Core Biology - Topic 1 Inheritance

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  • Created by: Katie
  • Created on: 14-04-13 22:08


  • Variration caused by genes is inherited variation
  • 3 main parts of most cells - cell membrane, cytoplasm, nucleus
  • Inside the nucleus - long strands of substance called DNA
  • Each strand forms a structure called a chromosome (with protein)
  • Chromosomes are divided into genes
  • Each chromosone carries a large number of genes that do particular jobs
  • Many genes control our variations e.g. eye colour, hair colour, face shape
  • Alleles - Different forms of the same gene
    • may contain slightly different instructions
    • these create the variations like eye colour
  • 2 copies of every chromosome in the body's cell nuclei
    • 2 copies of every gene
    • each copy could be a different allele
  • Different organisms have different numbers of chromosomes
    • humans have 23 pairs containing roughly 23,000 different genes in total
  • Many alleles for each gene so we all inherit slightly different characteristics
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Explaining Inheritance

  • All plants and animals produce gametes (sex cells)
  • Male gametes - sperm cells (animals) pollen grains (plants)
  • Female gametes - egg cells (animals and plants)
  • Gamates different to most body cells
    • only have one copy of each chromosome
    • therefore gametes only have one allee per gene
  • 2 gamates fuse together in sexual preproduction
    • the new organism contains two alleles for each gene (one each from male & female)
  • Each allele from the parents is either dominant or recessive
  • If a dominant is present, the recessive will have no effect
    • so a recessive characteristic is only seen if both alleles are recessive
  • Dominant alleles are shown with a capital letter - recessive same letter but lowercase
  • Genotype - the alleles in an organism
  • Phenotype - what the organism looks like
  • Homozygous - both alleles for gene are the same
  • Heterozygous - alleles are different
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Punnett Squares

  • Possible genotypes are shown in a Punnett square
  • To work out the probability of each phenotype occuring if 2 organisms breed
  • 3/4 chance of A characterisic - 0.75 / 75% / 3:1 probabilty


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Genetic Disorders

  • Genetic disorder - a disease caused by faulty alleles
    • faulty allele is recessive - suffers must have 2 copies
  • Sickle cell disease - sickle shaped red blood cells
    • suffers become tired easily and short of breath
    • painful joins when red blood cells stick together and block blood vessels
    • blocked blood vessels can sometimes be fatal
  • Cystic fibrosis - mutation with the gene that creates sweat, mucus and digestive juices
    • lungs get clogged with thick sticky mucus so difficulty breathing
    • results in debris entering lungs causing infections
    • mucus also blocks some tubes that carry enzymes to the small intestines
    • this lack on enzymes results in weight loss
  • Doctors use pedigree charts (key family tree) for:
    • understanding how genetic disorders are inherited
    • working out the probability of inheritance
  • Genetic screening can be done for couple's who know a genetic disorder is likely
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