Biology - B2.5 - Simple Inheritance in Animals and Plants

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B2.5.1 - Cell Division and Growth

  • Cell division - needed for growth or repairing damaged tissue
  • Mitosis - two identical cells produced from original cell
  • Chromosones - contain genes (alleles) passed on to new cell - copy is made before cell divides and one goes to each cell
  • Early stage of development - unspecialised cells - stem cells - most differentiate early and cell division is then only needed for growth
  • Plant cells can differentiate throughout the plants life
  • Asexual reproduction - mitosis from parent cell - same alleles as parent
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B2.5.2 - Cell Division in Sexual Reproduction

  • Cells in reproductive organs - divide by meiosis forming sex cells (gametes) - humans: sperm and ova
  • Gamete has only one chromosone from original pair - all cells are different from each other and parents
  • Sexual reproduction = variation as gametes from each parent fuse - half genetic information comes from each parent - new individual develops by cell repeatedly dividing by mitosis
  • Before division - copy of each chromosone made
  • Cell then divides forming 4 gametes
  • Each gamete has single set of chromosoneswith different combination of genes
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B2.5.3 - Stem Cells

  • Stem cells - unspecialised and found in the human embryo and bone marrow - stem cells in adult bone marrow can change into other cell types
  • Differentiate into all different types of body cells
  • Layers of cells in the embryo differentiate into cells the body needs
  • Hoped that human stem cells can be made to differentiate into many types of cell then used to treat conditions eg. paralysis
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B2.5.4 - From Mendal to DNA

  • Gregor Mendal - monk - worked out how characteristics were inherited - first person to suggest idea of separately inherited factors - now called genes
  • Took long time to accept Mendal's idea - didn't know about chromosones and genes until after he died
  • Genes - found on chromosones - chromosones made of DNA - long molecule with double helix structure - genes are short sections of DNA
  • Everyone, but identical twins, have different DNA - unique pattern can be identified by fingerprints
  • Genetic code - each gene codes for a particular combination (order) of amino acids which make a specific protein
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B2.5.5 - Inheritance In Action

  • Humans - 23 pairs of chromosones - one pair = sex chromosones
  • Females - **, Males - XY
  • Genes controlling same charateristics = alleles - if alleles masks effect of another, it's dominant, allele where effect is masked is recessive
  • Genetic diagrams show how alleles and characteristics are inherited
  • Genetic diagrams - biological models constructed to predict and explain inheritance of charateristics - punnet squares used to insert alleles symbols
  • Phenotype - physical appearanceof characteristic
  • Genotype - genetic makeup - which allele does individual inherit?
  • Homozygous - both alleles are the same - DD (homozygous dominant), dd (homozygous recessive)
  • Heterozygous - two alleles are different - Dd
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B2.5.6 - Inherited Conditions In Humans

  • Some genetic disorders are caused by dominant alleles, some recessive
  • Dominant - only has to inherit one dominant allele to have disorder - eg. polydactyly - extra fingers or toes - passed on by one parent with allele
  • Recessive - must inherit two recessive alleles to have disorder - eg. cystic fibrosis - affects cell membranes and causes thick sticky mucus which affects organs - must inherit recessive allele from both parents - can be passed on from parents who don't have it but are carriers
  • Genetic diagrams - see how disorder or allele was inheritted and predict if future offspring will inherit it
  • Parent is heterozygous for polydactyly - 50% of child inheriting it
  • Both parents heterozygous for cystic fibrosis - 25% of child inheriting it
  • Outcomes of genetic crosses shown on Punnet square
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B2.5.7 - Stem Cells and Embryos - Science and Ethi

  • Adult stem cells useful in treating disorders eg. lukaemia
  • Doctors investigated use of embryonic stem cells having potential to differentiate into wide variety of cells
  • Embryonic stem cells - taken from spare embryos from IVF, created from adult cells or taken from umbilical cord of new babies - used to grow new tissues and organs for transplant
  • People concerned about use of embryos - research is experimental and embryos have potential to be babies and are destroyed, embryos can't give permission and research is expensive
  • Embryo screening - tests to diagnose disorders before baby is born - results give parents choices about keeping the baby - some decide it's unethical to destroy embryo and keep affected baby
  • IVF embryos are screened - only healthy embryos implanted - unethical
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