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  • Inheritance in animals and plants
    • Cell division and growth
      • cell division is necessary for the growth or repair of damaged cells/tissues
      • Mitosis results in two identical cells being produced from the original cell
      • the chromosomes contain genes (alleles) which must be passed on to each new cell
        • a copy of each chromosome is made before the cell divides and one of each chromosome goes to each new cell
      • in early development of animal and plant embryos the cells are unspecialised (stem cells)
        • most animal cells differentiate and cell division is mainly for repair and replacement
      • plant cells can differentiate throughout the life of the plant as it grows
      • cells of offspring produced by asexual reproduction are produced by mitosis from the parent cell. they contain the same alleles as the parent
    • cell division is sexual reproduction
      • cells in reproductive organs divide by meiosis to form gametes (sperm and ova)
      • each gamete has one chromosome from each original pair, all cells are different from each other and the parent cell
      • results in variation as the gametes from each parent cell fuse. so half of the genetic info is from the father and half from the mother
      • at fertilisation, a single body cell with new pairs of chromosomes is formed
        • a new individual then develops by this cell repeatedly dividing by mitosis
    • Stem cells
      • unspecialised
      • are found in the human embryo and in adult bone marrow
      • change into all the different types of blood cell (nerve cell, muscle cell etc)
      • layers of cells in the embryo differentiate into all the cells the body needs
      • stem cells can be used to cure some disorders
    • DNA
      • Mendel was the first person to suggest the idea of separately inherited 'factors'
        • Mendel's 'factors' are now called genes
      • chromosomes are made of DNA which is a very long molecule with a double helix structure
        • Genes are short sections of DNA
          • Every individual, except identical twins, has different DNA
    • Inheritance
      • humans have 23 pairs of chromosomes, one pair are the sex chromosomes
        • females have two X chromosomes (XX) and males have an X and a Y chromosome (XY)
      • Genes controlling the same characteristics are called alleles
        • if an allele 'masks' the effect of another it is 'dominant'. the allele where the effect is 'masked' is 'recessive
          • some genetic disorders are caused by a dominant allele and some by recessive allele
            • the person only has to inherit one dominant allele to have the genetic disorder (polydactyly etc)
            • the person has to inherit two recessive alleles to have the disorder (cystic fibrosis etc)
    • science and ethics of stem cells
      • Embryonic stem cells can be used in medical research and treatments
      • Embryos can be screened for genetic disorders (embryo screening)
      • there are economic, social and ethical issues concerning the use of embryos and in embryo screening


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