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. Dimples = No Dimples = D d

Step 2: Write down the genotype of the parents Step 2: Write down the genotype of the parents. Remember that each parent has 2 alleles. Mom Dad d d D D

Step 3: Write the genetic cross. d d x D D

Step 4: Set up the Punnett Square Step 4: Set up the Punnett Square. A Punnett Square is a chart scientists use to show all the possible combinations of alleles that can result from a genetic cross. Remember, the Punnett Square shows all the possible outcomes that any 1 child may inherit. Then next time mom and dad have another child it is a new roll of the genetic dice.

d d D Dd Dd D Dd Dd

Step 5: Determine the phenotypes and genotypes of the offspring Step 5: Determine the phenotypes and genotypes of the offspring. Remember to label the ratios. Genotype Ratio DD : Dd : dd express in % % DD : % Dd : % dd Phenotype Ratio: Dimples : No dimples express in % % Dimples : % No dimples 4 100 4 100

Independent Practice PROBLEM: In humans, freckles are dominant over no freckles. A man heterozygous for freckles reproduces with a woman also heterozygous for freckles. What chance does their child have for no freckles?

**Show your work for Steps 1-5 below: 1. freckles = F no freckles = f 2. Mom = Ff Dad = Ff 3. Ff x Ff F f 4. F FF Ff ff f 5. Genotypic Ratio 1 FF : 2 Ff : 1 ff Genotypic Percent 25%FF : 50% Ff : 25% ff Phenotypic Ratio 3 freckles : 1 no freckles Phenotypic Percent 75% freckles: 25% no freckles