In human genetics, which is the correct pair of inherited sex chromosomes for a child to be a girl?

Introduction / Context:

Sex determination in humans is based on the combination of sex chromosomes inherited from the parents. Understanding which chromosomes come from the mother and which from the father is a fundamental concept in genetics. This topic is not only important for biology exams but also helps clear common misconceptions about who determines the sex of the child. The question asks specifically which combination of inherited chromosomes results in a female child.

Given Data / Assumptions:

• Humans have two types of sex chromosomes, X and Y.
• Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).
• The mother always contributes an X chromosome in her egg.
• The father contributes either an X or a Y chromosome in his sperm.
• The question focuses on the combination that produces a girl.

Concept / Approach:

In human reproduction, the female parent has genotype XX, so all of her eggs carry only the X chromosome. The male parent has genotype XY, so his sperm can carry either X or Y. When an egg is fertilised by an X bearing sperm, the resulting zygote has XX and will develop as a female. When an egg is fertilised by a Y bearing sperm, the zygote has XY and will develop as a male. Therefore, the key idea is that the father provides either X or Y, and the combination X from father and X from mother is required for a girl child.

Step-by-Step Solution:

Step 1: Note that the mother has XX and can only contribute X, never Y. Step 2: Note that the father has XY and can contribute either X or Y to the child. Step 3: Recognise that a female child has genotype XX. Step 4: Identify the option that combines X from father with X from mother. Step 5: Select X from father and X from mother as the correct pair for a girl child.

Verification / Alternative check:

Simple Punnett square diagrams in genetics chapters show the cross between XX (mother) and XY (father). When the egg with X meets sperm with X, the outcome is XX (female). When the egg with X meets sperm with Y, the outcome is XY (male). Teachers often emphasise that the sex of the child depends on which chromosome the father passes on, not on the mother. These repeated representations confirm that XX, specifically X from father and X from mother, produces a girl child.

Why Other Options Are Wrong:

Option A, X from father and Y from mother, is impossible because the mother does not have a Y chromosome to give. Option B, Y from father and X from mother, describes XY, which would produce a boy, not a girl. Option C, Y from father and Y from mother, is biologically impossible since the mother cannot provide Y. Option E, X from mother and no sex chromosome from father, does not represent a normal fertilisation event and is not a standard genotype for a living human child.

Common Pitfalls:

Many learners mistakenly think the mother can contribute either X or Y or that both parents equally decide the sex in terms of chromosome type. Others may not pay attention to the order in which the options list mother and father. To avoid confusion, it is useful to remember a simple rule: mother always provides X, father provides X or Y, and XX means girl while XY means boy. Drawing a quick Punnett square on rough paper during practice can also help fix this idea.

Final Answer:

The correct pair of inherited chromosomes for a child to be a girl is X from father and X from mother.