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HEREDITY

HEREDITY. Punnet Squares and Mendelian Genetics. OVERVIEW. Gregor Mendel used plants with True or Pure breeding TRAITS or ALLELES DOMINANT traits are always expressed. RECESSIVE traits must be present in two copies to be expressed.

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HEREDITY

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  1. HEREDITY Punnet Squares and Mendelian Genetics

  2. OVERVIEW • Gregor Mendel used plants with True or Pure breeding TRAITS or ALLELES • DOMINANT traits are always expressed. • RECESSIVE traits must be present in two copies to be expressed. • CAPITAL letters denote Genotype for Dominant alleles • Lower case letters denote recessive alleles.

  3. EXAMPLE EYE COLOUR • BROWN (B) eyes are dominant to • Blue (b) eyes in humans • Parents pass down only one ALLELE, B or b • A homozygous brown-eyed parent (BB) can pass down only the “B” allele. • A homozygous-blue (bb) eyed parent passes down only the “b” • Brown and blue are the PHENOTYPES

  4. SET UP THE PUNNET SQUARE • Make the Punnet Square Table • Identify the GENOTYPES OF THE parents: BB and bb • Identify the Gametes (one allele only) at each side of the table • Make the crosses. • All offspring have the Bb GENOTYPE and the Brown PHENOTYPE

  5. BB x bbOne Allele comes from each parent

  6. COMPLETE THE CROSSAll offspring have Bb genotype

  7. TEST CROSS • A homozygous recessive can be crossed with an unknown genotype. • The genotype of the unknown can be discovered by observing the offspring. • Eg. T = tall plant, t = short plant • A short plant (tt) is crossed with a tall plant of unknown genotype (Tt or tt).

  8. IF UNKNOWN IS TtHalf will be short

  9. IF Unknown is TT, all will be tall

  10. WHY? • Tall is dominant • Short plants can only result if the Tall parent contributed a “t” allele. • In the first result, all were Tall because each offspring received a Tall (T) allele from the unknown parent. • In the second case, the unknown parent contributed one short (t) allele.

  11. ASSIGNMENT • Complete Punnet Square Worksheets. • Submit for assessment. • See the rubric for a guide to assessment. • NOTE: Genotypic Ratio BB:Bb:bb • (Homozygous Dominant:Heterozygous:Homozygous Recessive) • PHENOTYPIC RATIO: (Dominant Trait:Recessive Trait)

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