Predicting the Traits of Offspring By 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 t. T.

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 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 tt