Solving Crosses

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Solving Crosses
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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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Example 1 Monohybrid

• 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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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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Example 2 Dihybrid

• In guinea pigs, the allele for short hair (S) is
  dominant to long hair (s), 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 Dihybrid

• Short hair dominant SS or Ss Long Hair
  recessive ss Black coat dominant
  BB or Bb Brown coat recessive bb
• SsBb x SsBb (gametes done by the FOIL method)
• SB, Sb, sB, sb and SB, Sb, sB, sb

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Example 2 Punnett Square
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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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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Incomplete Dominance

• blending of traits in heterozygote trait is
  controlled by both alleles
• Pink flowers
• RR red
• Rr pink
• rr white
• Ex Japanese 4 oclock (Mirabilis)
• RR X WW RW (pink)

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Codominance

• can see both alleles at the same time.
• ex Roan coats in horses
• Some white hairs, some red hairs
• ex erminette
• Black white speckeld chickens heterozygotes
• ex human protein for cholesterol level
• Heterozygotes produce 2 forms of the protein

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Multiple Alleles -still only 2 alleles, but more
possibilities

• 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/Polygenic Traits

• Multiple genes are involved in an interaction of
  these genes to produce a trait
• Examples
• _at_ least 3 genes involved in the reddish-brown
  pigment in the eyes of fruitflies
• Skin color, wide range in humans produced by more
  than 4 genes
• different combos of alleles very different
  phenotypes

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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