Introduction to Punnett Squares

INTRODUCTION TO PUNNETT SQUARES Objective: Learn how to solve and interpret the elements of a Punnett square, including dominant & recessive, percentages and hybrid & purebred . Materials: *Punnett Square Practice Problems, below *Pencil Vocabulary: ALLELES - the form a gene takes in regards to its physical characteristics (phenotype) DOMINANT - stronger trait GENES - the genetic make-up of a living thing, its’ DNA (genotype) GENETICS- the passing of traits from parent to offspring (the child) HYBRID - when an organism does not have identical alleles for a trait (Bw) (Ts) OFFSPRING - genes from the mother and father PREDICT- allows to see what the probability or likelihood that something will happen PUNNETT SQUARE - a tool used in genetics to help predict what traits will be passed down from one generation to the next. PUREBRED- when an organism has identical alleles for a trait (tt) or (BB) RECESSIVE - weaker trait TRAITS: physical characteristics of a living organism (color, size, shape) Directions: 1. Read the history of how Punnett squares are used in genetics. A scientist, Reginald C. Punnett, came up with a way to make potential genetic predictions by using a PUNNETT SQUARE. He invented a square that is divided into 4 smaller squares. The square has letters above and along the left side that represent a trait. This Punnett square, when used, can help determine possible traits to the offspring which is passed on from the parents. Sample of Punnett Square: NOTE: There are two concepts that are important when learning how to solve and understand PUNNETT SQUARES. They are dominant and recessive: a. Dominant- a stronger trait that dominates a recessive trait. When solving Punnett squares, a dominant trait is represented with a capital letter (Example: T, M, S). b. Recessive- a weaker trait that may get covered up by the dominant trait but is still present in the genes. When solving Punnett squares, a recessive trait is represented with a lowercase letter (Example: t, m, s). IMPORTANT NOTE: When a dominant trait is with a recessive trait, the dominant trait wins, strong beats weak. 2. GUIDED PRACTICE: Solve Punnett square and interpret the findings. SET UP THE SQUARE: For this example, we will use a black cat (BB) and a white cat (bb) that is going to have a kitten. In this case, black is the dominant color while white is the recessive color. For this example, we will place the dominant trait (alleles) on the top of the square and the recessive trait on the left sides of the square. GUIDED PRACTICE SQUARE # 1: Cross the first B with the first b INTERPRET SQUARE # 1: If B is dominant and b is recessive, what color would this be? If your answer was black, you are correct! Why is it black? Because the dominant trait/capital letter/allele is in the square #1 and the dominant trait wins, strong beats weak. GUIDED PRACTICE SQUARE # 2: Repeat the same process with the next square. Cross the second B with the first b INTERPRET SQUARE #2: What color is that square? If your answer was black, you are correct! Why is it black? Because the dominant trait/capital letter/allele is in the square #2 and the dominant trait wins, strong beats weak. GUIDED PRACTICE SQUARE # 3: Repeat the same process with the next square. Cross the first B with the second b INTERPRET SQUARE # 3: What color is that square? If your answer was black, you are correct! Why is it black? Because the dominant trait/capital letter/allele is in the square and the dominant trait wins, strong beats weak. GUIDED PRACTICE SQUARE # 4: Repeat the same process with the last square. Cross the second B with the second b. INTERPRET SQUARE # 4: What color is that square? If your answer was black, you are correct! Why is it black? Because the dominant trait/capital letter/allele is in the square and the dominant trait wins, strong beats weak. FINAL INTERPRETATION/CONCLUSION: Q: What color will the kitten be? A: If you said black, you are correct because all 4 squares have the dominant trait in it. Q: What is the probability of the kitten being that color? HINT: Think of each box as representing 25%. A: There would be a 100% chance of the kitten being black in color. 3. ADDITIONAL GUIDED PRACTICE: SET UP THE SQUARE: Let’s try the probability for height of a person. In this case S - tall (dominant) s - short (recessive) PRACTICE SQUARE # 1: Cross the first S with the first s INTERPRET SQUARE # 1: If S is dominant and s is recessive, is the first square tall or short? If your answer is tall, you are correct! Why is it tall? Because the dominant trait/capital letter/allele is in the square and the dominant trait wins, strong beats weak. PRACTICE SQUARE # 2: Repeat the same process with the next square. INTERPRET SQUARE # 2: What height is the next square? If you said short you are correct! Why is it short? Because there are only recessive traits/small letters/alleles in the square indicating there are no dominant traits and only recessive traits. Therefore, the height is short. PRACTICE SQUARE #3: Cross the first S with the second s. INTERPRET SQUARE # 3: What height is represented in this square? If your answer was tall, you are correct! Why is it tall? Because the dominant trait/capital letter/allele is in the square and the dominant trait wins, strong beats weak. PRACTICE SQUARE #4: Repeat the same process with the last square. INTERPRET SQUARE # 4: What height is the next square? If you said short you are correct! Why is it short? Because there are only recessive traits/small letters/alleles in the square indicating there are no dominant traits and only recessive traits. Therefore, the height is short. FINAL INTERPRETATION/CONCLUSION: Q: What will the final height of this organism be? A: Since 50% of the squares are tall, while 50% of the squares are short, there is a 50/50 chance the organism is tall OR short. 4. Distinguish between Purebred and Hybrid: PUREBRED - when an organism has identical alleles for a trait (ss) or (BB) HYBRID - when an organism does not have identical alleles for a trait (Bb) or (Ss) Let’s look back at our first example with the cats. We discovered the color of the offspring (kitten) was black. Would this kitten be considered a purebred or a hybrid? If you said hybrid you are correct, because the kitten does not have identical alleles (Bb in all 4 boxes). What about our tall versus short example… would this organism be considered a purebred or a hybrid for height? If you said hybrid you are correct, because the person does not have identical alleles (SS in 2 boxes and ss in 2 boxes) 5. Independent Practice: Solve each of the Punnett squares and answer the analysis questions for each trait. Remember, dominant traits (T) are stronger than recessive traits (t). TRAIT - Shape of head R - round head r - oval head * What will the shape of the head be? ____________________ * What is the probability of the head being round or oval (express it as a percentage)? ___________________________________________________________________ * Is the shape of the head a purebred or a hybrid? ______________ TRAIT - Eye color B - brown eye color b - blue eye color * What will the color of the eyes be? _________________________________________ * What is the probability of the eyes being brown or blue (express it as a percentage)? ___________________________________________________________________ * Are the color of the eyes a purebred or a hybrid? _____________________________ TRAIT - Length of eyelashes L - long eyelashes l - short eyelashes * What will the length of the eyelashes be? __________________________________ * What is the probability of the eyelashes being long or short (express it as a percentage)? ____________________________________________ * Are the length of the eyelashes purebred or a hybrid? ___________________ TRAIT - Size of nose N - large nose n- small nose * What will the size of the nose be? ___________________________________ * What is the probability of the nose being large or small (express it as a percentage)? ____________________________ * Is the size of the nose purebred or a hybrid? _________________________ TRAIT - Freckles F - no freckles f - freckles *Will there be freckles? __________________________________________ * What is the probability of there being freckles or no freckles (express it as a percentage)? __________________________________________ * Are the freckles purebred or hybrid? _______________________________ TRAIT - Hair type C - curly hair c - straight hair * Will the hair be curly or straight? __________________________________ * What is the probability of the hair being straight or curly (express it as a percentage)? __________________________________________ * Is the hair type purebred or hybrid? __________________________________ FOLLOW UP: 1. What is the only way a recessive trait can be passed onto the next generation? ______________________________________________________________________ CHALLENGE: Do you think you have more traits from your Mom or Dad? Answer the following questions with the name of the parent you believe you acquired that trait from. ● Hair color - Mom or Dad ● Eye color - Mom or Dad ● Shape of Head - Mom or Dad ● Hair Type - Mom or Dad ● Shape of Eyes - Mom or Dad Do you have more traits from one parent over another, or is it equal? _______ INDEPENDENT PRACTICE ANSWER KEY 100% of the squares will be for a round head. 100% of the eyes will be brown. 50% of the squares will be long eyelashes. 50% of the squares will be short eyelashes. 100% of the squares will be for a large nose. 100% of the squares will be for no freckles. 100% of the squares will be for curly hair.