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. RR r r

Step 3  Now you multiply the sides with the top to get the genotype of the 4 offspring boxes. r r RR Rr

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

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 (HO) or Heterozygous (HE) AA ____ Bb ____ Cc ____ Dd ____ Ee ____ ff ____ GG ____ HH ____ HO HE HO HE

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

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

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

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

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

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

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

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

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