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Genetics: Incomplete Dominance & Codominance. Biology 12. Joke of the day:. 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.
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Genetics: Incomplete Dominance & Codominance Biology 12
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.
PP = purple Pp = purple pp = white Review: Dominant/Recessive • One allele is dominant over the other (capable of masking the recessive allele)
PP Pp Pp pp Review Problem: • In pea plants, purple flowers (P) are dominant over white flowers (p) show the cross between two heterozygous plants. P p GENOTYPES: - PP (1); Pp (2); pp (1) - ratio 1:2:1 P p PHENOTYPES: - purple (3); white (1) - ratio 3:1
Exceptions to Mendel’s principles • Some alleles are neither dominant nor recessive. • Many traits are controlled by more than one gene (polygenic traits)
Incomplete dominance: • A situation in which neither allele is dominant. • When both alleles are present a “new” phenotype appears that is a blend of each allele. • The heterozygous phenotype is somewhere in between homozygous phenotype. • 2 alleles produce 3 phenotypes
CRCR = red CrCr = white CRCr = pink Incomplete Dominance: Rr = pink
R r r r Rr Rr rr rr Problem: Incomplete Dominance • Show the cross between a pink and a white flower. GENOTYPES: - Rr (2); rr (2) - ratio 1:1 PHENOTYPES: - pink (2); white (2) - ratio 1:1
Codominance: • Neither allele are dominant; both are expressed. • A cross between organisms with two different phenotypes produces offspring with has both phenotypes of the parental traits shown.
Example: codominance • In some chickens: Black Chicken x White Speckled Chicken
Example: • Roan coat color in horses and cattle are codominant for the alleles R and r. • Homozygous R individuals are red (or bay) • Homozygous r individuals are white • While heterozygous Rr individuals are roan (which means they have both white and red hair, giving them a lightened appearance.
What about the F2 generation? • Phenotype? • 1 red, 2 roan and 1 white • 1:2:1 • Genotype? • 1Hr Hr, 2 HrHw and 1Hw Hw
Which type of dominance… Codominance!
To do: • Page 145: questions 2, 3, 4, 5 • Practise sheet: Codominance and Incomplete dominance • Colouring sheet