Genetic Inheritance GCSE Biology

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  • 1. Basic geneticInheritanceGCSE BiologyOCROxford Cambridge Prospective StudentsBy yasmeen malik
  • 2. Chromosomes
  • 3. Chromosomes• In Nucleus• Coiled DNA• Humans have 46 chromosomes (23 pairs) in everybody cell• 20,000 genes (40,000 alleles)• Parents contribute a chromosome each to make apair• Gamete cells have 23 chromosomes• Basis of inheritance
  • 4. DNA• Molecule of DNA made from 2 strands ofnucleotides• Each nucleotide contains a nitrogenous base• A-adenine, T-thymin, C-cytosine, G-guanine• DNA can replicate itself exactly• Inheritance
  • 5. Gene mutation• Mutation is a change in the DNA of a cell• Sometimes during replication mistakes aremade = results in gene mutation• This alters the sequence of bases in a gene =gene codes for wrong protein
  • 6. Genes & Alleles• Pair of chromosomes• Gene is a section of moleculeof DNA that codes for aspecific protein• Each protein contributestowards body feature such aseye colour, skin pigmentation,blood group• 2 alleles make up a gene
  • 7. ChromosomesXY23XX23Fertilisation46ZygotecellSex chromosomesHaploid cell- halfno.of chromosomesZygote cellDiploid cell – fullset of chromosomes
  • 8. Back to genes & alleles• Gene for ear lobe attachment– Attached– Unattached• Gene for eye colour– Brown– Green• Dominant alleles• Recessive allelesAllelesAlleles
  • 9. Back to chromosome mutations• When cells divide sometimes they do notdivide properly = chromosome mutations• Result = cell death or, sometimes sex cellscontinue to divide• More or fewer chromosomes than normal• Downs syndrome
  • 10. Cell division - mitosis• Parent cell divides into daughter cells• Genetic identical cells• Chromosomes identical• Exact duplication• Body cells– Skin, gut....• Repair of cells, replacement of worn out cells• Asexual reproduction
  • 11. MitosisEg. 4 chromosomes(2 homologous pairs)Each chromosome copies itself2 new daughter cellsCopy of each chromosome
  • 12. MeiosisEg. 4 chromosome (2 pair ofhomologous pair)1st cellular division by meiosis2nd division by meiosis4 sex cells (gametes)With half no. Chromosomes1 chromosome each from eachpair of chromosomeMeiosis has 2 celldivisionsProduces 4 haploid cellswith half the number ofchromosomes from eachparent (23)Sexual reproduction
  • 13. Sexual reproduction and variation• Involves fusion of 2 sex cells to form zygote• Genetic variation in offspring• Identical twins form from same zygote• The zygote separates and each cell behaves as an individualzygote• Non-Id twins = fraternal twins – dev from different zygotes soare not identical
  • 14. Asexual reproduction• No fusion of sex cells• As part of the organism grows and breaksaway from parent organism• Mitosis cell division• Same genetic material• Plants• Humans – cells for repair, new growth,replacement
  • 15. Genes and environment• Environment factors can influence your genes• Diet, nutrition• Not grow to full potential as programmed ingenes• For example height– poor diet and lack of adequate calcium andvitamin D in the diet can lead to bones notgrowing to their full potential height
  • 16. Terms related to inheritance• Genotype– Genetic makeup of org. interms of alleles– Tt, TT, tt• Phenotype– Observable characteristic– Colour of eyes, tall orshort, blood group• Homozygous– Having two identical alleles– TT or tt• Heterozygous– Having two different alleles– Tt• Dominant– Allele expressed inindividual, Tt = T• Recessive– Allele only expressed ifthere are 2 recessivealleles, tt = t
  • 17. Monohybrid crossIs a single trait with two alleles A = dominant normala = recessive albinoGenotype parents: Aa x AaGametes: A a x A aMitosis:MonohybridCross:Genotype: AA, Aa, Aa, aaPhenotype: A , A, A, aPhenotype ratio: 3 : 1 (normal: albino)Genotype ratio: 1: 2: 1(normal : heterozygous for albino : albinoA aAaAA AaAa aa
  • 18. Pea plantsPhenotype of parents: tall x shortPure breed genotypes: TT x ttGametes : (T) (T) x (t) (t)Monohybrid cross: T Tt Tt Ttt Tt TtF1 genotype offspring: Tt Tt Tt TtHeterozygousF1 phenotype : 100% tall
  • 19. Pea plants F2Phenotype of parents: tall x tallHeterozygous genotypes: Tt x TtGametes : (T) (t) x (T) (t)Monohybrid cross: T tT TT Ttt Tt ttF2 genotype offspring: TT Tt Tt ttF2 phenotype : tall, tall, tall, shortRatio genotype: 1:2:1 (TT:Tt:tt)Ratio phenotype: 3:1 (tall:short)Homozygousrecessive
  • 20. genotype DD Dd ddHomozygousdominant2 dominantalellesHeterozygous1 dominant allele1 recessive alleleHomozygousRecessiveBoth alleles arerecessivePhenotype Polydactyl Polydactyl NormalD = polydactyl allele = dominantd = normal allele
  • 21. Inheritance of polydactylPhenotype of parents: poly x polyHeterozygous genotypes: Dd x DdGametes : (D) (d) x (D) (d)Monohybrid cross: D dD DD Ddd Dd ddgenotype offspring: DD Dd Dd ddphenotype :Ratio genotype: 1:2:1 (DD:Dd:dd)Ratio phenotype: 3:1 (poly:normal)Homozygousrecessive
  • 22. Co-dominance• So far, all genetic crosses involve completedominance, where one dominant allelecompletely masks the effect of a secondrecessive allele• There are many alleles that both contribute tothe phenotype• If two are expressed in the same phenotypethey are called co-dominant
  • 23. Co-dominanceexample of snapdragon flowerPhenotype of parents: red x whitePure breed genotypes: RR x WWGametes : (R) (R) x (W) (W)Monohybrid cross: R RW RW RWW RW RWF1 genotype offspring: RW, RW, RW, RW,HeterozygousF1 phenotype : 100% all pink flowers
  • 24. Co-dominanceexample of snapdragon flower...Phenotype of parents: pink x pinkgenotypes: RW x RWGametes : (R) (W) x (R) (W)Monohybrid cross: R WR RR RWW RW WWF2 genotype offspring: RR, RW, RW, WWF2 phenotype ration: 1: 2: 1red: pink : white
  • 25. Sex determinationPhenotype of parents: male x femalegenotypes: XY x XXGametes : (X) (Y) x (X) (X)Monohybrid cross: X YX XX XYX XX XYPhenotypes: 50% femaleRatio 50% male
  • 26. Question 11. Predict the genotype and phenotype ratios ofoffspring of pea plants from the following alleles ofthe parents genotype: T is dominant = Talla) TT x TTb) TT x Tt
  • 27. Question 22. In cattle, a pair of alleles control coat colour.Allele for black coat colour is dominant overthe red coat colour. Draw a genetic cross torepresent a cross between a pure breed blackbull and a pure breed red cow. ‘B’ allele isdominant for black, and ‘b’ allele for redcolour is recessivea) What is the genotype of the offspringb) What is the phenotype of offspring
  • 28. Question 33. Cows with the same genotype as offspring(Bb) were bred with bulls with samegenotype. Predict the genotype andphenotype ratios in their offspring.(B is dominant allele for black colour coat)
  • 29. Blood groups• Possible genotypes: IA IO , IB IO , IA IB , IO IO• Phenotypes: A B AB O• Sometimes a gene has more than two alleles =multiple alleles• The inheritance of human blood is an example• Gene ‘I’ = Immunoglobin• Allele IA = produces antigen A• Allele IB = produces antigen B• Allele Io = does not produce antigen A or B
  • 30. Inheritance of blood groupsPhenotype of parents: group AB group OGenotypes: IA IB x IO IOMonohybrid cross: IA IBIO IA IO, IB IOIO IA IO, IB IOgenotype offspring: AO : BO (1:1) (50 % each)phenotype : A B
  • 31. Inheritance of blood groupsPhenotype of parents: group A group BGenotypes: IA IO x IB IOMonohybrid cross: IA IOIB IA IB, IB IOIO IA IO, IO IOgenotype offspring: AB, B, A, Ophenotype : 25% chance of either
  • 32. Variation and selection• Natural selection– Mechanism by which new species arise– Allows different forms of species to survive indifferent areas– If environment changes and species can’t adapt tosurvive it may become extinct– Survival of the fittest = Darwin referred tobiological fitness. How well adapted an organismis to its environment
  • 33. Evidence for natural selection• The theory of natural selection proposes thatsome factor in the environment ‘selects’which forms of a species will survive toreproduce under those conditions• Forms that are not well adapted don’t survive• Therefore there is evidence of variation withinspecies
  • 34. Selective breeding• Breeding of only those individual withdesirable features• Artificial selection• Farmers breed stock animals and crops• They select animals that produce more milk,meat, eggs, resist disease, better qualityproduce• Modern selective breeding– Cloning plants and animals
  • 35. Variation• Is display of different characters in same species• Genetic variation = All animals of same specieswill look slightly different bec genes are different• Environmental variation = scars, nutrition,• Plants the environment factor plays a muchbigger role• Plants are affected by temp, sun, soil,• Plant in good quality soil is stronger than poorquality soil
  • 36. Reproduction in plants• Plants can reproduce sexually and asexually• The sexual organism of flowering plant are itsflowers which produce pollen and ovulescontaining flower gametes
  • 37. Asexual reproduction in Plants• Involves plants growing• New cells produced by mitosis• Genetically identical• Plant grows – breaks away – grows elsewhere• Gardeners can grow new plants from cuttings
  • 38. Sexual reproduction in Plants• Plants produce specialised haploid sex cells in their flowers• meiosis• Male sex cells are the pollen grains• Female sex cells are the ova• Pollination = fertilisation takes place• Self-pollination – pollen transported from anthers of aflower to stigma in same flower• Cross-pollination – pollen transported to different flower• Pollination by Insects or wind• Zygote develops into seed
  • 39. Pollination• Pollination leads to fertilisation1. Zygotes develop into embryonic plant withsmall root and shoot2. Ovule develops into cotyledons which will bethe food store3. Ovule wall becomes seed or testa4. Ovary wall becomes fruit coat
  • 40. Exam question papers• OCR paper 22– Question 8• OCR paper 32– Question 6• OCR specimen paper B731– Questions 4,12,13 & 14