Solving Crosses

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Solving Crosses
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Polleverywhere.com Bell-Ringer

• http//www.polleverywhere.com/multiple_choice_poll
  s/LTExNDcxODUyMjI

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Introduction to Punnett Squares

• http//www.youtube.com/watch?vY1PCwxUDTl8

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

____ ____
____
____
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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

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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?

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Lets 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?

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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

L l
l Ll ll
l Ll ll
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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?

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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

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Practice Determining Genotypes

• SSBb x Ssbb

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Lets 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?

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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

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Lets 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?

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Example 2 Punnett Square
HB Hb hB hb
HB HHBB HHBb HhBB HhBb
Hb HHBb HHbb HhBb Hhbb
hB HhBB HhBb hhBB hhBb
hb HhBb Hhbb hhBb hhbb
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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

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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.

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• Ssbb x ssbb

Sb Sb sb Sb
sb Ssbb Ssbb ssbb Ssbb
sb Ssbb Ssbb ssbb Ssbb
sb Ssbb Ssbb ssbb Ssbb
sb Ssbb Ssbb ssbb Ssbb
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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

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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

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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

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Continuous Variation

• Multiple genes are involved
• Examples
• Eye color
• Skin color
• Hair color

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Sex-linked Genes

• Present on the X chromosome
• More common in males
• When would a female have this phenotype?
• Examples
• Baldness
• Hemophilia

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Some Human Genetic Disorders Of Interest

• Cystic Fibrosis
• Sickle-cell Anemia
• Tay-Sachs Disease
• Phenylketonuria (PKU)
• Hemophilia
• Huntingtons Disease
• Muscular Dystrophy