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. During Meiosis, genes independently assort with each other.

Review: Dominant/Recessive One allele is dominant over the other (capable of masking the recessive allele) PP = purple Pp = purple pp = white

Review Problem: P P p PP Pp Pp pp GENOTYPES: - PP (1); Pp (2); pp (1) 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 PP Pp PHENOTYPES: Pp pp - 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

Incomplete Dominance: Rr = pink CRCR = red CrCr = white CRCr = pink

Problem: Incomplete Dominance Show the cross between a pink and a white flower. GENOTYPES: R r - Rr (2); rr (2) - ratio 1:1 r rr Rr 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