The protoplasm (overall living cell content) of a child is genetically and cellularly most exactly the same as that of which of the following relatives?

Introduction / Context:
In classical biology language, the term protoplasm refers to the living substance of the cell, including cytoplasm and nucleus. When we compare individuals, we usually think about similarities and differences in their genetic material and cell structure. A common examination question asks whose protoplasm is exactly the same as that of a given individual. This is essentially a question about who shares the same genetic makeup and extremely similar cellular composition.

Given Data / Assumptions:

• The question refers to the protoplasm of a child.
• Options include mother, father, grandfather, identical twin, and elder sibling (non-identical).
• We interpret "exactly the same" as having the same genetic information in their cells.
• We assume typical human reproduction, in which most siblings are not identical twins.

Concept / Approach:
Every person inherits half of their nuclear DNA from the mother and half from the father, so neither parent is genetically identical to the child. Grandparents share even less genetic identity. Non-identical siblings share a significant proportion of genes but still have unique combinations. Only identical twins, also called monozygotic twins, arise from the same fertilised egg that splits into two embryos. As a result, identical twins have the same genetic material in every cell, making their protoplasm essentially the same in classical exam language.

Step-by-Step Solution:
Step 1: Recall that a child receives 50 percent of its nuclear DNA from each parent. Step 2: Since the combination is unique, neither the mother nor the father has exactly the same genetic makeup as the child. Step 3: Grandparents contribute indirectly through parents, so they share even less of the genetic material compared with the child. Step 4: Ordinary siblings, born from different fertilisation events, share some genes but are genetically distinct individuals. Step 5: Identical twins originate from a single zygote that splits, so both individuals have the same set of chromosomes and genes in their cells. Step 6: Therefore, the protoplasm of a child is exactly the same as that of its identical twin.

Verification / Alternative check:
Genetics textbooks define identical twins as monozygotic and emphasise that they are genetically identical at conception, with identical nuclear DNA. They are widely used in twin studies to understand the roles of genes and environment, precisely because they start with the same genetic material. This is why exam questions about identical protoplasm point to identical twins rather than parents or ordinary siblings.

Why Other Options Are Wrong:
Mother: Shares half of the child's genes but is not genetically identical to the child. Father: Also contributes half the genes but has a different overall combination. Grandfather: Shares a smaller fraction of genes and is two generations apart. Elder sibling (non-identical): Has some genetic similarity but results from a different fertilisation event and is not identical.

Common Pitfalls:
Students sometimes confuse strong family resemblance with true genetic identity. They may think the child is exactly like the mother or father in terms of protoplasm because of physical similarities. Others may pick any sibling rather than specifically identical twin. To avoid these mistakes, link the phrase exactly the same protoplasm with identical twins, since they alone share the same original zygote and therefore the same genetic blueprints.

Final Answer:
The correct answer is Identical twin.