Transcript Chapter 10
Write everything that is underlined A. Dominant vs. Recessive Dominant traits: the ones you can see Recessive traits: are hidden by dominant Rule of Dominance: When a dominant trait is present the recessive trait is hidden Phenotype: The way an organism looks. No matter what genes are present, phenotype of a tall pea plant is tall and a short pea plant is short Genotype: The gene combination of an organism It consists of 2 alleles For example: Pure dominant, 2 dominant genes Pure recessive, 2 recessive genes Hybrid, 1 dominant and gene 1 recessive Homozygous: When the 2 alleles are the same 2 dominant genes = Homozygous dominant 2 recessive genes = Homozygous recessive Heterozygous: When the 2 alleles for a trait are different It’s a hybrid 1 dominant gene and 1 recessive gene Punnett Squares Punnett squares allow you to predict the ratios or percentages of offspring possible in a cross. Punnett SquareLettering Dominant Genes Always get capital letters. The letters will be the 1st letter of the trait. Recessive Genes Always get lower case letters. The letter used is the same as the one used for the dominant gene. Punnett Squares All problems will consist of 5 steps: 1. Labeling dominant and recessive letters 2. The Cross- Who are the parents 3. The Square- draw and fill in the square 4. Genotype ratios or percentages 5. Phenotype ratios or percentages Example Problem: Red flowers are dominant over purple flowers in a certain type of plant. What will the possible genotypes and phenotypes of the offspring of a hybrid red flower and a purple flower? There are five steps to this problem Step 1: Label the dominant and recessive traits R = Red flower r = Purple flower Step 2: Write the genotypes of each parent to be crossed Purple Hybrid Red (means heterozygous) Rr (since it is recessive it has to be homozygous) Rr x rr rr Step 3: Set up the punnett square. Each parent will give up one gene or the other so there will be one gene per box for each parent R r Rr r rr r Rr rr Step 4: Analyze the genotypes R r Rr r Rr r rr rr 2 of the 4 (50%) offspring will be pure recessive 2 of the 4 (50%) will be hybrids Step 5: Analyze the phenotypes R r Rr r rr r Rr rr 50% of the offspring will be red 50% of the offspring will be purple Now try one on your own Tall plants are dominant over short plants in a tomato plant. What will the possible genotypes and phenotypes of the offspring of a Homozygous Tall plant and a Homozygous short plant? Step 1: Label the dominant and recessive traits T = Tall plant t = short plant Remember, it’s NOT T and S Step 2: Write the genotypes of each parent to be crossed Homozygous Tall (homo means same) TT Homozygous Short (homo means same) TT x tt tt Step 3: Set up the punnett square. Write the parents first T t Tt T Tt Tt Tt t Step 4: Analyze the genotypes T t Tt T Tt Tt Tt t 4 of the 4 (100%) offspring will be heterozygous (or hybrid) for plant height Step 5: Analyze the phenotypes T t Tt T Tt Tt Tt t 100% of the offspring will be Tall because having at least one T makes it tall because T is dominant over short (t)