Question

Normal ears in a certain mammal are perky; mutants have droopy ears. In males of these mammals, the gene encoding perky ears is transcribed only from the female parent. This is because the gene from the male parent is silenced by methylation. If the maternal gene is mutated: a. male offspring have droopy ears. b. offspring have perky ears. c. male offspring have one droopy ear and one perky ear. d. the genetic mechanism is called alternative splicing. e. this is an example of posttranscriptional regulation.

Short answer

The correct answer is option a, male offspring have droopy ears because the gene for perky ears from the male is silenced, and the offspring can only express the trait from the mutated maternal gene.

Step-by-step solution

Understanding the genetic mechanism

Based on the information provided, the mechanism of inheritance for the perky ears trait in the given mammal is through maternal imprinting. In males, only the gene from the female parent is active because the gene from the male parent is silenced by methylation.

Analyzing the effect of the mutated maternal gene

Since only the maternal gene is expressed in males, if the maternal gene is mutated, this would mean that the male offspring cannot express the perky ears trait since the paternal gene is silenced. Thus, the males would express the phenotype of the mutation, which is droopy ears.

Eliminating incorrect options

The offspring cannot have perky ears as option b suggests because the functional gene from the male parent is silenced. The phenotype cannot be a mixture of perky and droopy ears in the males as per option c because only one of the parental genes is expressed in this scenario. Option d is incorrect because this scenario describes gene silencing due to methylation, not alternative splicing. Option e is also not applicable as the regulation is at the level of transcription and not posttranscription.

Identifying the correct answer

Given that the maternal gene is mutated and the paternal gene is silenced, the male offspring would express the droopy ears phenotype, so the correct answer is option a, male offspring have droopy ears.

Key concepts

Gene Silencing by Methylation

Gene silencing by methylation is a crucial process in the regulation of gene expression. Methylation is a chemical modification where a methyl group is added to DNA, specifically to the cytosine bases in the context of CG sequences, known as CpG islands. This modification can change the activity of a DNA segment without changing the sequence itself. When methylation occurs in the promoter region of a gene, it typically blocks the gene from being transcribed into RNA and thus, prevents the production of the corresponding protein.

In the context of the exercise, the gene from the male parent is silenced through methylation, meaning it does not contribute to the phenotype of the male offspring. If the maternal gene is functional, male offspring exhibit the 'perky ears' trait. However, if the maternal gene is mutated, the males inherit the phenotype associated with the mutation, which in this case is 'droopy ears'. This distinctly shows how methylation can dictate gene expression and ultimately, the traits inherited by offspring.

Maternal Imprinting and Methylation

Maternal imprinting is a type of gene silencing where the expression of certain genes is determined by the parent that contributed them. In mammals, this often involves methylation. Imprinted genes are critically important during development, and errors in imprinting or methylation patterns can lead to various genetic disorders.

Gene Expression in Inheritance

Gene expression in inheritance is fundamental for understanding how traits are passed from parents to offspring. Expression involves the processes through which genetic information is used to produce proteins, which are the workhorses of the cell and determine an organism's characteristics. Inheritance patterns can be influenced by whether a gene is turned 'on' or 'off,' which is controlled by mechanisms such as methylation.

When discussing maternal imprinting, as mentioned in the exercise, the inheritance pattern is such that only the gene from the female parent is active for certain traits, and the male's counterpart is silenced. This selective expression affects how traits are manifested in progeny. Therefore, the male offspring's phenotype depends only on the maternal allele of a gene. If it's mutated, regardless of the paternal allele, the mutation's effects will be visible in the progeny. This mechanism highlights how gene expression levels directly impact inheritance and phenotypic traits.

Phenotypic Expression of Mutations

The phenotypic expression of mutations refers to the observable physical or biochemical characteristics of an organism as a result of its genetic makeup. Mutations can vary in impact, from benign to causing significant changes in an organism's phenotype. In the exercise, we see a clear example of phenotypic expression: males inherit droopy ears due to a mutation in the maternally derived gene.

Understanding Dominant and Recessive Mutations

A mutation's effect can be dominant or recessive, influencing whether it will manifest when one or both copies of a gene are mutated. In the example of maternal imprinting, the paternal gene is silenced, and therefore any mutation in the maternal gene is expressed as the dominant phenotype in the male offspring. It's a demonstration of how a single genetic alteration can govern an observable characteristic. The phenotype ultimately provides visual evidence of how genetic information is expressed and can be used to trace inheritance patterns and mutations' impacts.