Incomplete Dominance and Codominance

Transcription

1 Incomplete Dominance and Codominance Name Define incomplete dominance Incomplete dominance can be remembered in the form of Red flower X white flower = pink flower The trick is to recognize when you are dealing with a question involving incomplete dominance. There are two steps to this: 1) Notice that the offspring is showing a 3rd phenotype. The parents each have one, and the offspring are different from the parents. 2) Notice that the trait in the offspring is a blend (mixing) of the parental traits. Try these sample problems below: 1. In blahblah birds, blue (B) feathers is incompletely dominant to white (W) feathers. The color of blahblah birds is determined by just two alleles. What are the genotypes of a blue blahblah bird and a white blahblah bird in the original cross? What is/are the genotypes of the F 1 generation? What is the phenotype of the F 1 generation? What would be the genotype and phenotype ratios of the F 2 generation?

2 2. A rooster with gray feathers is mated with a hen of the same phenotype. Among the offspring, 15 chicks are gray, 6 are black, and 8 are white. What is the simplest explanation for the inheritance of these colors in chickens? What would be the expected genotype and phenotype ratios from the mating of a gray rooster with a black hen? What phenotypes would you expect to see in the above animals if the alleles were codominant? 3. The flowers of the snapdragon plant can be red, pink, or white. Color is determined at a single locus. The genotype F R F R results in red flowers and F W F W results in white flowers. The heterozygote genotype of F R F W results in pink flowers. When the heterozygote has a different, intermediate phenotype compared to the homozygous dominant or homozygous recessive individuals, this is said to be incomplete dominance. 1. What kind(s) of gametes can a white male snapdragon of the P generation produce? 2. What kind(s) of gametes can a red female snapdragon of the P generation produce? 3. What is the expected genotype and phenotype ratios of a cross between and red snapdragon and white snapdragon? Complete the cross in the space below?

3 4. In a species of parrot, blue is dominant to yellow, but sometimes green feathers are found. This is incomplete dominance. Mate two green feathered birds together and determine the phenotype ratio. What is the expected genotype and phenotype ratios resulting from a cross between a blue rooster and blue hen? Codominance Define Codominance First let me point out that the meaning of the prefix "co-" is "together". Cooperate = work together. Coexist = exist together. Cohabitat = habitat together. In COdominance, the "dominant" traits appear together in the phenotype of hybrid organisms. A very common phenotype used in questions about codominance is roan fur in cattle and horses. Cattle can be red (H R H R = all red hairs), white (H W H W = all white hairs), or roan (H R H W = red & white hairs together). A good example of codominance. "The diversity of Texas Longhorn coloration is celebrated by many modern longhorn breeders. Texas Longhorns represent one of the best breeds in which to study the genetics of coat coloration, because much of the color variation that is known across all breeds of cattle exists in Texas Longhorns." Longhorns may be white (C W C W ), red (C R C R ) or roan (C R C W ). Roan longhorns have a mixture of both white hairs and red hairs due to a codominant gene. "A single copy can be expressed by just a few white hairs on the face or extremities, unevenly roaned patches, or an even mix of white and colored hairs all over the body. Two copies produce an almost white animal, with some pigment around the ears."

4 Try these sample problems below: 1. What would be the expected genotype and phenotype ratios in the F 1 generation if a red male longhorn was crossed with white female longhorn? 2. What would be the expected genotype and phenotype ratios in the F 2 generation if any of the longhorns in the F 1 generation were crossed? 3. What should the genotypes & phenotypes for parent cattle be if a farmer wanted only cattle with red fur?

5 4. A cross between a black cat & a tan cat produces a tabby pattern (black & tan fur together). a. What pattern of inheritance does this illustrate? b. What would be the phenotype of the F 1 generation if a black cat were crossed with a tan cat? Of the F 2 generation? c. What percent of kittens would have tan fur if a tabby cat were crossed with a black cat? 5. Define polygenic inheritance? 6. DefinePleiotropy? 7. What is epistasis? 8. Is it possible for two parents to have brown eyes and have a kid with blue or green eyes? If so, what are the chances? Show all the work!!! 9. Is it possible for one parent to have green eyes and one parent to have brown eyes and have a kid with blue eyes? If so, what are the chances. Show all the work!!!