Science Homework Help

Molecular bases of penetrance and expressivity

Mendel was performing his classical experiments with pea planta that have very clear-cut phenotypes. The plant was either with purple or white flowers, with yellow or green seeds, etc. However, when working with human disorders time to time we would observe certain traits that should be expressed but they are not, or they will be expressed but with varying degrees among the offspring [1, 2]. The advancement of the Molecular Genetics allowed for better understanding of the mechanisms underlying these phenomena [3].

Can you find examples of traits that show penetrance or varying expressivity not only in humans but in other organisms as well? Can you find explanation from a molecular point of view what the underlying mechanism is?

Here’s response 1 from classmate 3 days ago

Jacob Blancher

RE: Molecular bases of penetrance and expressivity

COLLAPSE

Certain traits that are not expressed when they should be or are expressed in varying degrees are known as incompletely penetrant. An example of incomplete penetrance in humans is osteogenesis imperfecta (OI) which affects our bones. OI is caused by a dominant mutation in one of the genes that creates collagen COL1A1 or COL1A2. Some people with the dominant are entirely asymptomatic (Miko). However, others with the dominant mutation can have any combination of various symptoms including weak bones, joints, teeth, and blueish tint in the whites of their eyes.

An example of incomplete penetrance in non-humans is polydactyly in cats. Polydactyly is the presence of extra toes on a cat’s paw and is caused by interbreeding between generations. Cats affected by polydactyly are known as Hemingway’s cats. This mutation has high penetrance, meaning cats with the gene are very likely to have extra toes, but the expression various. This means that the actual amount of extra toes is different among those affected by polydactyly. This degree of expression is controlled by the gene expression in tissue that surrounds the paws.

The underlying mechanism that causes incomplete penetrance is the sheer number of steps in gene expression. Anything that interferes with any part of the process is known as an epigenetic factor. These epigenetic factors could create an altered form of the protein that can not be activated, stop production entirely, or anything else that would silence the gene (Miko). The difference in expression could also be attributed to a difference in the amount of protein produced. Certain individuals may have the mutation gene up regulated or down regulated, meaning the gene is expressed more or less, respectively. This difference in protein concentration could also be what is causing the difference in symptoms.

Miko, Ilona. “Phenotype Variability: Penetrance and Expressivity.” Nature News. Nature Publishing Group, 2008. https://www.nature.com/scitable/topicpage/phenotype-variability-penetrance-and-expressivity-573/. 

here’s 2 Ashley Lavdas

RE: Molecular bases of penetrance and expressivity

COLLAPSE

Hello!

When Mendel was experimenting, he studied traits that ended up being completely penetrant, meaning that 100% of individuals with a specific genotype show the expected phenotype. Incomplete penetrance, however, occurs when there are progeny that have the expected phenotype, while others may not. In addition, expressivity can vary from person to person. Expressivity refers to the intensity of the expression of a phenotype given a genotype (Hartwell, 2018).

In humans, the most malignant form of eye cancer, known as retinoblastoma, is an example of incomplete penetrance and varying expressivity. Some people with retinoblastoma only have one eye affected, whereas others may have both eyes diseased. Retinoblastoma is about 75% penetrant. This means that only about 75% of people that have the mutant allele develop retinoblastoma. Usually, the incomplete penetrance and varying expressivity is a result of chance, but there are some cases where genes and/or environmental factors may play a role in the varying phenotypes (Hartwell, 2018).

An example of varying expressivity in dogs is the piebald coat color trait. There is an S locus with an MITF gene that is responsible for determining the white spotting of the coats in dogs. At the S locus there are two main alleles, S (nonwhite) and s (piebald or white). The piebald expressivity varies from limited to extensive depending on the organism. This phenotype is often observed in Beagle and Fox Terrier breeds. They often have colored heads but patches of white on the body (Piebald, White Spotting Dog Coat Color- S Locus).

References:

Hartwell, L. (2018). Genetics: From genes to genomes (Sixth edition). McGraw-Hill Education.

Piebald, White Spotting Dog Coat Color – S Locus. AnimaLabs©. (n.d.). http://www.animalabs.com/shop/dogs/piebald-white-s…