Test crosses, Incomplete Dominance, and Codominance Genetic Exceptions Test crosses, Incomplete Dominance, and Codominance

Test Cross Used to determine an unknown dominant genotype. Offspring that show a dominant phenotype could have either a homozygous dominant (RR) or heterozygous (Rr) genotype… How can the second allele be determined?

Test cross example In mice, black (R) is dominant to white (r) Take a black mouse with an unknown dominant genotype and cross it with a homozygous recessive (rr) white mouse, then observe the color of the offspring. R ? R ? Rr r r Rr R r r r If any offspring are white, the unknown allele is ______. No white offspring then unknown allele is _____. R r

Incomplete Dominance According to Mendel’s Law of Dominance; if dominant allele is present then the dominant trait will be expressed. However...there are always exceptions to the rules!!! Sometimes in the offspring; both the dominant(R) and recessive(r)determine phenotype Phenotype that results is a “blending effect” of the dominant and recessive proteins. Alleles themselves do not combine. Two pieces of DNA, the genes, cannot combine.

MICROSCOPIC PETALS PANSY PETAL ROSE PETAL COMPLETE DOMINANCE INCOMPLETE DOMINANCE PIGMENT CELLS ALL THE SAME PROTEIN PIGMENT CELLS MORE THAN ONE PROTEIN PANSY PETAL ROSE PETAL

Example: Flower Color R R R r Rr Rr R r RR Rr r r Rr Rr Rr rr Red (R) Blue (r) R R R r Rr Rr R r RR Rr r r Rr Rr Rr rr All F1= Heterozygous (Rr) F2 = 1 RR : 2 Rr : 1 rr All Heterozygous genotypes will show the blending effect of phenotypes.

SNAPDRAGONS ILLUSTRATE INCOMPLETE DOMINANCE GENERALLY; MOST ‘BLENDED’ FLOWER COLORS ILLUSTRATE INCOMPLETE DOMINANCE! SURF

CO-DOMINANCE aka: Shared Dominance. Two dominant alleles code two different proteins. R’ = the codominant allele No blending of characteristics. All dominant proteins are coded.

Example 1: Coat color R= brown R’= black R R’ R R’ RR RR’ RR’ R’R’

How are they similar? How are they different? They are both co-dominant for black and white…. The cow’s color placement is random. The panda has genes that code for color placement.

Example 2: Sickle Cell R R’ R R’ R= Round R’= Sickle Persons with RR’ suffer from milder symptoms than R’R’ because they have some normal cells R R’ R R’ RR RR’ RR’ R’R’

NORMAL RR SICKLE CELL RR’

Example 3: Blood Types IA IB and i(type O) A is codominant to B 3 alleles code for blood type it is polygenetic co-dominant IA IB and i(type O) A is codominant to B Both are expressed: Type AB A is dominant to O : Type AA or AO B is dominant to O : Type BB or BO O is recessive: Type OO

Taste Receptor: Phenylthiocarbamide (PTC) The “taster” gene is co-dominant to the “non-taster” gene. People with two copies of the taster gene taste the chemical compound more intensely than those with only one copy. Chemicals similar to PTC are found in plants of the mustard family. (Broccoli, mustard, etc.) This gene is found among many other taster genes on chromosome 7.

www.loofli-rpa.genetic.org 25-50 mild 0-24 moderate Negative score= severe www.loofli-rpa.genetic.org

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