Genetic Crosses

1
Genetic Crosses

• Section 9.2

2
Genotype

• The genetic makeup of an organism
• Consists of the alleles that the organism
  inherits from its parents
• Example white flowering pea plants
• (recessive trait pp)

3
Phenotype

• The appearance of an organism as a result of its
  genotype
• Example pea plants that are PP or Pp
• will have purple flowers

4
Homozygous

• Both alleles of a pair are alike
• May be homozygous dominant (PP) or homozygous
  recessive (pp)

5
Heterozygous

• The two alleles in the pair are different
• Example a pea plant that is heterozygous for
  flower color
• would be Pp (purple flowers)

6
Probability

• The likelihood that a specific event will occur
• May be expressed as a decimal, a percentage, or a
  fraction

7

• Probability number of times an event is
• expected to
  happen_____________
• number of
  opportunities for an event
• to happen

8

• Example yellow pea seeds appeared 6022 times and
  
• green pea seeds appeared 2001
  times in the F2
• Total (6022 2001 8023)
• What is the probability that the dominant trait
  will appear in a similar cross?
• P 6022
  .75 (or 75 or ¾ )
• 8023
  

9
Monohybrid Cross

• A cross between individuals that involves one
  pair of contrasting traits
• Example crossing a pea plant that is pure for
  white flowers (pp) with one that is pure for
  purple flowers (PP)

10
Punnett Square

• Diagram used to predict the outcome of certain
  crosses
• Predicts the probability of inherited traits

11
Homozygous X Homozygous

• white flower
• (pp)
• p p
• purple
• flower (PP) P Pp Pp
• P Pp Pp

12
Homozygous X Heterozygous

• black coat
• (Bb)
• B b
• black coat
• (BB) B BB Bb
• B BB Bb

13
Heterozygous X Heterozygous

• black coat
• (Bb)
• B b
• black coat
• (Bb) B BB Bb
• b Bb bb

14
Testcross

• An individual of an unknown genotype is crossed
  with a homozygous recessive individual
• Can be used to determine the genotype of any
  individual whose phenotype is dominant
  (example BB or Bb)

15
Incomplete Dominance

• Occurs when two or more alleles influence the
  phenotype
• Example four oclock flowers
• RR red
• Rr pink
• rr white

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Incomplete Dominance in Four Oclock Flowers

• Pink flower
• (Rr)
• R r
• Pink flower
• (Rr) R RR Rr
• r Rr rr
• Phenotypic ratio 1 red 2 pink1 white

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Codominance

• Occurs when both alleles for a gene are expressed
  in a heterozygous offspring
• Neither allele is dominant or recessive
• Example roan horses R red coat
• (RR) R white

18
Dihybrid Cross

• A cross between individuals that involves two
  pairs of contrasting traits
• Must consider how the four alleles from each
  parent can combine

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Homozygous X Homozygous

• wrinkled green
• 
  (rryy)
• ry ry
  ry ry
• round RY RrYy RrYy RrYy
  RrYy
• yellow RY RrYy RrYy
  RrYy RrYy
• (RRYY) RY RrYy RrYy
  RrYy RrYy
• Ry RrYy RrYy
  RrYy RrYy

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Heterozygous X Heterozygous

• round yellow
• (RrYy)
• RY Ry
  rY ry
• round RY RRYY RRYy RrYY RrYy
• yellow Ry RRYy RRyy RrYy
  Rryy
• (RrYy) rY RrYY RrYy rrYY
  rrYy
• ry RrYy Rryy
  rrYy rryy

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Four Phenotypes

• 9/16 round yellow
• (genotypes RRYY, RRYy, RrYY, and RrYy)
• 3/16 round green
• (genotypes RRyy and Rryy)
• 3/16 wrinkled yellow
• (genotypes rrYY and rrYy)
• 1/16 wrinkled green
• (genotype rryy)

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STUDY!

• STUDY! STUDY! STUDY! STUDY!
• STUDY! STUDY! STUDY! STUDY!
• STUDY! STUDY! STUDY! STUDY!
• STUDY! STUDY! STUDY! STUDY!
• STUDY! STUDY! STUDY! STUDY!