incomplete dominance & codominance section 8-4 in book pgs. 177- 179
TRANSCRIPT
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Incomplete Dominance & Codominance
Section 8-4 in Book Pgs. 177-179
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Review of Mendel’s Principles
Genes are passed parents offspring; get one allele from each parent
During Meiosis, the alleles for a gene segregate from each other.
During Meiosis, genes independently assort with each other.
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Exceptions to Mendel’s principles
Sometimes, there is no dominant or recessive gene, or the trait is controlled by many alleles or genes.
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In Incomplete Dominance, every genotype has its own phenotype. (One allele not completely dominant over the other.) Third phenotype that is a blending of the parental traits. (2 alleles produce 3 phenotypes.) Result: Heterozygous phenotype somewhere in
between homozygous phenotype.
Incomplete Dominance
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1. Incomplete Dominance
Examples: Trait: Flower Color
Expressions: Red x White Pink
RR= Red; RW= pink; WW= white
straight hair, wavy, curly
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Incomplete Dominance
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In codominance, neither allele are dominant; both are expressed. A cross between organisms with two different phenotypes produces offspring with both phenotypes of the parental traits shown.
Codominance
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2. Codominance Both alleles contribute to the phenotype.
Example: In come chickens
Black Chicken x White Speckled Chicken
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YOU tell me which type of dominance…
Codominance!
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Type of Dominance?
Incomplete Dominance!
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Type of Dominance?
Incomplete Dominance!
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Test Cross (Back Cross)…
Used to determine whether an individual is homozygous dominant or heterozygous.
RULES: Always cross unknown genotype with a homozygous
recessive. Observe (count) large numbers of offspring to insure
accuracy in determining the unknown genotype. Then…
if ANY offspring show the recessive trait, the unknown genotype is heterozygous
if ALL the offspring have the dominant trait, the unknown genotype is homozygous dominant
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Example:
TT or Tt You would cross both with tt. What would you expect the outcome to
be?