chapter 9 notes
TRANSCRIPT
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Fundamentals of Genetics
Chapter 9
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Mendel’s Legacy
• Genetics is the study of how characteristics are transmitted from parents to offspring– Founded with the work of Gregor Mendel, an
Austrian monk
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Gregor Mendel
• Mendel conducted experiments at the monastery with pea plants
• He researched heredity – the transmission of characteristics from parents to offspring
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Mendel’s Garden Peas
• Mendel observed seven characteristics of pea plants
• Each characteristic occurred in two contrasting traits – genetically determined variants of a characteristic
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Mendel’s Garden Peas
Characteristic• Position of flowers along
stem
• Height of plant
Trait
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Mendel’s Garden Peas
Characteristic• Pod Appearance
• Pod color
Trait
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Mendel’s Garden Peas
Characteristic• Seed Texture
• Seed color
• Flower color
Trait
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Mendel’s Methods
• Mendel controlled pollination– Self-pollination – pollen fertilizes egg cells on the
same flower– Cross-pollination – pollen fertilizes egg cells on
flowers of different plants
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Mendel’s Experiments
• P generation – parent generation• F1 (first filial) generation – offspring of the P
generation• F2 (second filial) generation – offspring of the
F1 generation
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Mendel’s Experiments
• First he grew true-breeding plants– A plant that is true-
breeding, or pure, for a trait always produces offspring with that trait when they self-pollinate
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Mendel’s Experiments
• Next, he cross-pollinated true-breeding parents of the P generation
• What do you think the F1 generation looked like?
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Mendel’s Experiments
• The F1 generation was all purple flowers!
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Mendel’s Experiments
• Next, he let the flowers from the F1 generation self-pollinate
• What do you think the F2 generation looked like?
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Mendel’s Experiments
• The F2 generation came out with 75% purple flowers and 25% white flowers!
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Mendel’s Results and Conclusions
• Mendel’s observations led him to hypothesize that something within the pea plants controlled the characteristics observed– He called these controls factors
• Because the characteristics had two alternative forms, he reasoned that a pair of factors must control each trait
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Recessive and Dominant Traits
• Dominant – trait that masks the factor for the other trait
• Recessive – trait that is masked by the factor for the other trait
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The Law of Segregation
• The law of segregation states that a pair of factors is segregated, or separated, during the formation of gametes
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The Law of Independent Assortment
• The law of independent assortment states that factors separate independently of one another during the formation of gametes
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Support for Mendel’s Conclusions
• A gene is a segment of a chromosome that controls a particular hereditary trait
• The different forms of a gene are called alleles– Capital letters are used to represent dominant
alleles– Lowercased letters are used to represent recessive
alleles
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GENETIC CROSSESSection 2
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Genotype and Phenotype
• Genotype – an organism’s genetic makeup (PP, Pp, pp)
• Phenotype – an organism’s physical appearance (purple flower, white flower)
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Genotype and Phenotype
• Homozygous – both alleles of a pair are alike (PP = homozygous dominant and pp = homozygous recessive)
• Heterozygous – two alleles are different (Pp)
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Probability
• Probability – the likelihood that a specific event will occur
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Predicting Results of Monohybrid Crosses
• In a monohybrid cross, one characteristic is tracked
• Biologists use a Punnett square to predict the probable distribution of inherited traits in the offspring
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Ex 1: Homozygous X Homozygous
• TT x tt (T = tall, t = short)
• Possible Genotypes– 100% Tt
• Possible Phenotypes– 100% tall
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Ex 2: Homozygous X Heterozygous
• BB X Bb (B = black hair, b = brown hair)
• Possible Genotypes– 50% BB, 50% Bb
• Possible Phenotypes– 100% black
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Ex 3: Heterozygous X Heterozygous
• Bb X Bb(B = black hair, b = brown hair)
• Possible genotypes– 25% BB, 50%Bb, 25% bb
• Possible phenotypes– 75% black hair, 25%
brown hair
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Ex 4: Testcross
• How could you determine whether a black guinea pig is homozygous (BB) or heterozygous (Bb)?
• By completing a testcross – an individual with an unknown genotype is crossed with a homozygous recessive individual
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Ex 5: Incomplete Dominance
• Incomplete dominance occurs when the phenotype of heterozygous individual is in between the two homozygous phenotypes
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Ex 5: Incomplete Dominance
• RW X RW (R = red, W = white)
• Possible genotypes and phenotypes– 25% RR, red– 50% RW, pink– 25% WW, white
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Ex 6: Codominance
• In codominance, both alleles contribute to the phenotype
• In some varieties of chicken, the allele for black feathers is codominant with the allele for white feathers– Heterozygous chickens are speckled with black
and white feathers
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Ex 6: Codominance
• FWFW X FBFB (F = feathers, B = black, W = white
• Possible genotypes and phenotypes– 100% FWFB, speckled
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Predicting Results of Dihybrid Crosses
• A dihybrid cross is a cross in which two characteristics are tracked
• More complicated than monohybrid crosses because more combinations of alleles are possible
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Dihybrid Homozygous X Homozygous
• rryy X RRYY (R= round, r= wrinkled, Y= yellow, y= green)
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Dihybrid Heterozygous X Heterozygous
• RrYy X RrYy