Biology Revision. Essay

3767 words - 15 pages

*14.1 meiosis, Medelian inheritance*14.2 variation*14.3 population genetics*14.4 classification*14.5 ecosystems*14.6 photosynthesis*14.7 food chains and webs*14.8 respiration*14.9 carbon and nitrogen cycles*14.10 deforestationMedelian ExperimentHe noticed pea plants were either full sized (tall) or very short (dwarf)He used self fertilisation to find PURE BREEDING PLANTSThen produced offspring by FERTILISATIONAll plants grew tallSo he took the offspring and crossed them with each other.The offspring were ¾ Tall and ¼ DwarfExplanationEach plant has 2 genesGenes give instructions to organisms-'grow tall' 'grow short'If a plant has both type of gene the GROW TALL gene is DOMINANT the GROW SHORT is RECESSIVETo show crosses we use symbols P-parents T-grow tall t-grow shortGAMETES: T & T x t & tF1: Tt all offspring are TALL2nd cross Tt x TtGAMETES: T t x T tF2: TT, Tt, Tt, tt ¼ will be DWARFPunnett squares/diagrams1.Mendel's experiment (second part) using PUNNETT SQUARES2.Showing CODOMINANCE-Feather colour in chickens is controlled by CODOMINANT ALLELES C -allele for white feather C -allele for black3.Showing MULTIPLE ALLELES-Blood group is controlled by a gene which has 3 alleles- I & I are co dominant and I is recessive4.SEX LINKAGE- X chromosomes are LARGE, y chromosomes are SMALL-Male humans XY Women XX5.DIHYBRID INHERITANCE-All previous examples dealt with the inheritance of a single characteristic e.g. MONOHYBRID INHERITANCE-With DIHYBRID INHERITANCE we are looking at the inheritance of a pair of characteristics.MeiosisLegend:Illustration of the process by which a single parent diploid cell (Both homologous chromosomes) divides to produce four daughter haploids cells (One homologous chromosome of the pair).Meiosis is the type of cell division by which germ cells (eggs and sperm) are produced. Meiosis involves a reduction in the amount of genetic material.Meiosis comprises two successive nuclear divisions with only one round of DNA replication.Four stages can be described for each nuclear division.*First division of meiosisoProphase 1: Each chromosome duplicates and remains closely associated. These are called sister chromatids. Crossing-over can occur during the latter part of this stage.oMetaphase 1: Homologous chromosomes align at the equatorial plate.oAnaphase 1: Homologous pairs separate with sister chromatids remaining together.oTelophase 1: Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair.*Second division of meiosis: Gamete formationoProphase 2: DNA does not replicate.oMetaphase 2: Chromosomes align at the equatorial plate.oAnaphase 2: Centromeres divide and sister chromatids migrate separately to each pole.oTelophase 2: Cell division is complete. Four haploid daughter cells are obtained.One parent cell produces four daughter cells. Daughter cells have half the number of chromosomes found in the original parent cell and with crossing over, are genetically...

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