Punnett Square Notes1. A Punnett Square is a grid for organizing

genetic information. It is VERY similar to how you predict probability.

2. A Punnett Square indicates the PROBABILITY of what the offspring from two individuals will look like.

3. What are the possibilities of the offspring between two hybrid tall pea plants?

4. What is the genotype (alleles) for a hybrid tall pea plant?

5. What alleles can one parent possibly give to their offspring?

6. What alleles can the other parent give to their offspring?

Punnett Square Notes

• Place each allele at the top (male) or side (female) of the punnett square. (drag and drop the colored letters)

• Fill in the punnett square by reading the alleles down & across. (drag and drop the purple letters)

• The squares represent the possible genotypes of the offspring from these two individuals. It does NOT mean that there are four tall offspring

Punnett Square Notes

Tt

Tt T

t

T t

TT Tt

Tt tt

Punnett Square Notes

• The square has just given you a probability of getting possible traits in the offspring

• Probability of having a tt offspring (phenotype = .25) = 25%

• Probability of having a Tt offspring (phenotype = .50) = 50%

• Probability of having a TT offspring (phenotype = .25) = 25%

Punnett Square Notes

• Example What are the offspring possibilities between a hybrid widow peaked dad and a non-widow peaked mom ?

• Dad is Ww • Mom is ww

Example

• Solve the cross

Dad W W

Mom

w

w

Questions

• Genotype for mom = • Genotype for dad = • Probability of Ww offspring ( ? ) =

___• Probability of WW offspring ( ? ) =

___• Probability of ww offspring ( ? ) =

___

Types of Punnett SquaresThere are two different types of punnett

squares: monohybrid and dihybrid.• A monohybrid cross is exploring the

possibilities of only one trait. For example, what will the offspring look like when a green and yellow podded plant are crossed. You are ONLY looking at one trait-pod color.

• A dihybrid cross is exploring the possibilities of TWO traits. For example, what will the offspring look like when a yellow, wrinkled pea plant is crossed with a green, smooth pea plant. You are looking at two traits- pea color and pea texture.

Monohybrid Cross

• Green (Gg) x Yellow (gg) G g

g

g

Question: are both parents hybrids?

Dihybrid CrossYellow, wrinkled (Yyrr) x Green, Smooth (yyRr)

Yr Yr yr yr

yR

yr

yR

yr

Question: Are the parents dihybrids?

Dihybrid Cross

• For a dihybrid cross, you must think of all of the possible combination of alleles.

• For example, a yellow wrinkled plant (Yyrr) can offer the possible allele combinations: Yr, yr.

• This is easily determined by using the foil method the first times the third, the first times the forth, the second times the third, and the second times the forth

Problems

• What are the possible allele combinations for a green, smooth plant (yyRr)?

• What are the possible allele combinations for a yellow, smooth plant (YyRr)?

• What are the possible allele combinations for a green, wrinkled plant (yyrr)?

Test cross• A Punnett Square is used to

determine the genotype of an unknown organism. For example, you have a tall pea plant but you do not known if it is homozygous (TT) or heterozygous (Tt). To determine this, you test cross the plant with a homozygous recessive plant- or a SHORT plant. If the unknown is Tt, you will have some short offspring, if it is TT, you will have all tall offspring

Test cross

• Tall plant (TT or Tt) x short plant (tt)

t

t

t

t

T T T t

100% Tall offspring 50% Tall offspring