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 Rr r Rr Rr Rr r

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 r Rr Rr Rr r

Ratios Genotypic Phenotypic Ask your self: Ask yourself: 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 AA ____ Bb ____ Cc ____ Dd ____ Homozygous (HO) or Heterozygous (HE) AA ____ Bb ____ Cc ____ Dd ____ Ee ____ ff ____ GG ____ HH ____ HO HE HE HO HE HO HE HO

Determine Phenotype Purple Brown Purple Brown White Blue Purple flowers are dominant to white flowers PP _____________ Pp _____________ pp _____________ Brown eyes are dominant to blue eyes BB ____________ Bb ____________ bb ____________ Purple Brown Purple Brown White Blue

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 PP Ss Pp ss pp

Problem #1 Step 1 Draw punnett square 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?

Problem #1 Step 2 Determine the genotype of parents, and place alleles on punnett square 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? ________ x ________ Ss Ss S s S s

Problem #1 Step 3 Multiply sides to get possible offspring 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? ________ x ________ Ss Ss S s Ss ss S s SS Ss

Problem #1 Step 4 Interpret the possible offspring 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? ________ x ________ Ss Ss S s Ss ss S s SS Ss 3/4

Problem #2 Step 1 Draw punnett square Step 4 Interpret the possible offspring Step 3 Multiply Step 2 Determine Parent genotypes 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? ________ x ________ Rr rr R r Rr rr r r Rr rr 2:2

Problem #3 Step 3 Multiply Step 2 Determine Parent genotypes Step 1 Draw punnett square Step 4 Interpret the possible offspring Squidward is purebred for light blue skin, which is the dominant trait for body color. He recently married a nice girl who has light green skin, which is a recessive trait. Create a Punnett square to show the possibilities that would result if Squidward and his new bride had children. What percentage of Squidward’s offspring could be green? ________ x ________ BB bb B B Bb Bb b b Bb Bb 0%

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 Gremlin 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 YyRr

Step 4 Write all of the possible combinations of alleles on the sides and top of the punnett square. YR Yr yR yr YR Yr yR yr YR Yr yR yr

Step 5 Go through the results and determine what the phenotype of each offspring would be YR Yr yR yr YYRR Yellow round YYRr Yellow, round YyRR Yellow, round YyRr Yellow, round YR YYRr Yellow, round YYrr Yellow,wrinkled YyRr Yellow, round Yyrr Yellow, wrinkled Yr YyRR Yellow, round YyRr Yellow, round yyRR Green, round yyRr Green, round yR YyRr Yellow, round Yyrr Yellow, wrinkled yyRr Green, round Yyrr Green, wrinkled yr

Step 6 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 yr YYRR Yellow round YYRr Yellow, round YyRR Yellow, round YyRr Yellow, round YR YYRr Yellow, round YYrr Yellow,wrinkled YyRr Yellow, round Yyrr Yellow, wrinkled Yr YyRR Yellow, round YyRr Yellow, round yyRR Green, round yyRr Green, round yR YyRr Yellow, round Yyrr Yellow, wrinkled yyRr Green, round Yyrr Green, wrinkled 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- Outside- Inside- Last- Gremlin TtYy TY Ty tY ty

Problem #4 Step 6 Answer question(s) Step 1 Draw punnett square Step 3 and 4 Determine gametes and place on square Step 2 Determine genotypes Step 5 Analyze results Suppose the alpha male and alpha female of a wolf pack are black with blue eyes and normal colored with brown eyes, respectively. Assume further that normal coat color (N) is dominant to black (n) and brown eyes (B) are dominant to blue (b). The female is also heterozygous for both traits. What fraction of the offspring could have the following genotypes? ________ x ________ nnbb NnBb nb nb nb nb NnBb normal, brown NnBb normal, brown NnBb normal, brown NnBb normal, brown NB Bb Bb Nnbb normal, blue Nnbb normal, blue Nnbb normal, blue Nnbb normal, blue Nb nnBb black, brown Bb nnBb black, brown nnBb black, brown Bb nnBb black, brown nB nnbb black, blue nnbb black, blue nnbb black, blue nnbb black, blue nb NNBB: NnBb: nnbb: 0/16 4/16 4/16