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How to Make a Punnett Square A step by step method
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What is a Punnett Square? A punnett square is a diagram used to try to predict the outcome of a cross between 2 parents. It does not tell us what will happen, but it does tell us the likelihood that something will happen.
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Monohybrid Crosses These are crosses that only look at a single trait Ex: hair color, eye color, albinism Ex: What are the chances that my baby will have brown eyes?
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Step 1 Draw a box and divide it into 4 squares. These squares represent the possible offspring
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Step 2 Determine the genotypes of the parents. ____x_____ Place the alleles for the genotypes of the parents on the sides and top of the square. As a general rule, we put the dad on the top and the mom on the side. RR r r
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Step 3 Now you multiply the sides with the top to get the genotype of the 4 offspring boxes. r r RR Rr
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Step 4 Now you can see what the genotypes of the offspring would be. In this case there is a 100% chance of the offspring being heterozygous for the trait because all 4 out of 4 boxes are capital, lower case. r r RR Rr
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Ratios Genotypic Ask your self: How many homozygous dominants? How many heterozygous? How many homozygous recessives? Phenotypic Ask yourself: How many dominant phenotypes? How many recessive phenotypes?
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Let’s Practice Homozygous (HO) or Heterozygous (HE) AA ____ Bb ____ Cc ____ Dd ____ Ee ____ ff ____ GG ____ HH ____ HO HE HO HE
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Determine Phenotype Purple flowers are dominant to white flowers PP _____________ Pp _____________ pp _____________ Brown eyes are dominant to blue eyes BB ____________ Bb ____________ bb ____________ Purple White Brown Blue Brown
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For each phenotype list the genotype Straight hair is dominant to curly. ____________ straight ____________ straight ____________ curly Pointed heads are dominant to round heads. ____________ pointed ____________ pointed ____________ round SS Ss ss PP Pp pp
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Problem #1 In pea plants, spherical seeds (S) are dominant to dented seeds (s). In a genetic cross of two plants that are heterozygous for the seed shape trait, what fraction of the offspring should have spherical seeds? Step 1 Draw punnett square
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Problem #1 Determine the genotype of parents, and place alleles on punnett square Step 2 ________ x ________ In pea plants, spherical seeds (S) are dominant to dented seeds (s). In a genetic cross of two plants that are heterozygous for the seed shape trait, what fraction of the offspring should have spherical seeds? Ss SsSs
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Problem #1 Multiply sides to get possible offspring Step 3 ________ x ________ In pea plants, spherical seeds (S) are dominant to dented seeds (s). In a genetic cross of two plants that are heterozygous for the seed shape trait, what fraction of the offspring should have spherical seeds? Ss S s SS ssSs
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Problem #2 ________ x ________ SpongeBob SquarePants recently met SpongeSusie Roundpants at a dance. SpongeBob is heterozygous for his square shape, but SpongeSusie is round (recessive). What is the ratio of round to square pants? Ss ss S s s ss s ssSs ss 2 Round : 2 Square
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Problem #3 ________ x ________ What is the probability of producing offspring that have short whiskers from a cross of two long-whiskered seals, one that is homozygous dominant and one that is heterozygous? What is the outcome for long whiskers and short whiskers? LL Ll L l LL Ll LL 100% Long, 0% Short
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Problem #4 ________ x ________ In pea plants, purple flowers are dominant over white flowers, which are recessive. Cross two homozygous dominant parents. What is the outcome for this cross between two homozygous dominant parents? PP P P PP 100% Purple Flowers
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Problem #5 ________ x ________ In pea plants, round peas are dominant and wrinkled peas are recessive. Cross a heterozygous parent with a homozygous round pea plant. What is the outcome for this cross? Rr RR R r R RR R RR RrRR Rr 100% Round Pea Plants
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