How to make a punnett square A step by step method
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 that are heterozygous for the seed shape trait, what fraction of the offspring should have spherical seeds? S s Step 2 Problem #1 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 In Multiply sides to get possible offspring Ss ss S Step 3 SS Ss Problem #1 3 Interpret Multiply Determine Parent genotypes Draw punnett square Step 1 42 the possible offspring SquarePants rr Rr x ________ recently met SpongeSusie ________ Roundpants at a dance. R r SpongeBob is heterozygous for his square shape, but SpongeSusie is round Rr rr (recessive). What is the ratio of round to square pants? r r SpongeBob 2:2 Rr rr Dihybrid crosses Dihybrid crosses are punnett squares used to predict the probability of an offspring having 2 traits at the same time. Ex: What are the chances of my baby having brown eyes AND blond hair. Step 1 Draw a box and divide it into 16 squares. Step 2 Determine the genotypes of the parents Ex: cross two pea plants that are heterozygous for yellow peas and round peas. Yellow is dominant to green and round is dominant to wrinkled. YyRr x YyRr Step 3 Determine all of the possible gamete combinations for each parent by using the F.O.I.L. or Gremlin method FOIL= first, outside, inside last Ex: YyRr First- YR Outside- Yr Inside- yR Last- yr Gremlin YyRr Step 4 Write all of the YR possible combinations of alleles on the sides YR and top of the Yr punnett square. YR Yr yR yr yR yr Yr yR yr Step 5 Do the cross Remember to keep all of one letter together and all of the 2nd letter together. YR Yr yR yr YR YYRR YYRr YyRR YyRr Yr YYRr YYrr YyRr Yyrr yR YyRR YyRr yyRR yyRr yr YyRr Yyrr yyRr yyrr Step 6 Go through the results and determine what the phenotype of each offspring would be Yr yR yr YYRR YYRr YyRR YyRr Yellow round Yellow, round Yellow, round Yellow, round YYRr YYrr YyRr Yyrr Yellow, round Yellow,w rinkled Yellow, round Yellow, wrinkled YyRR YyRr yyRR yyRr Yellow, round Yellow, round Green, round Green, round YyRr Yyrr yyRr Yyrr Yellow, round Yellow, wrinkled Green, round Green, wrinkled YR YR Yr yR yr Step 7 Figure out the phenotypic ratio or answer whatever question they ask you. 9:3:3:1 Example: what are the chances of having a yellow and wrinkled pea? 3/16 Yr yR yr YYRR YYRr YyRR YyRr Yellow round Yellow, round Yellow, round Yellow, round YYRr YYrr YyRr Yyrr Yellow, round Yellow,w rinkled Yellow, round Yellow, wrinkled YyRR YyRr yyRR yyRr Yellow, round Yellow, round Green, round Green, round YyRr Yyrr yyRr Yyrr Yellow, round Yellow, wrinkled Green, round Green, wrinkled YR YR Yr yR yr Let’s Practice Determine the gametes for the following cross: heterozygous tall plant with heterozygous yellow seeds with another of the same genes. Tall and yellow are dominant F.O.I.L TtYy First-TY Outside- Ty Inside- tY Last- ty Gremlin TtYy Problem #4 bt bt bt bt 5 6 Fill Analyze in4the results square Determine gametes and place on square Step 3 17 Draw punnett square 2 and Determine Answer question(s) genotypes In horses, black is dependent upon a dominant gene (B), ________ BBTT x ________ bbtt and chestnut color upon its BT BT BT BT recessive allele (b). BbTt BbTt BbTt BbTt chestnut, chestnut, chestnut, chestnut, The trotting gait is due to a trotting Bb trotting trottingBbtrotting dominant allele, (T), the BbTt BbTt BbTt BbTt chestnut, chestnut, chestnut, chestnut, pacing gait to its recessive trotting trotting trotting trotting allele, (t). BbTt BbTt BbTt BbTt Bb Bb chestnut, chestnut, chestnut, chestnut, Cross a homozygous black, trotting trotting trotting trotting homozygous trotting horse BbTt BbTt BbTt BbTt chestnut, chestnut, chestnut, chestnut, with a chestnut, pacing trotting trotting trotting trotting horse. What percentage of offspring could be chestnut and pacing? 0% Problem #5 5 6 Fill Analyze in4the results square Determine gametes and place on square Step 3 17 Draw punnett square 2 and Determine Answer question(s) genotypes Suppose the alpha male and alpha female of a wolf pack nnbb x ________ NnBb ________ are black with blue eyes and nb nb nb nb normal colored with brown NnBb NnBb NnBb NnBb eyes, respectively. Assume normal, normal, normal, normal, further that normal coat color brown brown brown Bb brown Bb (N) is dominant to black (n) Nnbb Nnbb Nnbb Nnbb normal, normal, normal, normal, and brown eyes (B) are blue blue blue blue dominant to blue (b). The nnBb nnBb nnBb nnBb Bb Bb black, black, black, black, female is also heterozygous for brown brown brown brown both traits. nnbb nnbb nnbb nnbb What fraction of the offspring black, black, black, black, blue blue blue blue could have the following genotypes? nb nB Nb NB NNBB: 0/16 NnBb: 4/16 nnbb: 4/16 Problem #6 fs fS Fs FS Fill in4the square Determine gametes and place on square Step 3 15 Draw punnett square 2 and Determine genotypes Carrion beetles lay their eggs in dead animals and then FfSs x ________ FfSs ________ bury them in the ground until FS Fs fS fs they hatch. Assume that the FFSS FFSs FfSS FfSs preference for fresh meat (F) is fresh, fresh, fresh, fresh, shallow Bb shallow shallowBbshallow dominant to the preference FFSs FFss FfSS for rotted meat and that the fresh, fresh, fresh, shallow deep shallow tendency to bury the meat shallow (S) is dominant to the Bb Bb tendency to bury the meat deep. Suppose a female and male carrion beetle heterozygous both traits mate. What is the phenotypic ratio? Problem #6 Step 7 5 Answer 6 Fill in thequestion(s) Analyze results square Carrion beetles lay their eggs in dead animals and then bury them FfSs x ________ FfSs ________ in the ground until they hatch. Assume that the preference for FS Fs fS fs fresh meat (F) is dominant to the FFSS FFSs FfSS FfSs preference for rotted meat and fresh, fresh, fresh, fresh, shallow Bb shallow shallowBbshallow that the tendency to bury the FFSs FFss FfSS Ffss meat shallow (S) is dominant to fresh, fresh, fresh, fresh, the tendency to bury the meat shallow deep shallow deep deep. Suppose a female and FfSS FfSS ffSS ffSs Bb Bb fresh, fresh, rotted, rotted, male carrion beetle heterozygous shallow shallow shallow shallow both traits mate. What is the fS Fs FS fs phenotypic ratio? FfSS Ffss ffSs ffss fresh, shallow fresh, deep rotted, shallow rotted, deep Fresh/shallow: fresh/deep: rotted/shallow: rotted/deep 9:3:3:1
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