Predicting the Traits of

OffspringBy Reginald PunnettBy Reginald Punnett

Punnett Squares

• Punnett Squares are used to predict the probability of certain traits by showing the ways that different alleles can combine

 

• In a Punnett square, capital letters represent dominant alleles.

• Lower case letters represent recessive alleles.

Genotype and Phenotype

• genotype- two letters that represent the two alleles that an organism has for a trait. Example: Tt would mean that it has one dominant and one recessive allele.

• phenotype- the way an organism looks or acts as a result of its genotype

• The two alleles for each trait come from the organism’s parents.

Homos and Heteros

• Homozygous means that an organism has two alleles that are the same for a trait.

 

• Heterozygous means that an organism has two different alleles for a trait.

Making a Punnett Square

• 1. Write the letters representing the alleles from one parent across the top of the box.

• 2. Write the letters from the other parent down the side of the box.

• 3. Fill in the boxes with the alleles from each parent.

• The letters in the boxes represent the genotypes of the possible offspring the parents could produce.

Stuff To Remember• The letter that is used to represent the

alleles will be the first letter of the dominant trait. For example, if tall plants are dominant over short plants, the letter T is used, not the letter S. Lowercase t would represent the short allele.

• If an organism is heterozygous, the capital letter always comes first. For example: Tt NOT tT.

Example Problem

• In pea plants, tall plants are dominant to short plants. Show the genotypes and phenotypes that are produced when two heterozygous, tall plants are crossed with each other.

• Write the genotype of one parent across the top.

• Write the genotype of the other parent down the side.

• Fill in each box with the letter above and to the left of it.

T t

T TT Tt

t Tt tt

• What is the chance that the offspring will be tall?

• 75% (3 out of 4)• What is the chance

that the offspring will be short?

• 25% (1 out of 4)

T t

T TT Tt

t Tt tt