I NCOMPLETE D OMINANCE AND C O -D OMINANCE N OTES

I NCOMPLETE D OMINANCE AND C ODOMINANCE 2

I NCOMPLETE D OMINANCE 3 Recall in Mendel’s pea-plant crosses, one allele was completely dominant over another but in the world of genetics things aren’t always so clear-cut & “simple”. In some organisms, an individual can display a trait that is intermediate between two parents. Neither is dominant. A cross between organisms with two different phenotypes produces offspring with a third phenotype that is a blending of the parental traits It often appears to be a mixture of the two traits. (Heterozygous) rr

It’s like mixing paints, + will make. We end up with something in-between. We can still use the Punnett Square to solve problems involving incomplete dominance. The only difference is that the letters we use are both lowercase because neither trait dominates the other. This condition is known as incomplete dominance. RedWhitePink

I NCOMPLETE DOMINANCE The most common example of incomplete dominance is a snapdragon’s flower color. Example: snapdragons (flower) red (rr) x white (ww) Alleles: r = red flower w = white flower r r ww

W HAT HAPPENS WHEN … A red snapdragon crosses with a white snapdragon? Notice: There is only one trait, but the alleles are coded for by two different letters. Neither is dominant, neither will be capitalized. red (rr) x white (ww) Trait: flower color Alleles: rr = red flower ww = white flower r r ww w w w w r r r r All of the offspring are “rw” What is the phenotype?

PINK!

H OW DOES THIS HAPPEN ? 8

S O WHAT WILL THE STEPS LOOK LIKE ? Step 1: Trait(s) Step 2: Alleles Step 3: Cross Step 4: Punnett Step 5 Genotype Step 6 Phenotype

C ROSS A RED SNAPDRAGON WITH A WHITE SNAP DRAGON. Step 1: Trait(s): flower color Step 2: Alleles: rr = red, ww=white, rw=pink Step 3: Cross: rr x ww Step 4: Punnett Step 5 Genotype rr, red – 0 ww, white – 0 rw, pink – 4 (100%) 0:0:4 Step 6 Phenotype Red – 0 White – 0 Pink – 4 (100%) 0:0:4 r r ww rwrwrwrw rwrwrwrw

H OW DO YOU KNOW IF IT IS AN INCOMPLETE DOMINANCE PROBLEM ? There will be clues in the word problem. Example: “Neither trait is dominant” “blending occurs” “a new phenotype appears” You will have only one trait but different letters will represent the single trait. The offspring will show a 3 rd phenotype. The parents each have one, and the offspring are different from the parents. The trait in the offspring will be a blend (mix) of the parental traits

O THER INCOMPLETE DOMINANT TRAITS … Hair texture is also considered to be incompletely dominant. Straight hair is homozygous Curly hair is homozygous But neither is dominant over the other. Wavy hair is heterozygous and is intermediate between straight and curly hair. In Humans…

S HORT HORN CATTLE ARE THE SAME. The coat color of short horn cattle can be red, white or roan. rr=red, ww=white, rw=roan Cross a roan male with a white female.

C ROSS A ROAN MALE ( BULL ) WITH A WHITE FEMALE SHORT HORN. Step 1: Trait(s) Step 2: Alleles Step 3: Cross Step 4: Punnett Step 5 Genotype Step 6 Phenotype

C ODOMINANCE A number of traits are the result of 2 types of alleles that are equally dominant. Such traits are said to be codominant for that trait. The prefix “co” is “together”. Ex. Cooperate = work together, Coexist= exist together Cohabitate=habitat together

C ODOMINANCE When an individual is heterozygous for such traits, the resulting phenotype or expression of these two traits is a combination, because both traits are expressed equally. This creates a 3 rd phenotype. Both of the parents’ traits can be seen. The alleles in this case will both be represented by capital letters.

C ODOMINANCE APPEARS AS … Spots Stripes

In some chickens, the gene for feather color is controlled by codominance. The allele for black is B (i B ) and the allele for white is W (i W ). The heterozygous phenotype is known as erminette-BW (i BW )

S O WHAT WILL THE STEPS LOOK LIKE ? Step 1: Trait(s) Step 2: Alleles Step 3: Cross Step 4: Punnett Step 5 Genotype Step 6 Phenotype

Step 1: Trait(s) - Color Step 2: Alleles – B (Black), W (White), BW (Erminette) Step 3: BB x WW Step 4: Punnett Step 5 Genotype Step 6 Phenotype i B BW-4 (4:0, 100%) Erminette-4 (4:0, 100%) i W i B i W i B i W Cross a black rooster with a white hen.

E XAMPLES In horses, chestnut and white coat colors are codominant. Heterozygous horses have a blend of both colors, which is a golden tan color. Such heterozygous horses are known as palominos.

Step 1: Trait(s) - Step 2: Alleles – Step 3: Cross - Step 4: Punnett Step 5 Genotype Step 6 Phenotype i i i i i Cross a chestnut with a palomino.

B ALL P YTHONS These reptiles have an extensive list of recessive, dominant and co-dominant traits! RECESSIVE: Albino Axanthic Caramel Albino Clown Genetic Stripe Ghost (Hypo) Lavender Albino Piebald (Pied) DOMINANT: Pinstripe Spider CO-DOMINANT: Butter – Blue Eyed Leucistic Cinny – Super Cinny Enchi – Super Enchi Fire Ball – Black Eyed Leucistic Lesser – Blue Eyed Leucistic Mojave – Blue Eyed Leucistic Pastel – Super Pastel Yellow Belly – Ivory