Punnett Squares: Codominance and Incomplete Dominance

Expressed means… The gene is “on” or “activated” and the protein will be produced. We observe expressed genes as phenotypes. Hair Color Expressed Eye Color Expressed

Phenotype for Eye Color Subject A Subject B Subject C Genotype for Eye Color BB Bb bb Phenotype for Eye Color What eye color gene is expressed? What eye color is unexpressed? What is Ryan Reynolds’ phenotype for eye color?

Phenotype for Eye Color Subject A Subject B Subject C Genotype for Eye Color BB Bb bb Phenotype for Eye Color The examples we have seen so far are simple: you either have brown eyes or blue eyes. It is often more complicated than just one gene being dominant over another. Sometimes, genes are considered codominant. This means both alleles are expressed equally; the phenotype will be a combination of both alleles.

Blood types are a great example of co-dominance! Red blood cell Blood types are a great example of co-dominance! Your blood type results from a protein that is found in the membrane of your red blood cells. If you have gene IA, then you make protein A. If you have gene IB, then you make protein B. If you have gene i, then you do not make a protein. If you have both gene IA and gene IB, then you make BOTH proteins. These alleles are co-dominant because they are both always expressed! 5

Blood types are Codominant If a person has genotype IAIA, their blood type (phenotype) will be ______ If a person has genotype IAi, their blood type (phenotype) will be ______ If a person has genotype IBIB, their blood type (phenotype) will be ______ If a person has genotype IBi, their blood type (phenotype) will be ______ If a person has genotype ii, their blood type (phenotype) will be ______ If a person has genotype IAIB, their blood type (phenotype) will be ______ A A B B O AB 6

Codominance Examples % A = _____ % O = _____ 100% % A = _____ IA i IA i IA IA IA i IA IB i IA IB IB IAIA IAi IAIB IBi IAIA IAIA IAIB IBi IAIA IAi IAi ii IAIB IAIB IAIB IBi % A = _____ % O = _____ 100% % A = _____ % B = _____ % O = _____ % AB = _____ 25% % A = _____ % B = _____ % AB = _____ 50% % A = _____ % B = _____ % O = _____ % AB = _____ 0% 0% 25% 0% 50% 25% 50% 0% 25% 50% 7

More Codominance Examples IB i i i IA IA IA i IB i IA IB IAi IAi i IA IB IBIB IBi IAi IAi IAIA IAi IBi ii IBi IBi IAi IAi IAIB IBi % B = _____ % O = _____ 75% % A = _____ % B = _____ % O = _____ % AB = _____ 50% % A = _____ % O = _____ 100% % A = _____ % B = _____ % O = _____ % AB = _____ 50% 25% 0% 50% 25% 0% 0% 0% 25% 8

Incomplete Dominance R R r R r → RR rr Rr As we have seen, not all gene combinations are as simple as having one dominant gene and one recessive gene. Based on the diagram, how would you define incomplete dominance? R R r R r → X RR rr Rr

R r R r R R R r R r r r Incomplete dominance Occurs when a combination of genes produces an intermediate of the two traits. One trait is not completely dominant, so the heterozygous phenotype is a mixture. R r R r R R R r R r r r

Animals, like these horses, often exhibit incomplete dominance in the color of their hair or skin. This produces a range of shades from light to dark. Photo by David Blaikie

Dark Brown D d D d DD Dd Palomino Dd dd Cream

Dark Brown d d D d Dd Dd Palomino dd dd Cream

R R r R r R r R r R r 100% pink flowers Examining Incomplete Dominance 1.A species of flowers exhibits incomplete dominance between the allele for red flowers and the allele for white flowers. If a plant homozygous for red flowers is crossed with a plant that is homozygous for white flowers, what percentage of offspring will have pink flowers? R r R r KEY: 100% pink flowers

R r R r R R R r R r r r 25% red flowers 50% pink flowers Examining Incomplete Dominance R r 2. If two heterozygous, pink plants are crossed, what percentage of each color of flower will appear in the offspring? R r R R R r R r r r 25% red flowers 50% pink flowers 25% white flowers KEY:

Genotype for Flower Color Phenotype for Flower Color Complete the following Punnett squares about incomplete dominance and answer the questions go with each cross. Subject A Subject B Subject C Genotype for Flower Color RR Rr rr Phenotype for Flower Color Red Pink White r r R r R r R R R r Rr Rr R r RR Rr r Rr rr R r RR RR rr rr Rr rr Rr rr Rr Rr 50% 3 50% 50%

Codominance vs. Incomplete Dominance Assume each chicken below is the results of a cross between a parent with black feathers and a parent with white feathers. Based on the definitions we have learned, which breed shows codominance between feather color and which shows incomplete dominance. Chicken with Gray Feathers Chicken with Black & White Feathers Incomplete Dominance An intermediate trait appears in the offspring. Codominance Both traits appear in the offspring.