individual has an extra copy of chromosome number 21, giving a total of 47 chromosomes in each cell. - broad forehead, short neck and limbs, almond shaped eyes with folded eyelids, thick protruding tongue and varying degrees of mental retardation and heart defects at birth. d) Turner’s syndrome - infertile female has only 44 + X0 chromosomes. - low levels of sex hormones very little development of secondary sexual characteristics. e) Klinefelter’s syndrome - male has 44 + XXY chromosomes. - underdeveloped sex organs. - a low level of testosterone, sparse body bod y hair and developed breasts.



f) Polyploidy - addition of one or several whole sets of chromosomes in an organism. - three sets-triploid - common among flowering plants and food plants like apples, tomato and wheat to produce bigger fruit with more nutritional value. g) change in the chromosomal structure involving rearrangement of whole blocks of genes on a chromosome. h) may result in an alteration in the number of genes or in the sequence of whole sets of genes in chromosomes.



The importance of variation in the survival of a species

1. Provides the necessary adaptations for them to compete and also to survive together in new environments that arise.


Biology Form 5: Chapter 6 (Variation)

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