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Transcript
How to Make a Punnett Square A step by step method 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. 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? Step 1 Draw a box and divide it into 4 squares. These squares represent the possible offspring 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. R r r R Step 3 Now you multiply the sides with the top to get the genotype of the 4 offspring boxes. R R r Rr Rr r Rr Rr Step 4 Now you can see what the genotypes of the offspring would be. r In this case there is a 100% chance of the offspring being r heterozygous for the trait because all 4 out of 4 boxes are capital, lower case. R R Rr Rr Rr Rr 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? Let’s Practice Homozygous AA ____ HO Bb ____ HE HE Cc ____ HE Dd ____ (HO) or Heterozygous (HE) Ee ____ HE ff ____ HO HO GG ____ HO HH ____ Determine Phenotype Purple flowers are dominant to white flowers Brown eyes are dominant to blue eyes Purple PP _____________ Purple Pp _____________ White pp _____________ Brown BB ____________ Brown Bb ____________ Blue bb ____________ For each phenotype list the genotype Straight hair is dominant to curly. Pointed heads are dominant to round heads. SS PP ____________ straight ____________ pointed ____________ straight ____________ pointed Ss Pp ss pp ____________ curly ____________ round Problem #1 Step 1 In Draw punnett square 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? Problem #1 Determine the genotype of parents, and place alleles on punnett square Ss Ss In pea plants, ________ x ________ spherical seeds (S) are dominant to dented S s seeds (s). In a genetic cross of two plants S that are heterozygous for the seed shape trait, what fraction of the offspring should s have spherical seeds? Step 2 Problem #1 Step 3 In Multiply sides to get possible offspring 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 ________ Ss x ________ S s S SS Ss s Ss ss Problem #2 SpongeBob SquarePants ss Ss x ________ recently met SpongeSusie ________ Roundpants at a dance. S s SpongeBob is heterozygous for his square shape, but SpongeSusie is round s Ss ss (recessive). What is the ratio of round to square pants? s Ss ss 2 Round : 2 Square Problem #3 What is the probability of Ll LL x ________ producing offspring that ________ have short whiskers from a L L cross of two long-whiskered seals, one that is homozygous dominant and L LL LL one that is heterozygous? What is the outcome for long whiskers and short l Ll Ll whiskers? 100% Long, 0% Short Problem #4 In pea plants, purple flowers PP PP x ________ are dominant over white ________ flowers, which are P P recessive. Cross two homozygous dominant parents. What is the P PP PP outcome for this cross between two homozygous dominant parents? P PP PP 100% Purple Flowers Problem #5 In pea plants, round peas RR Rr x ________ are dominant and wrinkled ________ peas are recessive. Cross a R r heterozygous parent with a homozygous round pea plant. What is the outcome R RR Rr for this cross? 100% Round Pea Plants R RR Rr