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Punnett Squares: Predicting Hereditary Traits with Genotypes and Phenotypes

Learn how to use Punnett squares to predict the probability of inheriting certain traits in offspring by understanding genotypes and phenotypes. Explore different scenarios and calculate the chances of specific traits being expressed.

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Punnett Squares: Predicting Hereditary Traits with Genotypes and Phenotypes

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  1. #50 Using a Punnett Square

  2. Vocabulary • Heredity- The passing on of traits from parent to offspring • Genetics- The study of heredity • Gene-Units of heredity; segments of DNA on chromosomes • Dominant-A trait that is shown when at least 1 dominant allele is present , represented by a capital letter (ex. B)“stronger trait” • Recessive-A trait that is only shown if 2 recessive alleles are inherited , represented by a lower case letter (ex. b) “weaker trait” • Allele-different forms of the same gene; represented by a letter (“B” or “b”) • Genotype-Gene makeup of an organism (RR, Rr, or rr) • Phenotype- Physical appearance (brown hair, green eyes) • Homozygous- genotypes with the same alleles; either 2 dominant or 2 recessive • Heterozygous-genotypes with different alleles; one dominant and one recessive • Punnett Square- a tool to predict the probability of inheriting traits

  3. What is a PUNNETT SQUARE? • A tool to predictthe probability of certain traits in offspring that shows the different ways the allelesof each genotypecan combine • A way to show possible phenotypes & genotypes

  4. What is a PUNNETT SQUARE? • Lettersstand for dominant and recessive alleles • An uppercaseletter stands for a dominantallele • Lowercaseletters stand for recessivealleles

  5. Using a PUNNETT SQUARE To set up a Punnett square, draw a large square, and then divide it into 4 equal sections (also squares). It should look something like this:

  6. Using a PUNNETT SQUARE Now you need two parents to mate, ones with a known genotype For example, a red flower (genotype Rr) and a white flower (genotype rr). Rr x rr

  7. Using a PUNNETT SQUARE Place one of the parents on top, and one on the left. You should get a something similar to this:

  8. Using a PUNNETT SQUARE Finally, take each letter in each column and combine it with each letter from each row in the corresponding square. You should now have a picture close to this:

  9. Using a PUNNETT SQUARE • The two-letter combinations are the possible genotypes of offspring • They are: Rr, Rr, rr, and rr genotypes • From this it is possible to determine the probability (chance) that a flower will have a red phenotype (2/4 or 50%) or a white phenotype (2/4 or 50%)

  10. 1. Cross a homozygous dominant tall plant with a homozygous recessive short plant • Recessive trait = short • What letters would you use to represent this cross? • Dominant trait = tall • T = tall • t = short

  11. TT • What is the genotype of a homozygous recessive short plant? • What is the genotype of a homozygous dominant tall plant? • tt • TT x tt • Genotype % TT % Tt % tt 0 t t 100 Tt Tt T T 0 • Phenotype % tall % short 100 Tt Tt 0

  12. Dominant trait = brown hair #2. Cross a heterozygous brown haired rabbit with a homozygous recessive white haired rabbit. • Recessive trait = white hair • What letters would you use to represent this cross? • B = brown • b = white

  13. Bb • What is the genotype of a homozygous recessive white haired rabbit? • What is the genotype of a heterozygous brown haired rabbit? • bb • Bb x bb • Genotype % BB % Bb % bb 0 b b 50 Bb Bb B b 50 • Phenotype % brown % white 50 bb bb 50

  14. 3. Cross a homozygous dominant long haired guinea pig with a heterozygous long haired guinea pig. • Dominant trait = long hair • Recessive trait = short hair • What letters would you use to represent this cross? • L = long • l = short

  15. LL • What is the genotype of a heterozygous long haired guinea pig? • What is the genotype of a homozygous dominant long haired guinea pig? • Ll • LL x Ll • Genotype % LL % Ll % ll 50 L l 50 LL Ll L L 0 • Phenotype % long % short 100 LL Ll 0

  16. 4. Cross a heterozygous wrinkly elephant with another heterozygous wrinkly elephant. • Dominant trait = wrinkly skin • Recessive trait = smooth skin • What letters would you use to represent this cross? • W = wrinkly • w = smooth

  17. Ww • What is the genotype of a heterozygous wrinkly-skinned elephant? • What is the genotype of a heterozygous wrinkly-skinned elephant? • Ww • Ww x Ww • Genotype % WW % Ww % ww 25 W w 50 WW Ww W w 25 • Phenotype % wrinkly % smooth 75 Ww ww 25

  18. A plant that is homozygous dominant for round seeds is crossed with a heterozygous plant. What is the probability that the offspring will have wrinkled seeds? • R = round • r = wrinkled RR x Rr • Genotype % RR % Rr % rr 50 R r 50 RR Rr R R 0 • Phenotype % round % wrinkled 100 RR Rr 0

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