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Mendel and Heredity Chapter 8 Ms. Hogg, Biology
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The Origins of Genetics Heredity – the passing of characteristics from parent to offspring – Before DNA and chromosomes were discovered, heredity was one of the greatest mysteries of science!
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Gregor Mendel The scientific study of heredity began with Gregor Mendel – Austrian monk – Carried out experiments with garden peas – He was the 1 st to develop rules that accurately predict patterns of heredity. – “Father of Genetics”
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Pea Plant
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7 Characteristics Mendel Studied:
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Pollen Transfer in Mendel’s Experiments
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Mendel’s Hypotheses #1 For each inherited characteristic, an individual has two copies of the gene – one from each parent.
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Mendel’s Hypotheses #2 There are alternative versions of genes. – These different “versions” are called alleles. – An individual receives one allele from each parent.
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Mendel’s Hypotheses #3 When two different alleles occur together, one of them may be completely expressed, while the other may have no observable affect on the organisms appearance. – Dominant = expressed trait – Recessive = trait that is NOT expressed
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Mendel’s Hypotheses #4 When gametes are formed, the alleles for each gene in an individual separate independently of one another.
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Terms of Genetics Homozygous = two alleles of a gene are the SAME – Example: BB or bb Heterozygous = two alleles of a gene are DIFFERENT – Example: Bb
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Genotype = the set of alleles that an individual has for a trait – Example: BB Phenotype = the physical appearance of a characteristic – Example = Brown hair Terms of Genetics
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Punnett Square A punnett square is a diagram that predicts the outcome of a genetic cross Monohybrid cross:
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The Hardy-Weinberg Principle p 2 + 2pq + q 2 = 1 You can use this equation to predict genotype frequencies in a population.
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The Hardy-Weinberg Principle p 2 + 2pq + q 2 = 1 The sum of allele frequencies must always equal 1. p + q = 1
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Exceptions… The Hardy-Weinberg principle holds true for any population as long as evolutionary forces are not acting… – Mutations – Gene flow – Nonrandom mating – Genetic drift – Natural selection
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A Review… Monohybrid Cross
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A Review… Dihybrid Cross NOTE: Must use the “FOIL” method to correctly set up the parent genotype combinations on the Punnett Square
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Incomplete Dominance An organism can display a phenotype that is intermediate between the two parents Example: Snapdragon flowers RR = red Rr = pink rr = white
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Incomplete Dominance
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Codominance Two dominant alleles can be expressed at the same time Example: Human Blood groups (ABO blood groups) Type A = I A I A or I A i Type B = I B I B or I B i Type AB = I A I B (Universal Recipient) Type O = ii(Universal Donor)
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Codominance – ABO Blood Groups
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Rh Factor Source: NobelPrize.org
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Human Blood Type Frequency Source: RedCrossBlood.org
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Sex-linked Traits In sex-linked inheritance, the gene responsible for the disease is located on the X chromosome. Usually, the abnormal gene is recessive. – For these reasons, the resultant disorder is called an X-linked recessive disease
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Sex-linked Disorders Color blindness – A person with the most common form of color blindness cannot distinguish red from green hues. Hemophilia – In hemophilia, the blood does not clot properly.
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Color blindness Tests
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Hemophilia
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Pedigrees
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Polygenic Traits Polygenic traits are controlled by two or more genes, usually on different chromosomes. Examples of Polygenic Traits: – Height, skin color, weight, – Many common diseases, such as heart disease, are caused by polygenic genes and environmental factors (Lifestyle choices such as diet and exercise)
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Human skin color is controlled by at least eight different loci on different chromosomes...
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