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Genes & Inheritance Prepared by Ms W.S.Kwan What are chromosomes? • Chromosomes appear as very thin & thread-like structures called Chromatins • Chromosomes are made up of DNA & protein • Each cell in each type of organism has a definite number of chromosomes • Each cell has Diploid Number of chromosomes: 1 set of Paternal Chromosomes + 1 set of Maternal Chromosomes gene controlling skin colour • gene controlling tongue rolling • Gene A gene is a short length of DNA on a chromosome It is a unit controlling an inherited character • DNA controls the formation gene controlling of proteins, such as eye colour enzymes & hormones gene controlling blood group chromosome • Hence, genes control all the metabolic activities of the cells Homologous chromosome - Chromosomes always exist in pairs ‘Diploid’ in body cells (somatic cells] - Each human body cell has 46 chromosomes (i.e. 23 pairs of homologous chromosomes) allele for white skin colour allele for dark skin colour • Members of homologous chromosomes carry the same genes (controlling the same character) in same loci • But the genes on the members of homologous chromosomes may be in different forms (effects) Alleles • Alternative forms of genes on the same position of the homologous chromosomes which control the same character but have different expressions e.g. dark skin / white skin In sexual reproduction Gametes (sex cells) Male gamete Sperm Fertilization Female gamete) Egg Zygote Role of Gametes: transfer the genetic information from the parents to the offspring What would happen if gametes (sex cells)are formed by mitosis? Mitosis Fertilization Mitosis Male (2n) Female (2n) Sperm (2n) Egg (2n) Zygote (4n) Sperm/Egg (4n) Fertilization Zygote (8n) Meiosis Another type of nuclear division Meiosis Producing cells with chromosome number half of the parent cell Two nuclear divisions – Meiosis I and meiosis II Four haploid daughter cells are produced First Meiotic Division • Chromosomes become visible • Nuclear membrane disappears Homologous chromosomes pair up & duplicate • Homologous chromosomes pair up & duplicate • Crossing-over may occur between homologous chromosomes (not in mitosis) genetic materials exchanged Homologous chromosomes line up in the middle of cell randomly The 2 members of each homologous chromosome separate from each other and move towards the opposite poles of the cell Nuclear membrane reforms Followed by cytoplasmic cleavage Second Meiotic Division Equal separation of chromatids of each chromosome 4 daughter cells form Each daughter cell has a half number of chromosome as the parent cell Summary of meiosis: One diploid parent cell produces 4 haploid daughter cells Occurrence of meiosis Plants: anthers and ovules Mammals: testes and ovaries Formation of pollen grains by meiosis Comparison between Mitosis & Meiosis Mitosis Meiosis Number of division 1 2 No. of daughter cells produced 2 4 Somatic (body) cells Gametes (sex cells) Growing tissues Reproductive tissues (gonads) Type of cells produced Occurrence Comparison between Mitosis & Meiosis Mitosis Pairing of homologous chromosomes No Chromosome number of daughter cells Same as parent cell diploid [2n) Role For Growth, Replacement & asexual reproduction Meiosis Yes Half of parent cell haploid [n) For Gamete Formation Significance of meiosis Produce haploid gametes which combine to restore the diploid conditions after fertilization. Allow genetic variations in the offspring Sources of genetic variation • Random assortment of chromosomes in meiosis • Random fusion of gametes during fertilization • Cross fertilization • Crossing-over in meiosis ~END~