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Solving Crosses

Solving Crosses. Polleverywhere.com Bell-Ringer. http://www.polleverywhere.com/multiple_choice_polls/LTExNDcxODUyMjI. Introduction to Punnett Squares. http://www.youtube.com/watch?v=Y1PCwxUDTl8. Punnett squares - probability diagram illustrating the possible offspring of a mating. Ss X Ss.

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Solving Crosses

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  1. Solving Crosses

  2. Polleverywhere.com Bell-Ringer • http://www.polleverywhere.com/multiple_choice_polls/LTExNDcxODUyMjI

  3. Introduction to Punnett Squares • http://www.youtube.com/watch?v=Y1PCwxUDTl8

  4. Punnett squares - probability diagram illustrating the possible offspring of a mating. Ss X Ss gametes

  5. Steps for Solving a Genetics Problem: • Trait – dominant = A (AA or Aa) Trait – recessive = a (aa) • ___________ x ___________ • Punnett Square • Answer questions based on results from Punnett Square

  6. Monohybrid Crosses • Cross that involves one pair of contrasting traits • Solve using Punnett Square • Sample problems: • Rr x rr • RR x rr • Rr x Rr • Rr x RR

  7. Ratios • Genotypic Ratio: the probability that you will see an individual with a certain genotype • Ex. PP : Pp : pp  What is genotypic ratio? • Phenotypic Ratio: the probability that you will see an individual with a certain phenotype • Ex. Purple Flowers : White Flowers  What is the phenotypic ratio?

  8. Let’s Solve Together • Short hair (L) is dominant to long hair (l) in mice. What is the genotype and phenotype ratio of a heterozygous short-haired mouse crossed with a long-haired mouse?

  9. Example 1: Monohybrid • Short hair = dominant = L (LL or Ll) long hair = recssive = l • Ll x ll (heterozygote parent = Ll) • Punnett Square: • Genotype ratio: ½ Ll: ½ ll • Phenotype ratio: ½ short hair: ½ long hair

  10. Monohybrid Practice • In mice, Brown hair (B) is dominant to black hair (b). Researchers crossed a homozygous dominant brown-haired mouse with a black-haired mouse. What are the genotypic and phenotypic ratios for this cross?

  11. Dihybrid Crosses • Involves two pairs of contrasting traits • Pea shape and pea color • Coat length and coat color in rodents • Plant height and flower color

  12. Practice Determining Genotypes • SSBb x Ssbb

  13. Let’s Solve Together • In guinea pigs, the allele for short hair (H) is dominant to long hair (h), and the allele for black hair (B) is dominant over the allele for brown hair (b). What is the probable offspring phenotype ratio for a cross involving two parents that are heterozygotes for both traits?

  14. Determining Dihybrid Genotypes • Short hair = dominant = HH or Hh Long Hair = recessive = hh Black coat = dominant = BB or Bb Brown coat = recessive = bb • HhBb x HhBb (gametes done by the FOIL method) • HB, Hb, hB, hb and HB, Hb, hB, hb

  15. Let’s Solve Together • In guinea pigs, the allele for short hair (H) is dominant to long hair (h), and the allele for black hair (B) is dominant over the allele for brown hair (b). What is the probable offspring phenotype ratio for a cross involving two parents that are heterozygotes for both traits?

  16. Example 2: Punnett Square

  17. Example 2: Answer the Question • What is the probable offspring phenotype ratio for a cross involving two parents that are heterozygotes for both traits? • 9/16 Black, short coats • 3/16 Black, long coats • 3/16 Brown, short coats • 1/16 Brown, long coats

  18. Practice Problem • What are the probable offspring for a cross involving a heterozygous short, brown haired guinea pig and a long, brown haired guinea pig? • Show your Punnett Square and the percentage of individuals with the different phenotypes.

  19. Ssbb x ssbb

  20. Other Types of Heredity Patterns • Incomplete Dominance – blending of traits in heterozygote. • Pink flowers • RR = red • RW = pink • WW = white • RW x RW Show Punnett Square

  21. Other Types of Heredity Patterns • Codominance – can see both alleles at the same time. • Roan coats in horses • RR= red hair • RW= some red and some white hairs (roan) • WW= white hair • RW x RW

  22. Multiple Alleles • Blood Types in Humans • Single gene, but four phenotypes • Type A  can be AA or Ao • Type B  can be BB or Bo • Type AB  only AB (codominant pattern here) • Type O  only oo (both recessive) • All 3 blood types are dominant to O

  23. Continuous Variation • Multiple genes are involved • Examples • Eye color • Skin color • Hair color

  24. Sex-linked Genes • Present on the X chromosome • More common in males • When would a female have this phenotype? • Examples: • Baldness • Hemophilia

  25. Some Human Genetic Disorders Of Interest • Cystic Fibrosis • Sickle-cell Anemia • Tay-Sachs Disease • Phenylketonuria (PKU) • Hemophilia • Huntington’s Disease • Muscular Dystrophy

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