Mendelian & Non-Mendelian Genetics - Mendel and the...

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Mendel and the Garden Pea The tendency for traits to be passed from parent to offspring is called heredity Gregor Mendel (1822-1884) The first person to systematically study heredity Austrian monk who studied science and mathematics Worked with garden peas in his monastery
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Mendel chose the garden pea for several reasons Mendel’s Experimental System 1. Many distinctive varieties were available 2. Small and easy to grow 3. Short generation time and lots of offspring 4. Both male and female reproductive organs are enclosed within the pea flower
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Mendel selected seven characteristics to study, each of which had two distinguishable traits He let each variety self-fertilize for many generations to ensure it was true-breeding He crossed individuals from two different varieties that differ in only one trait P (parental) generation = Pure bred lines Mendel’s Experimental Design
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Mendel’s Experimental Design How Mendel conducted his experiments
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What Mendel Observed For all seven pairs of contrasting traits studied, Mendel observed the same results 1. The F 1 generation showed only one of the two parental traits He called it the dominant trait The recessive trait was not expressed 2. The F 2 generation showed an ~ 3:1 ratio of the dominant:recessive parental traits
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Mendel let the F 2 plants self-fertilize for another generation He concluded from the results that the 3:1 ratio is a disguised 1:2:1 ratio
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Mendel Proposes a Theory Mendel proposed a simple set of hypotheses 1. Parents do not transmit traits directly to their offspring They do so via factors (now termed genes ) 2. Each parent contains two copies of the factor governing each trait If the two copies are the same, the individual is called homozygous If the two copies are different, the individual is called heterozygous
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Mendel Proposes a Theory Mendel proposed a simple set of hypotheses 3. Alternative forms of a factor lead to different traits Alternative forms are called alleles The appearance of an individual is its phenotype The genetic composition of an individual is its genotype 4. The two alleles that an individual possesses do not affect each other 5. The presence of an allele does not ensure that its trait will be expressed in the individual
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Each trait is determined by the inheritance of two alleles: one maternal and one paternal These alleles, present on chromosomes, are distributed to gametes during meiosis Analyzing Mendel’s Results Genes on homologous chromosomes
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Analyzing Mendel’s Results Consider Mendel’s cross of purple-flowered with white-flowered pea plants P (dominant) allele Purple flowers p (recessive) allele White flowers Using these conventions, the above cross can be symbolized as PP X pp
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A Punnett square is a grid structure that enables the calculation of the results of simple genetic crosses Punnett Squares Gametes p P P p Gametes
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