Incomplete Dominance, Codominance, Polygenic Traits, and Epistasis

In some guinea pigs, having hair is associated with the presence of a dominant allele “H.” Hairless guinea pigs do not have the dominant allele “H.” This is a Mendelian trait. 1. Show a Punnett square with a Mendelian cross between two guinea pigs that are Hh x Hh.

2. According to your work, complete the following phenotype ratio: ___3___Have Hair: ___1____Hairless 3 According to your work, complete the following genotype ratio: ___1___HH: ___2____ Hh: ___1___hh

The traits covered in this video are non-Mendelian traits, unlike #1. What does it mean for a trait to be non-Mendelian? A non-Mendelian trait may “not follow the {Mendelian} rules.” An example: in Mendelian inheritance, having a dominant allele means the dominant trait will show. This may not be the case in non-Mendelian traits.

Describe how incomplete dominance and codominance, two non-Mendelian traits, are different from each other. In incomplete dominance, the dominant allele is not completely expressed when the recessive allele is present. This can make it seem like an “in-between” kind of phenotype when both a dominant and recessive allele are together. In codominance, both alleles are dominant, which can mean a phenotype where both alleles are expressed.

There are a variety of ways to represent the alleles for incomplete dominance, codominance, and other non-Mendelian traits. Many times, there are different pros and cons for how alleles are represented as well as different preferences. How do you plan to represent the alleles for incomplete dominance and codominance, and how are you planning to keep them separate in your mind?

It is important to understand the difference of incomplete dominance and codominance when one is writing the alleles. As the video emphasizes, importance is to know what alleles represent regardless of how one chooses to write them!

Pleiotropy, not discussed in the video, is when just one gene can affect several traits! How is this vocabulary term different from a polygenic trait, which is discussed in the video? A polygenic trait is a trait is affected by MANY genes. (Example: human height) This is nearly the opposite of pleiotropy, which means that ONE gene can affect many traits.

Codominance can be observed in some breeds of chicken. Black chickens can result from BB alleles. White chickens can result from WW alleles. A chicken with alleles BW can be speckled with black and white. 8. Show a Punnett square with a Mendelian cross between two chickens that are BW x BW.

According to your work, complete the following phenotype ratio: ___1___Black: ____2___Speckled: ___1____White 10. According to your work, complete the following genotype ratio: ___1__BB: ____2___BW: ___1____WW

Incomplete dominance can be observed in snapdragons. Snapdragon flowers that have two RR alleles have a red phenotype. Snapdragon flowers with a rr have a white phenotype. Snapdragon flowers that are Rr are pink. 11. Complete the two Punnett squares in diagram at right. 12. This is a non-Mendelian trait, but how could this be different if the trait was Mendelian?

If this was a Mendelian trait, the presence of even just one dominant allele would mean that the snapdragons would be red. All Rr offspring would be red. That is not the case for this nonMendelian trait as the dominant allele is not completely expressed when the recessive allele is present due to incomplete dominance. All Rr offspring are pink.

Epistasis is when one gene really depends on another gene for it to be expressed. In this graphic, there is a gene (CC, Cc, or cc) - that controls whether the pigment will even be expressed in the llama wool in the first place. If a llama has the genotype cc, the gene will not allow the other gene for wool color (BB , Bb, or bb) to even be expressed. Llamas with “cc” gene are therefore white.

Apply the Vocab! The below graphic is a follow-up from the graphic in #13. Circle genotypes below that you would expect to result in white coloration and explain why.

The gene “cc” can result in pigment not being expressed in the llama due to epistasis explained in #13. All llamas with the “cc” gene will not have pigment expressed. That is a total of 4 llama genotypes in Punnett square above.