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

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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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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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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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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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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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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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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