Determining Probability of Genetic Outcomes Featuring the Punnett Square 8 th Grade Science – K. Franklin – 8/25/10

Problem: What will the outcome be if a tall plant is crossed with a short plant?

Phenotype Is the physical appearance of a trait. The plant on the left has a Tall phenotype. The plant on the right has a Short phenotype.

Alleles or Genes Are inherited “factors” (Mendel’s term) that control traits for each characteristic. Living things have 2 for each trait. (One from each parent) We use letters to represent alleles or genes. (t) lower case letters = recessive traits (T) capital letters = dominant traits

“Tallness” is a dominant trait “shortness” is a recessive trait T = Tall t = short

Genotype Is the combination of alleles (type of genes). TT 2 dominant alleles (purebred or “true breeder”) Homozygous Dominant Tt 1 dominant and 1 recessive (hybrid like f 1 ) Heterozygous tt 2 recessive alleles (purebred or “true breeder”) Homozygous Recessive

We’ll use a Punnett Square as a graphic organizer to help us figure out the probability of outcomes.

1 st : The Parent’s alleles Go on the outside Parent 1 Parent 2

1 st : Write the parent’s alleles correctly. Parent 1 Parent 2 T T t t Purebred tall Purebred short

2nd : Bring the alleles from Parent 1 down. Parent 1 Parent 2 T T t t T T T T

3rd : Bring the alleles from Parent 2 over. Parent 1 Parent 2 T T t t T T T T tt tt

4th : Look at the different possibilities of outcomes. Parent 1 Parent 2 T T t t T T T T tt tt

What is the Probability of a tall plant? T T t t T T T T tt tt 1. Look for the capital T 2. Count the number of squares with a dominant allele. 4 out of 4 possibilities = a 100% chance of a tall plant. The phenotype for this genotype is TALL Heterozygous

What is the Probability of a short plant? T T t t T T T T tt tt 1. Look for a double recessive (tt) 2. Count the number of squares with double recessive alleles. 0 out of 4 possibilities = a 0% chance of a short plant. Heterozygous

What are the possible outcomes if the hybrids are crossed? T T t t Now the first generation will serve as “parents” of the next generation.

2 nd : Bring the alleles from parent 1 down. T T t t T t t T

3 rd : Bring the alleles from Parent 2 over to the right. T T t t T T T T t t t t

4 th : Check the probability T T t t T T T T t t t t Homozygous Dominant Homozygous Recessive Heterozygous Heterozygous 1 out of 4=25% 2 out of 4=50%

What is the probability of a short Phenotype? T T t t T T T T t t t t Tall Homozygous Recessive Heterozygous 1 out of 4=25% 3 out of 4=75% Tall Short Homozygous Dominant

What is the probability of a Homozygous Recessive Genotype? T T t t T T T T t t t t Homozygous Dominant Homozygous Recessive Heterozygous 1 out of 4=25%

More Practice Read all about it! Textbook pp Use one of these websites for additional practice as a class or for enrichment at home: – res.htmlhttp://www2.edc.org/weblabs/Punnett/punnettsqua res.html –

Now You Try! Complete the worksheet that your teacher gave you!