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Punnett Squares: Dominant & Recessive Traits
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Gregor Mendel is often called the “Father of Genetics”.
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He studied the heredity of pea plants & was the first to identify dominant & recessive traits.
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Our focus will be on the following 3 traits: Height, Color, and Pea Texture.
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Our focus will be on the following 3 traits: Height Color Pea Texture
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A CAPITAL letter is used to represent a dominant allele while a lower case letter represents a recessive allele.
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TraitDominant trait Recessive trait Height Color Pea texture Tall (T) Short (t) Green (G) Yellow (g) Smooth (R) Wrinkled(r)
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The physical appearance or trait that is visible, like freckles or dimples, is known as the phenotype.
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The actual gene combination or alleles that determine a trait is known as the genotype.
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Offspring inherit one allele of a genotype from each parent.
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If an organism carries two dominant alleles (TT) or two recessive alleles (tt) for a trait they are said to be purebred or homozygous.
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“homo” = same
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An organism that carries both a dominant allele & a recessive allele (Tt) for a trait is called a hybrid or heterozygous.
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“hetero” = different
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In a heterozygous mix, the dominant trait will mask the recessive trait.
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Some traits are determined by several genes & are therefore hard to predict in offspring, for example, height, hair color & eye color in humans.
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AllelesGenotypePhenotype GG Gg gg Homozygous dominant green Heterozygousgreen Homozygous recessive yellow
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A table called a Punnett square can be used to predict the genotypes & phenotypes offspring will have based on the genotypes or traits of the parents.
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**Unless noted otherwise, use the following rules:
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Mom’s genes will be first in a cross (ladies first) & should go on the left of a Punnett square. Dad’s genes will be last in a cross & should go on the top of a Punnett square. Capital letter is written BEFORE the lowercase letter in a heterozygous genotype.
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G = green g = yellow GG X Gg MomDad
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Mom Dad G G G g GGG g G G G g
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GG X gg Mom Dad G = green g = yellow
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Mom Dad G G g g GgG g G g G g
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Genotypes (%) GG: 0% Gg: 100% gg: 0% Phenotype ratio: 4:0 G = green g = yellow
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Gg X Gg Mom Dad G = green g = yellow
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Mom Dad G g G g GGG g G g g g
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Genotypes (%) GG: 25% Gg: 50% gg: 25% Phenotype ratio: 3:1 G = green g = yellow
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Mom Dad R R r r RrR r R r R r
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Genotypes (%) Homozygous dominant: 0% Homozygous recessive: 0% Heterozygous: 100% Phenotype ratio: 4:0 R = Smooth r = Wrinkled
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Mom Dad R r R r RRR r R r r r
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Genotypes (%) Homozygous dominant: 25% Homozygous recessive: 25% Heterozygous: 50% Phenotype ratio: 3:1 R = Smooth r = Wrinkled
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Mom Dad R r r r RrR r r r r r
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Genotypes (%) Homozygous dominant: 0% Homozygous recessive: 50% Heterozygous: 50% Phenotype ratio: 2:2 R = Smooth r = Wrinkled
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Mom Dad R R R R RRR R R R R R
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Genotypes (%) Homozygous dominant:100% Homozygous recessive: 0% Heterozygous: 0% Phenotype ratio: 4:0 R = Smooth r = Wrinkled
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