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Chapter 5 Lesson 1 Mendel and His Peas
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Gregor Mendel Young priest that worked in the garden at a monastery in Vienna. Considered the “Father of Genetics” Observed traits in hundreds of pea plants.
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Mendel’s Work - Vocabulary
Traits Physical characteristics of organisims Heredity Passing of traits from parent to offspring Genetics Scientific study of heredity
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Mendel’s Peas Why was Gregor Mendel lucky he chose peas to study?
Many traits exist only in two forms Peas produce many offspring in one generation.
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How did Mendel make his crosses?
White Plant Purple Plant
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True Breeding (Purebred) Plant
Mendel’s Experiments True Breeding (Purebred) Plant One that always produces offspring with the same form of a trait as the parent i.e. purebred short plants will only produce short offspring.
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Purebred purple plants will only produce purple plants
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White flower would mean the parents are not purebred
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Mendel’s Experiments Mendel crossed plants with opposite forms of a trait i.e. purple flowers crossed with white flowers Look at Figure 3 on page 152 Explain why all offspring are purple in the first generation (F1) What happens in the second generation (F2)
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Mendel’s Experiments Mendel studied flower color followed by six other “opposite” traits. Look at table 1 on page 154. What do you notice about the ratio of traits in the second generation (F2)
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Let’s Look at another example
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Genes and Alleles Genes Alleles Factors that control traits
Different forms of a gene One allele is inherited from each parent. Alleles can be dominant or recessive
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Dominant and Recessive Alleles
Dominant allele Always seen if present Indicated by a capitol letter (T) Recessive allele Masked or hidden if dominant allele is present Indicated by a lower case letter (t)
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Hybrid vs Purebred Hybrid Purebred
Organism has two different alleles for the trait – one dominant and one recessive. Purebred Organism has two identical alleles for a trait – two recessive OR two dominant.
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Alleles are represented with capitol and lower case letters.
T = Tall allele (dominant) t = Short allele (recessive) Purebreds TT – two dominant alleles – plants will be tall tt – two recessive alleles – plants will be short Hybrid Tt – one dominat allele and one recessive allele – plants will be tall.
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Phenotypes and Genotypes
Physical appearance or visible trait Example is flower color, stem height etc. Genotype The genetic makeup or allele combination Examples are TT or Tt Genotypes determine phenotypes
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Homozygous and Heterozygous
An organism that has two identical alleles for a trait TT – homozygous dominant tt – homozygous recessive Heterozygous An organism that has two different alleles for a trait Tt – heterozygous – will show the dominant trait
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Probability Probability Example: coin toss
The likelihood that a particular event will occur Example: coin toss
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Punnett Squares Punnett Squares
chart that shows all possible combinations of alleles that can result from a genetic cross. Used to determine the probability of a particular outcome.
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Using a punnett square Tt t T T Tt t TT Tt Tt tt Tall Tall Tall Short
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Codominance Codominance – alleles are neither dominant or recessive therefore both alleles are expressed in the offspring
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Codominance The heterozygous offspring will express (show) both phenotypes
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What is the rabbit’s phenotype for fur color? _________
Consider a rabbit with black fur and the allele combination (Bb). What is the rabbit’s phenotype for fur color? _________ What is the rabbit’s genotype for fur color? __________ Is the rabbit heterozygous or homozygous for the fur color trait? _______________ If black is dominant over white, what genotype would produce a white rabbit? _______ If fur color in rabbits was a codominant trait instead of dominant/recessive trait, what color fur would the (Bb) rabbit have? ________________ Black (Bb) heterozygous (bb) White and black 23
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