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Bio- Genetics: Incomplete Dominance Name:_______________________Hr:____ So far, we’ve discussed only Complete Dominance, meaning that only two phenotypes are possible for three different genotypes. H=hairy toes Genotypes HH Hh hh h=non-hairy toes Phenotypes hairy toes hairy toes non-hairy toes Something new! Incomplete Dominance occurs when one allele is not completely dominant over the other allele for a trait. This means that the heterozygous phenotype is somewhere in between the two homozygous phenotypes. For instance: Snapdragons (flowers) R=red Genotypes RR Rr rr r=white Phenotypes red pink** white Follow the same steps in order to solve problems: 1. Assign letters to represent the genes 2. Determine the parental phenotypes and genotypes. 3. Predict the outcome of a cross between the possible gametes from the two parents. 4. State phenotypic and genotypic ratios present in the offspring. 5. Don’t forget to answer the question within the problem. Example: Cross a homozygous dominant snapdragon with a homozygous recessive snapdragon. Assume incomplete dominance. Step 1: RR=red rr=white Rr=pink Step 2: Parents’ phenotypes: red (homozygous dominant) x white (homozygous recessive) Parent’s genotypes: RR x rr Step 3: Step 4: R R r Rr Rr r Rr Rr Genotypic Phenotypic Ratio Ratio 100% Rr 100% pink Example: Shape of a radish is controlled by gene that shows incomplete dominance. There are two alleles for the gene, round shape and long shape. The phenotypes possible in the radishes are round, oval, or long. Suppose you have seeds for round and oval plants, and that you cross these plants. What would be the phenotypes present in the next generation of plants produced from this cross? Step 1: RR=round rr=long Rr=oval Step 2: Parents’ phenotypes: round x oval Parent’s genotypes: RR x Rr Step 3: Step 4: R R R RR RR r Rr Rr Genotypic Ratio Phenotypic Ratio 50% RR 50% Rr 50% round radishes 50% oval radishes Example: Using the same information from the problem above regarding radishes, cross two oval radishes. What is the probability of getting round radishes from this cross? Step 1: RR=round rr=long Rr=oval Step 2: Parents’ phenotypes: oval x oval Parent’s genotypes: Rr x Rr Step 3: Step 4: R r R RR Rr r Rr rr Genotypic Ratio Phenotypic Ratio 25% RR 50% Rr 25% rr 25% round radishes 50% oval radishes 25% long radishes Step 5: The probability of getting round radishes from this cross is 25%. Bio- Genetics: Codominance Name:_______________________Hr:____ Codominance is when BOTH alleles contribute to the phenotype of the organism. Therefore, in this case a heterozygous phenotype will express both alleles separately and simultaneously. (You will see both alleles in the phenotype.) For example: In shorthorn cattle, red coat color is codominant with white coat color. The heterozygous condition results in a color known as ROAN, which is intermingled red and white hairs. (Roan is not pink) Remember the letters we assign are arbitrary (we make them up; you can choose anything). Let’s say R=red and r=white (white is not recessive; it is still dominant) Genotypes RR Rr rr Phenotypes red roan** white Follow the same steps in order to solve problems: 1. Assign letters to represent the genes 2. Determine the parental phenotypes and genotypes. 3. Predict the outcome of a cross between the possible gametes from the two parents. 4. State phenotypic and genotypic ratios present in the offspring. 5. Don’t forget to answer the question within the problem. Example: A roan bull is mated with a red cow. Show the genotypes and phenotypes of the offspring. Step 1: RR=red rr=white Rr=roan Step 2: Parents’ phenotypes: roan x red Parent’s genotypes: Rr x RR Step 3: Step 4: R r R RR Rr R RR rr Genotypic Phenotypic Ratio Ratio 25% RR 50% Rr 25% rr 25% red offspring 50% roan offspring 25% white offspring Example: In rabbits of black and white coloration, F is the allele for fur color. If F= black fur allele f= white fur allele then… FF= Black rabbit ff=White rabbit Ff=Black AND white rabbit What is the probability of getting black bunnies from mating a white rabbit with a black and white rabbit? Step 1: FF=black ff=white Ff=black and white Step 2: Parents’ phenotypes: black x black and white Parent’s genotypes: FF x Ff Step 3: Step 4: F F F FF FF f Ff Ff Genotypic Phenotypic Ratio Ratio 50% FF 50% Ff 50% black bunnies 50% black and white bunnies Step 5: The probability of getting black bunnies is 50%. Practice Problems: The interactions of genes that cause sickle-cell anemia show another example of C0-DOMINANCE. Sickled cells (abnormally shaped red blood cells) are unable to carry oxygen properly and often clump up in the blood vessels. The physical symptoms of anemia are the result of all sickled cells. Those symptoms include fatigue, shortness of breath, tingling of extremities, and blueness of the skin. However, individuals who are heterozygous (Ss) produce both normal red blood cells (RBCs) and sickled red blood cells. Neither allele is dominant—both are expressed in the phenotype. This is called sickle cell trait. SS— Normal RBCs ss— Sickled RBCs Ss--- Sickled RBCs and Normal RBCs 1. Suppose a man with sickle cell anemia marries a woman with the sickle cell trait. What are the predicted genotypes and phenotypes of their children? 2. Suppose a woman with normal RBCs marries a man with the sickle trait. Will any of their children have sickle cell anemia? Will any of their children carry the sickle cell trait? 3. You are a genetic counselor and a couple comes in to visit you. They are very concerned that they will have a child with sickle cell anemia, because they both have the sickle cell trait. What will you say to them? What is the chance that their child will have sickle cell anemia? Sickle cell trait? Have normal RBCs? 4. In pigs, having a curly tail is dominant over having a straight tail. If a piglet is heterozygous for the tail gene, his or her tail will be crooked. Suppose a heterozygous pig mates with a straight-tailed pig. What is the probability of having curly tail piglets? 5. In an island bird population, short, crushing beaks are incompletely dominant over long, delicate beaks. The heterozygotes have medium length beaks. Show the genotypic and phenotypic ratios of a cross between two birds with short, crushing beaks.
