Punnett Squares Follow this step by step procedure EVERY time you set up a punnett square! Genetics Word Problem A brown eyed woman who is heterozygous for the trait has a child with a man who has blue eyes. The brown eye allele is dominant over blue eyes. What is the probability that their child will have blue eyes. Step 1 Assign a letter to the alleles involved. – I always try to pick letters that look different from capital to lowercase – It does not matter what letter you choose as long as the same letter is used for the same gene – Assign a capital letter for dominant alleles, lowercase for recessive – WRITE IT OUT! EVERYTIME!!! Step 1 Gene - eye color Alleles – brown = B blue = b Step 2 Using the information given in the word problem, determine the parental genotypes (genetic make-up) – Sometimes you are given the parents’ phenotype (physical appearance), genotype (hetero/homozygous), or even the offspring genotype or phenotype – The important thing to remember is you can always find the parents’ genotype from given info! Step 2 Mother is heterozygous with brown eyes – Bb Father HAS blue eyes which is recessive so his genotype MUST be – bb Write it like this: Bb x bb Step 3 Determine the types of gametes that each parent will contribute – Remember each parent can only pass one of the alleles to the offspring – Which one does it pass??? We don’t know, so all genetic possibilities have to be tested – It will be helpful if we construct our actual square here. Step 3 Parent one on top row Bb x bb Parent two on side Step 3 Bb x bb Parent one on top row B b Parent two on side Step 3 Bb x bb Parent one on top row B Parent two on side b b b Step 4 Fill out the table Go slow so you do not make silly mistakes! Step 4 Bb x bb Parent one on top row B Parent two on side b b b Step 4 Bb x bb Parent one on top row B Parent two on side b b Bb b Step 4 Bb x bb Parent one on top row Parent two on side b b B b Bb bb Step 4 Bb x bb Parent one on top row Parent two on side B b b Bb bb b Bb Step 4 Bb x bb Parent one on top row Parent two on side B b b Bb bb b Bb bb Step 5 Determine the ratio or probability! – Give the answer in whatever form the question asks for – In this example it asks for the probability (that is a fraction or percentage) 2 out of 4, or 2/4 = ½ or 50% Step 4 Bb x bb Parent one on top row Parent two on side B b b Bb bb b Bb bb The question asked for the probability of blue eyes. How many offspring would have blue eyes? Out of how many? You Try! You are an expert in genetics called to testify at a divorce case. A man with brown eyes is questioning the paternity of a child who was born by his estranged wife who also has brown eyes. The man claims it cannot be his baby because it has blue eyes. Give your expert testimony! What parental genotypes would cause this scenario? Show your punnett square that explains how or if this can happen. Does this PROVE that the baby is his? What would be the next step in granting or refusing the man a divorce based on infidelity?