advertisement

Hands On 5 Steps to Solving Monohybrid Crosses First, analyze the problem and highlight important information PROBLEM: Dimples are dominant over no dimples. A father is homozygous dominant for dimples and a mother is homozygous recessive for dimples. What are the chances that their child will have dimples? Step 1: Choose a letter to represent a trait. (hint: Dominant trait named dimples starts with “D”, recessive gets the same but small letter) Dimples = D No dimples = d Step 2: Write down the genotype of the parents. Remember that each parent has 2 alleles. Mom Dad d d DD Find coins to represent each parents’ alleles. Step 3: Write the genetic cross. dd x DD Step 4: Set up the Punnett Square. Write in the parent alleles on the Punnett Square. Like a multiplication table, it does not matter BUT we find it helpful to place mom’s alleles across the top and dad’s down the side of the Punnett Square. What is a Punnett Square? A Punnett Square is a tool like a multiplication table which scientists use to show all the possible combinations of alleles that can result from a genetic cross. The Punnett Square shows all the possible outcomes that any 1 child may inherit. Each time mom and dad have another child it is a new roll of the same genetic dice. Place coins to represent each parents’ alleles directly on top of the written allele. For each fertilization move mom’s “allele” and dad’s “allele” to join in the Punnett Square box. d d D Dd Dd D Dd Dd Step 5: Determine the phenotypes and genotypes of the offspring. Genotype(s): Dd Phenotype(s): Dimples Now let’s talk about probability numbers... The Punnett Square has 4 boxes in it. Each box represents ¼ or 25% probability to occur. For this mating, the 4 boxes representing possible offspring get the genotype of Dd. So there is 100% chance (4 x 25%) that offspring phenotype will have dimples. Do you like broccoli or brussel sprouts? In humans, being a taster of the chemical PTC, a bitter component found in broccoli and brussel sprouts, is dominant over being a non-taster (recessive). Can you taste PTC? Let’s find out… PTC = phenylthiocarbamide Problem 2 First, analyze the problem and highlight important information A man heterozygous for this trait of tasting PTC marries a female who is homozygous recessive (non-taster) for this trait. Use a Punnett square to predict the genotypes and phenotypes of any offspring. Step 1: Choose a letter to represent a trait. Taster = T Non Taster = t Step 2: Write down the genotype of the parents. Remember that each parent has 2 alleles. Mom Dad tt Tt Find coins to represent each parents’ alleles. Step 3: Write the genetic cross. Tt x tt Step 4: Set up the Punnett Square. Write in the parent alleles on the Punnett Square. Like a multiplication table, it does not matter BUT we find it helpful to place mom’s alleles across the top and dad’s down the side of the Punnett Square. Place coins to represent each parents’ alleles directly on top of the written allele. For each fertilization move mom’s “allele” and dad’s “allele” to join in the Punnett Square box. t t T Tt Tt t tt tt Step 5: Determine the phenotypes and genotypes of the offspring. Genotype(s): Tt and tt Phenotype(s): Taster and non-tasters Now let’s talk about probability numbers… The Punnett Square has 4 boxes in it. Each box represents ¼ or 25% probability to occur. For this mating, 2 offspring boxes receive the genotype of Tt There is 50% chance (2 x 25%) that phenotype will be tasters. 2 boxes receive the genotype of tt so there is 50% chance (2 x 25%) that offspring phenotype will be non-tasters.