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  1. title Genetics

  2. Genetics 22.1 What is genetics? 22.2 Chromosomes, genes and DNA 22.3 How are genes passed on from generation to generation? 22.4 Studying the pattern of inheritance 22.5 How to solve problems involving monohybrid inheritance? 22.6 Sex determination in humans 22.7 How to study human inheritance? 22.8 Variation among individuals of the same species 22.9 Why are we all different? 22.10 Significance of variations

  3. 22.1 What is genetics? Why are you similar to your parents in many ways? Passing on of characteristics from one generation to the next is the process is the study is heredity inheritance genetics

  4. 22.2 Chromosomes, genes and DNA chromosome nucleus chromatids cell DNA (helical) gene

  5. Gene • short segment of aDNA at specific location of chromosome • basic unit of heredity • controls inheritable characteristics

  6. twisted together into a helix deoxyribonucleic acid D N A carries genetic information that determines the sequence of amino acids in proteins types of proteins and enzymes control • body characteristics • metabolic activities

  7. e.g.human:46chromosomes; cat: 38 chromosomes DNA molecules special proteins Chromosome specific number of chromosomes in each species same numberin all body cellsexcept sex cells

  8. Chromosomes chromosomes in pairs (homologous chromosomes) Body cells diploid either paternal or maternal chromosome Sex cells (gametes) haploid Two haploid gametes combine to form one diploid cell

  9. 22.3 How are genes passed on from generation to generation? starting point of life male gamete female gamete zygote haploid gametes • each carries half of the genes which determine the characteristics of the parent • as vehicles of inheritance

  10. Stages in meiotic cell division I 1 diploid cell 4 haploid gametes meiotic cell division meiosis (nuclear division which reduces the chromosome number by half) followed by division of cytoplasm Stages in meiotic cell division

  11. Stages in meiotic cell division II

  12. Occurrence of meiotic cell division flowering plants • male gametes in pollen grains • egg cells in ovules animals • sperms in testes • ova in ovaries

  13. The significance of meiosis • Halving of chromosome numbers in gametes • produces haploid gametes • the diploid number of chromosomes can be restored at fertilization • Independent assortment • produces gametes with different genetic make-up • variations among offspring of the same species

  14. The two possible combinations of chromosomes in the gametes formed from only 2 pairs of chromosomes during meiosis combination 1 either Imagine the enormous number of combinations in humans which have 23 pairs of chromosomes! combination 2 or

  15. Carry out Practical 22.1 Observation of meiosis in a testis squash of the grasshopper or in photomicrographs

  16. 22.4 Studying the pattern of inheritance Monohybrid inheritance • inheritance of a characteristic which is controlled by only one pair of alleles for each gene • first studied by Gregor Mendel who investigated the inheritance of two contrasting characters (tall & short stems) in garden peas

  17. About the experiment dwarf (pure breeding) tall (pure breeding) produced offspring only tall

  18. About the experiment dwarf (pure breeding) tall (pure breeding) produced offspring only dwarf

  19. About the experiment dwarf (pure breeding) tall (pure breeding) cross-pollination & fertilization first filial (F1) generation all tall (1064) self-pollination & fertilization of F1 second filial (F2) generation 787 tall 277 dwarf 3 : 1 ???

  20. Other characters studied Character studied Shape of seed coat Colour of cotyledons Colour of seed coat Shape of pods inflated smooth yellow grey  Cross    constricted All show similar pattern of inheritance. wrinkled green white F1 all smooth all yellow all grey all inflated Ratio of characters in F2 2.96 : 1 3.01 : 1 3.15 : 1 2.95 : 1

  21. Interpretation of experimental results • These characteristics are controlled by pairs of genes. tall dwarf  parents TT tt

  22. Interpretation of experimental results • Each gamete receives only one gene (allele) from each pair. tall dwarf  parents TT tt gametes T t

  23. Interpretation of experimental results • All offspring (F1) are tall because T gene is dominant to t gene. tall dwarf  parents TT tt gametes T t F1 Tt(tall)

  24. Interpretation of experimental results • F1 generation can produce 2 types of gametes (T or t). tall dwarf  parents TT tt gametes T t F1 Tt(tall) gametes T t

  25. Interpretation of experimental results • There are 4 possible combinations of gametes when random fertilization happens. tall dwarf  parents TT tt gametes T t  F1 Tt(tall) Tt(tall) gametes T t T t F2 TT Tt Tt tt tall dwarf ratio 3 : 1

  26. Common terms in genetics • Allele tall dwarf  parents • a length of DNA controlling a certain character TT tt which may have 2 or more alternate forms gametes T t  F1 Tt(tall) Tt(tall) • each form of a gene is called an allele gametes T t T t e.g. T : the allele for tallness t : the allele for dwarfness F2 TT Tt Tt tt tall dwarf ratio 3 : 1

  27. Common terms in genetics • Phenotype tall dwarf  parents • the appearance of a character of an organism TT tt gametes T t e.g. the phenotype of the F1 generation is tall  F1 Tt(tall) Tt(tall) gametes T t T t F2 TT Tt Tt tt tall dwarf ratio 3 : 1

  28. Common terms in genetics • Genotype tall dwarf  parents TT tt • the genetic make-up of an organism in relation to the gene being investigated gametes T t  F1 Tt(tall) Tt(tall) e.g. the genotype of the F1 generation is Tt gametes T t T t F2 TT Tt Tt tt tall dwarf ratio 3 : 1

  29. Common terms in genetics • Homozygote tall dwarf  parents TT tt • an organism whose genotype contains two identical alleles for a particular characteristic gametes T t  F1 Tt (tall) Tt (tall) e.g. the tall parent plants (TT) or the dwarf plants (tt) gametes T t T t F2 TT Tt Tt tt homo= the same tall dwarf ratio 3 : 1

  30. Common terms in genetics • Heterozygote tall dwarf  parents TT tt • an organism whose genotype contains two different alleles for a particular characteristic gametes T t  F1 Tt (tall) Tt (tall) e.g. the tall plants with Tt make-up in F1 generation gametes T t T t F2 TT Tt Tt tt hetero = different tall dwarf ratio 3 : 1

  31. Common terms in genetics • Dominant tall dwarf  parents • a dominant gene can express itself or produce its effect in both homozygous and heterozygous conditions TT tt gametes T t  F1 Tt (tall) Tt (tall) e.g. T represents a dominant gene which causes the plants to be tall in either the homozygous (TT) or heterozygous (Tt) condition gametes T t T t F2 TT Tt Tt tt tall dwarf ratio 3 : 1

  32. Common terms in genetics • Recessive tall dwarf  parents • a recessive gene can only express itself in a homozygous condition TT tt gametes T t  e.g. t represents a recessive gene which causes the plant to be dwarf when in a homozygous (tt) condition F1 Tt (tall) Tt (tall) gametes T t T t F2 TT Tt Tt tt tall dwarf ratio 3 : 1

  33. Common terms in genetics • Hybrid tall dwarf  parents • an individual which results from crossing two homozygous parents which are genetically different TT tt gametes T t  F1 Tt (tall) Tt (tall) e.g. the F1 generation gametes T t T t F2 TT Tt Tt tt tall dwarf ratio 3 : 1

  34. Examples of monohybrid inheritance Alleles Phenotype Dominant Recessive Characteristic • In maize plants Colour of the seeds in a cob dark-coloured e.g. purple light-coloured e.g.yellow • In fruit flies Length of wingAppearance of abdomen long vestigial broad narrow • In mice Coat colourEar size black brown normal short

  35. Examples of monohybrid inheritance Dominant characteristics Recessive characteristics brown eye blue eye • Eye colour lobed ear lobeless ear • Ear lobes tongue can be rolled tongue cannot be rolled • Tongue rolling normal pigment • Skin pigment no pigment (albinism) straight thumb curved thumb • Thumb shape Examples of human characteristics that are inherited in a Mendelian manner

  36. 22.5 How to solve problems involving monohybrid inheritance? all can be solved by genetic diagrams and Punnett squares We are often provided with the phenotypes(P) and genotypes(G) of the parents, and then asked to: • deduce P and/or G of the offspring and their P and/or G ratios • determine the probability that an offspring will have a certain P or G • predict the number of offspring that will have a certain P or G

  37. Genetic diagrams Consider a monohybrid cross between two humans which are heterozygous for the presence of ear lobes. Let L = allele for lobed ear (dominant) l = allele for lobeless ear (recessive) • Choose a symbol to represent the alleles of the character of the parents.

  38. Genetic diagrams Let L = allele for lobed ear (dominant) • a State the genotypes • and phenotypes of • the parents. l = allele for lobeless ear (recessive) Lobed ear Lobed ear  Ll Ll Parents b Label the genotypes ‘Parents’. c Write ‘’ between the phenotypes and genotypes of the parents to stand for a cross. always write dominant allele first

  39. Genetic diagrams Let L = allele for lobed ear (dominant) • a Write down the • possible alleles of • the male and female • gametes. l = allele for lobeless ear (recessive) Lobed ear Lobed ear  Ll Ll Parents Label them ‘Gametes’. Gametes L l L l b Add lines as shown.

  40. Genetic diagrams Let L = allele for lobed ear (dominant) • a Show the results of • possible random • crossing of the • gametes by using • lines as shown. l = allele for lobeless ear (recessive) Lobed ear Lobed ear  Ll Ll Parents Label the offspring ‘F1’ generation. Gametes L l L l b State the phenotypes of the offspring underneath the genotypes. LL Ll Ll ll F1 Lobed ear Lobeless ear

  41. Punnett squares • Draw four boxes as shown.

  42. Punnett squares • Write down the possible alleles from the female along the top, one above each box. Also do the same along the left side for the male, one next to each box. (female) and (male). Label them L l L l

  43. Punnett squares • The products of the various possible combinations after fusions are written in the appropriate boxes. L l LL Ll L Ll ll l

  44. Punnett squares • State the phenotypes and genotypes of the offspring. L l LL Ll L lobed ear lobed ear Ll ll l lobed ear lobeless ear

  45. The outcome of any particular cross is totally unrelated to that of any other. It is possible to predict the proportion of offspring that will have a certain phenotype or genotype. The use of genetic diagrams or Punnett squares only gives the expected results. from heterozygous couples, 3/4 and 1/4 of offspring will have lobed ears and lobless ears respectively You should note that:

  46. Offspring first parent  second parent phenotypic ratio genotypic ratio 1 BB  BB 2 BB  Bb 3 Bb  Bb 4 BB  bb 5 Bb  bb 6 bb  bb Types of monohybrid crosses • up to 6 types for any pair of alleles e.g. eye colour in human (B:dominant allele for brown eyes, b: recessive allele for blue eyes) all brown eyes all BB all brown eyes BB : Bb = 1 : 1 brown : blue = 3 : 1 BB : Bb: bb = 1 : 2 : 1 all Bb all brown eyes Bb : bb = 1 : 1 brown : blue = 1 : 1 all blue eyes all bb

  47. Types of monohybrid crosses If you are familiar with the 6 types of crosses, you should be able to: • Find the phenotypes or genotypes of parents if result of certain crosses is provided. • State the dominant or recessive characters for a certain phenotype if the phenotypes of the parents and offspring are provided.

  48. How to find out the genotype of an organism with a particular dominant phenotype a dominant phenotype 2 possible genotypes homozygous dominant heterozygous dominant How can we find out the genotype accurately ? By test cross.

  49. How to find out the genotype of an organism with a particular dominant phenotype Example: To identify the genotype of a tall plant Case 1 If the genotype is TT organism to be tested homozygous recessive organism tt TT Gametes T t Tt F1 genotype phenotype tall (all the offspring are tall)

  50. How to find out the genotype of an organism with a particular dominant phenotype Example: To identify the genotype of a tall plant Case 2 If the genotype is Tt organism to be tested homozygous recessive organism tt Tt Gametes T t t tt Tt F1 genotype phenotype tall dwarf 1 : 1 ratio