Genetic%20Crosses

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Genetic Crosses

• How to predict the probable genetic makeup and
  appearance of offspring resulting from specified
  crosses.

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Genotype

• The genetic makeup of an organism. Consists of
  the alleles that the organism obtains from its
  parents. Ex. Blue eyes is bb and brown eyes
  could be BB or Bb.

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Phenotype

• The appearance of an organism as a result of its
  genotype. What the characteristic of the organism
  looks like. i.e. does it have blue eyes or brown
  eyes.

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Homozygous

• When both alleles of a pair are alike. An
  organism may be homozygous dominant (BB) or
  homozygous recessive (bb).

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Heterozygous

• When the two alleles in the pair are different.
• Ex. Bb

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Predicting Results

• Monohybrid cross A cross between individuals
  that involves 1 pair of contrasting traits. Ex. A
  cross between a blue eyed person and a brown eyed
  person. A Punnett square is used.

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Homozygous X Homozygous

• A monohybrid cross between a person homozygous
  for brown eyes and a person homozygous for blue
  eyes.

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Homozygous X Heterozygous

• A monohybrid cross between a person homozygous
  for blue eyes and a person heterozygous for brown
  eyes.

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Heterozygous X Heterozygous

• A monohybrid cross between a person heterozygous
  for brown eyes and a person heterozygous for
  brown eyes.

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Genotypic Ratio

• The ratio of the genotypes that appear in
  offspring.

• Homozygous X homozygous 4 Bb
• Homozygous X Heterozygous
• 2 Bb 2bb
• Heterozygous X heterozygous 1BB2Bb1bb

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Phenotypic Ratio

• The ratio of the offsprings phenotypes or what
  they look like.

• Homozygous X homozygous 4 brown
• Homozygous X Heterozygous
• 2 brown and 2 blue
• Heterozygous X heterozygous 3 brown and 1 blue

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Incomplete Dominance

• When offspring have a phenotype that is in
  between that of the parents. Ex. Four oclock
  flowers. RR is red, Rr is pink and rr is white.

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Four OClock Flowers
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Codominance

• Occurs when both alleles for a gene are expressed
  in a heterozygous offspring. Neither allele is
  dominant or recessive nor do they blend in the
  phenotype. Ex. The color roan coat color. The
  genes for both white coat and red coat are
  expressed.

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Roan Coat Color
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Dihybrid Crosses

• A cross between individuals that involves two
  pairs of contrasting traits.

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Homozygous X Homozygous

• This Punnett square shows a dihybrid cross
  between a pea plant that is homoozygous for green
  pods and yellow seeds (GGYY) and a pea plant that
  is homozygous for yellow pods and green seeds
  (ggyy).

• Pod color
• G (green) and
• g (yellow)
• Seed color
• Y (yellow) and
• y (green)

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Dihybrid Cross

• All plants will be have the same genotype.
• Heterozygous for both traits. (GgYy).
• All plants will have the same phenotype. All will
  be green pods and yellow seeds.

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Heterozygous X Heterozygous

• This Punnett square shows a dihybrid cross
  between a pea plant that is heterozygous for
  green pods and yellow seeds (GgYy) and a pea
  plant that is heterozygous for green pods and
  yellow seeds (GgYy).

• Pod color
• G (green) and
• g (yellow)
• Seed color
• Y (yellow) and
• y (green)

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Dihybrid Cross

• 9 different genotypes
• 9/16 green pods yellow seeds. 3/16 green
  pods, green seeds. 3/16 yellow pods, yellow
  seeds. 1/16 yellow pods green seeds.
• 4 different phenotypes.
• 9 Green pods, yellow seeds
• 3 Green pods, green seeds
• 3 Yellow pods,yellow seeds
• 1 Yellow pods, green seeds