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#50 Using a Punnett Square

Learn how Punnett squares predict offspring traits with genotype combinations based on dominant and recessive alleles.

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#50 Using a Punnett Square

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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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