Non-Mendelian Genetics: Incomplete Dominance and Co-Dominance

WHAT COLOR OFFSPRING WOULD YOU GET WHEN YOU CROSS A PURE (HOMOZYGOUS) RED SNAPDRAGON WITH A PURE WHITE SNAPDRAGON?

ODDLY ENOUGH, NEITHER RED NOR WHITE Snapdragon flower color is controlled by Incomplete Dominance and a new 3rd phenotype is seen.

INCOMPLETE DOMINANCE IS A BLENDING Like Paint, the RED Pigment “MIXES” with the WHITE to create PINK-FLOWERED offspring

INCOMPLETE DOMINANCE CAN BE SEEN IN: → Human hair → Horses + Chestnut Cremello Palomino Many flowers Human hair → + Curly Straight Wavy

ALLELE NOTATION HOW DO YOU WRITE OUT THE GENOTYPE WHEN BOTH ALLELES ARE DOMINANT?

YOU MUST USE DIFFERENT LETTERS Choose different letters to represent each form (In this case we’ll use “W” for the white allele and “R” for the Red allele). Use CAPITAL letters. This is INCOMPLETE DOMINANCE. RR WW RW

SO…THE CROSS BETWEEN PURE SNAPDRAGONS LOOKS LIKE THIS RW RW W RW RW W

RW 1 RR : 2 RW : 1 WW 1 Red : 2 Pink : 1 White YOU’VE SEEN A CROSS BETWEEN PURE SNAPDRAGONS, NOW DO THE PUNNETT SQUARE FOR A CROSS BETWEEN TWO HYBRIDS (HETEROZYGOTES). What are the alleles of a heterozygous snapdragon? What is the ratio of genotypes? What is the ratio of phenotypes? RW 1 RR : 2 RW : 1 WW 1 Red : 2 Pink : 1 White

WHAT KIND OF GENETICS DO THESE ORGANISMS EXHIBIT?

CODOMINANCE → The same ratios as Incomplete Dominance occur: A ratio of 1:2:1 for both genotype and phenotypes of a monohybrid cross → Different Phenotype: The two original phenotypes are combined to give a SPOTTED or MULTICOLORED phenotype.

NOTATION FOR CODOMINANCE Choose a letter to represent the trait. Choose different letters to represent each of the alleles. Take the letters representing the alleles and turn them into superscripts. Combine the two. Remember to use capital letters.

WRITING OUT THE GENOTYPE We’re going to use C to represent color of a Four O’Clock flower. We can choose “P” to represent pink and “Y” to represent yellow. Spotted flowers can be a combination of pink and yellow. So, we write the genotypes as: CPCP CYCY CPCY

CODOMINANCE EXAMPLES:

LET’S PRACTICE Tabby cats exhibit codominance between tan and black fur. Can a tan cat be heterozygous? Why or Why not? If you mate a tabby with a black cat, what is the ratio of phenotypes in their offspring? Draw the Punnett square to provide evidence for your answer.

A cross between 2 tabbies (the heterozygotes - CBCT) results in 1 black : 2 tabbies : 1 tan cat CBCB CBCT CBCT CTCT

Set up a Punnett Square showing Incomplete Dominance between straight hair and curly hair.

Set up a Punnett Square showing CoDominance between Red Camelias and White Camelias.

What did we learn today?