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Variation and Inheritance
National 5 Biology Key Area 5: Variation and Inheritance National 4 Biology Key Area 4 Genetic Information
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By the end of the lesson you will be able to …..
Carry out monohybrid crosses from P to F1 and F1 to F2 Define the terms: Allele Dominant Recessive
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Gregor Mendel Born in 1822 Augustinian monk and natural science teacher “Father of Genetics” His experiments on inheritance in pea plants are the basis of all modern day genetics.
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Gregor Mendel Originally studied inheritance in bees but he bred a strain of hybrid honeybees so vicious the abbot ordered them to be destroyed. True story allegedly. His abbot ordered them destroyed an suggested a less dangerous organims and Mendel switched to pea plants
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What did he actually do? He used pea plants because they have many traits that exist in only two forms. E.g. tall or short plants. Always crossed (or bred) plants with opposite forms of a trait, for example, tall plants and short plants.
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What did he actually do? Mendel didn’t know it but ...
He always started his experiment with ‘true-breeding’ parents He noticed that some traits were dominant over others. e.g. purple flowers showed up more often than white He concluded there were two types of ‘trait’: dominant and recessive Mendel didn’t know it but ... he was studying how different types of genes (alleles) were passed from one generation to the next
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Alleles Genes come in pairs (one from mum, one from dad).
Different forms of a gene are called alleles
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e.g. Brown eyes are dominant over blue eyes
Alleles Not all alleles are created equal…. e.g. Brown eyes are dominant over blue eyes Dominant alleles always get their way like neilson Recessive alleles only ‘stand up’ for themselves when there is two of them. Dominant alleles are always expressed in an organisms phenotype Recessive alleles are only expressed in an organisms phenotype if the organism has two of them.
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Representing Alleles Trait Phenotypes DOMINANT recessive
Alphabet letters Dominant alleles are CAPITAL letters Recessive alleles are lower case letters Usually first letter of DOMINANT trait Genotypes are simply represented by the two letters of the alleles Trait Phenotypes DOMINANT recessive B Eye Colour BROWN/green b Seed colour (Peas) Y y YELLOW/green
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Alleles Genes come in pairs (one from mum, one from dad).
Different forms of a gene are called alleles Letters are used to represent the allele’s involved in a genetic cross. An upper case letter = DOMINANT A lower case letter = recessive
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Monohybrid Cross – seed colour
Parental Generation “P” X Phenotype yellow green yy Genotype YY Genotype of Gametes all Y all y Draw punnet square on board all Yy F1 (1st generation) Heterozygous
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X Yy Yy Y and y Y and y Phenotype yellow yellow Genotype Gametes
F2 (2nd generation)… In order to work out the second generations possible genotypes we need to use a punnet square again.
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Y y YY Yy yy F2 Genotype’s YY Yy Yy yy Phenotype’s
F2 (2nd generation)… Y y YY Yy yy F2 Genotype’s YY Yy Yy yy Draw punnet square on board yellow, yellow, yellow, green Phenotype’s How would you express this as a ratio?
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