Incomplete Dominance & Co-dominance

Review of Mendelian Genetics Law of Independent Assortment Alleles of genes on nonhomologous chromosomes assort independently during gamete formation. Law of Segregation The two alleles for each gene separate during gamete formation.

Mendelian Genetics: Dominant & Recessive Review Mendel’s experiments demonstrated complete dominance - offspring always resembled one of the two parents One allele is DOMINANT over the other (because the dominant allele can “mask” the recessive allele) Purple Purple White

Mendel continued All Mendel’s plants displayed dominant or recessive alleles. In other words, only one trait showed up.

There are some crosses where both alleles of a pair are expressed in a heterozygote Snapdragons, an example of incomplete dominance HUH? http://www.dobermann-review.com/info/genetics/mendels_genetic_laws/Gregor%20Mendel.jpg

Non-Mendelian Genetics Incomplete Dominance Codominance Multiple Alleles Polygenic Traits Sex-Linked Traits

Incomplete Dominance a third (new) phenotype appears in the heterozygous condition as a BLEND of the dominant and recessive phenotypes. Neither allele is completely dominant over the other allele Result: A heterozygous phenotype Example: Red (FR) x White (FW) = Hybrid Pink (FRFW)

Neither Red (FR) or White (FW) is dominant over the other. example: when a homozygous red flower (FRFR) is crossed with a homozygous white flower (FWFW), the alleles blend to make a hybrid (FRFW) with pink flowers FRFR = red FWFW= white FRFW = pink

= Blue- Feathered (CBCW) Andalusian Chickens Incomplete dominance Neither Black (CBCB) nor White (CWCW) are dominant black-feathered (CBCB) x white-feathered (CWCW) = Blue- Feathered (CBCW)

Incomplete Dominance Example Questions: In snapdragons, colour has incomplete dominance. If you cross a purebred red snapdragon (FRFR) with a purebred white (FWFW) snapdragon what are the phenotypic and genotypic ratios? What are the phenotypic and genotypic ratios for the F2 generation of the example? Indicate the ratios that would be expected if the following crosses are made: a) red x pink b) white x white c) pink x pink

Codominance Two equally dominant alleles are expressed at the same time. Heterozygous phenotype will have both phenotypes visible

Codominance Example #1: Speckled Chickens CBCB= black feathers CwCw = white feathers CBCw = black & white speckled feathers Notice – NO GRAY! NO BLEND! Each feather is either black or white

Codominance Example #2: Rhodedendron RRRR = allele for red flowers RwRw= allele for white flowers Cross a homozygous red flower with a homozygous white flower.

Codominance Example #3: Roan cattle Cattle can be: red (CRCR – all red hairs) white (CwCw – all white hairs) roan (CRCw – red and white hairs together)

Codominance Example #4: Appaloosa horses Gray horses (HGHG) are codominant to white horses (HwHw). The heterozygous horse (HGHw) is an Appaloosa (a white horse with gray spots). Question: Cross a white horse with an appaloosa horse.

Codominance Example Questions: A red cow (CRCR) was crossed with a roan cow (CRCw). What proportion of the offspring would be roan? If you cross 2 roan cows (CRCw), what proportion of the offspring will be white cows?