Question

As described in Chapter 22 , researchers have been able to clone mammals by fusing a cell having a diploid nucleus (i.e., a somatic cell) with an egg that has had its nucleus removed. A. With regard to maternal effect genes, would the phenotype of such a cloned animal be determined by the animal that donated the egg or by the animal that donated the somatic cell? Explain. B. Would the cloned animal inherit extranuclear traits from the animal that donated the egg or from the animal that donated the somatic cell? Explain. C. In what ways would you expect this cloned animal to be similar to or different from the animal that donated the somatic cell? Is it fair to call such an animal a clone of the animal that donated the diploid nucleus?

Short answer

A. The phenotype of a cloned mammal, with regard to maternal effect genes is determined by the animal that donated the egg. B. The cloned animal would inherit extranuclear traits from the animal that donated the egg. C. While the cloned animal would be largely genetically similar to the animal that donated the somatic cell, differences stemming from maternal effect genes and extranuclear traits exist. It is 'fair' to refer to these animals as clones given their identical nuclear DNA, but this term may not encompass the inherent complexity of the organism's genetics.

Step-by-step solution

Discuss Maternal Effect Genes

Maternal effect genes are genes whose expression in the offspring is determined by the mother's gene, meaning that the phenotype of a cloned animal would be most influenced by the animal that donated the egg. These phenotypes may be attributed to cytoplasmic factors or to early or rapid gene transcription of the zygote.

Explain the Inheritance of Extranuclear Traits

Extranuclear traits, also known as cytoplasmic inheritance or non-Mendelian inheritance, are traits that are inherited independently of the nucleus' genetic material. In this case, mitochondrial and chloroplast DNA, which is only passed down from the mother, would provide inherited traits. Thus, the cloned animal would inherit extranuclear traits from the animal that donated the egg.

Analyze the Similarities and Differences

The cloned animal would be genetically identical to the animal that donated the somatic cell in terms of nuclear DNA, hence manifesting similar traits. However, differences owing to the influence of the maternal effect genes and extranuclear traits exist that are inherited from the animal that donated the egg. Furthermore, environmental factors and randomness during DNA replication may also introduce variations.

Evaluate the Term 'Clone'

Since the cloned animal has identical nuclear DNA to the somatic cell donor, it is conventionally adequate to call it a clone. However, considering non-genomic components such as maternal effect genes and extranuclear traits, the term 'clone' may mask the complexities involved in the overall genotype and phenotype of the cloned animal.

Key concepts

Maternal Effect Genes

Maternal effect genes are fascinating because their influence is not derived from the genes of the offspring itself. Instead, their effect comes from the genetic information of the mother. In the context of cloning, this becomes particularly intriguing. When an egg cell is used, particularly one that has had its nucleus removed for cloning purposes, it still leaves certain genetic footprints. The egg donor’s cytoplasmic factors, as well as genes that are rapidly transcribed early in development, play a critical role.
As such, the phenotype of the cloned mammal is partially determined by the egg donor through these maternal effect genes. This means that while the cloned animal may have nuclear DNA matching the somatic cell donor, early traits influenced by maternal effect genes are reflective of the animal that provided the egg. This is an essential detail of how maternal influences can subtly yet significantly impact the clone’s phenotype.

Extranuclear Inheritance

Extranuclear inheritance deals with genetic information that exists outside the traditional nucleus, primarily in organelles like mitochondria. Mitochondria have their own DNA, which is inherited only from the mother, as egg cells contribute almost all of the cytoplasm in a fertilized egg. In cloning, the egg donor becomes the source of all extranuclear genetic material.
This means that any traits controlled by mitochondrial DNA, such as certain metabolic functions, will be inherited from the egg donor and not from the somatic cell donor. Even though the cloned animal will have nuclear DNA identical to the somatic cell donor, its mitochondrial traits will reflect those of the egg donor. This highlights a critical aspect of inheritance that goes beyond the nuclear genome and plays a substantial role in the clone's characteristics.

Somatic Cell Nuclear Transfer

Somatic cell nuclear transfer (SCNT) is the cornerstone of cloning technology. It involves the integration of a nucleus from a somatic cell, which contains the complete set of DNA, into an enucleated egg. This procedure is essential for creating organisms that are genetically identical to the somatic cell donor in terms of nuclear DNA.
Although SCNT results in an organism with identical nuclear genetic material to the donor, other factors complicate the notion of a "clone." Elements like maternal effect genes and mitochondrial DNA introduced by the egg donor can cause discrepancies in phenotype, as well as potential variations due to environmental influences and epigenetic factors. Therefore, while the term "clone" implies complete genetic identity, it is nuanced by the involvement of inherited extranuclear elements and maternal effects, making the cloned animal slightly different in certain aspects.