How to Punnett Squares Biology 103 Punnett Squares

How to: Punnett Squares Biology 103

Punnett Squares: - determine the probability of the possible genotypes and phenotypes of two organisms offspring

Words to Know • Homozygous: 2 of the same alleles for the same trait • Heterozygous: 2 different alleles for the same trait • Dominant: the gene that is expressed; capital letter • Recessive: the gene that is masked; lower-case letter

Example: In humans brown eyes are dominant to blue eyes. 1. Homozygous Dominant BB (same capital letter) 2. Heterozygous Dominant Bb (one capital and one lowercase letter [different]) 3. Homozygous Recessive bb (same lower-case letter)

YOU DO IT!! Q. If black fur is dominant to white fur, what are the possible genotypes for a black furred rabbit? A. BB for homozygous dominant and Bb for heterozygous dominant

YOU DO IT!! Q. If long tails are dominant to short tails in squirrels the what would the genotype for a short tailed squirrel be? A. tt for homozygous recessive

Monohybrid Cross A monohybrid cross is a cross of only one trait. It is the simplest and fastest.

Example 1: If brown eyes are dominant to blue eyes and a homozygous brown eyed male mates with a blue eyed female, what are the possible genotypes and phenotypes of the offspring?

Step 1: Perform the cross of the parents. BB X bb

Step 2: Draw a square with four boxes in it.

Step 3: Figure and place the Parents genotypes. Other Parent Mom or Dad B b b B

Step 4: Perform the cross and write down the possible phenotypes of the offspring b b B B

Step 4: Perform the cross and write down the possible phenotypes of the offspring All offspring are heterozygous dominant for brown eyes B B b Bb Bb

YOU DO IT!! Q. In horses, brown fur is dominant to chestnut color fur. What would the possible genotypes and phenotypes be for the offspring if a two heterozygous brown furred horses mated?

A. 1 -homozygous brown furred 2 -heterozygous brown furred 1 -chestnut B b B BB Bb bb

Dihybrid Cross A dihybrid cross is a cross between two organisms that differ in two traits instead of just one. Ex. A homozygous tall/homozygous green

Example 2 If tall is dominant to short and purple is dominant to white, what would the phenotypes of the offspring be if a heterozygous tall/heterozygous purple plant is crossed with a heterozygous tall/heterozygous purple plant?

Step 1: Write down all given information 2 Traits Height~Tall=dominant ~short=recessive Color~purple=dominant ~white=recessive So our cross would be… Tt. Pp X Tt. Pp This tells me … Homozygous tall = TT Heterozygous tall = Tt Short (must be homozygous) = tt Homozygous purple = PP Heterozygous purple = Pp White (must be homozygous) = pp

Hints 1. Look for the “with” in the problem. ~this is where the cross separates the two parents If a heterozygous tall/heterozygous purple plant is crossed with a heterozygous tall/heterozygous purple plant? Parent #1 Tt. Pp X Tt. Pp Parent #2

Step 2: Separate the Gametes Tt. Pp X Tt. Pp TP ~take each parent individually and produce 4 gametes ~Each gamete must contain a “t” or “T” and a “P” or “p” Hint: Use the “FOIL” method!!!

Step 2: Separate the Gametes Tt. Pp X Tt. Pp TP Tp ~take each parent individually and produce 4 gametes ~Each gamete must contain a “t” or “T” and a “P” or “p” Hint: Use the “FOIL” method!!!

Step 2: Separate the Gametes Tt. Pp X Tt. Pp TP Tp t. P ~take each parent individually and produce 4 gametes ~Each gamete must contain a “t” or “T” and a “P” or “p” Hint: Use the “FOIL” method!!!

Step 2: Separate the Gametes Tt. Pp X Tt. Pp TP Tp t. P tp ~take each parent individually and produce 4 gametes ~Each gamete must contain a “t” or “T” and a “P” or “p” Hint: Use the “FOIL” method!!!

Step 3: Draw a 16 square punnett square

Step 4: Place gametes on top and side of square TP TP Tp t. P tp

Step 5: Perform the crosses TP Tp t. P tp TP TTPp Tt. PP Tt. Pp Tp TTPp TTpp Tt. Pp Ttpp t. P Tt. Pp tt. PP tt. Pp tp Tt. Pp Ttpp tt. Pp ttpp

Step 6: Count your offspring Tall Purple IIIII Tall White III Short Purple III TP Short White I Tp 9: 3: 3: 1 t. P tp TP TTPp Tt. PP Tt. Pp Tp TTPp TTpp Tt. Pp Ttpp t. P Tt. Pp tt. PP tt. Pp tp Tt. Pp Ttpp tt. Pp ttpp

YOU DO IT!! In pea plants, green pod color is dominant to yellow pod color and tallness is dominant to shortness. You cross a short/yellow pea plant with a homozygous tall/heterozygous green plant. What are the offspring phenotype and genotype possibilities? Q.

A. Tall Green IIII Tall Yellow IIII 1: 1 or 4: 4 or 8: 8 tg Ttgg X TTGg tg tg tg TG Tt. Gg Tt. Gg Tg Ttgg Ttgg